214
IAEA-TECDOC-429 REACTOR WATER CHEMISTRY RELEVANT TO COOLANT-CLADDING INTERACTION A TECHNICAL DOCUMENT ISSUED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1987
IAEA-TECDOC-429
REACTOR WATER CHEMISTRYRELEVANT TO
COOLANT-CLADDING INTERACTION
A TECHNICAL DOCUMENT ISSUED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1987
REACTOR WATER CHEMISTRY RELEVANT TOCOOLANT-CLADDING INTERACTION
IAEA, VIENNA, 1987IAEA-TECDOC-429
Printed by the IAEA in AustriaSeptember 1987
PLEASE BE AWARE THATALL OF THE MISSING PAGES IN THIS DOCUMENT
WERE ORIGINALLY BLANK
The IAEA does not normally maintain stocks of reports in this series.However, microfiche copies of these reports can be obtained from
IN IS ClearinghouseInternational Atomic Energy AgencyWagramerstrasse 5P.O. Box 100A-1400 Vienna, Austria
Orders should be accompanied by prepayment of Austrian Schillings 100,-in the form of a cheque or in the form of IAEA microfiche service couponswhich may be ordered separately from the INIS Clearinghouse.
FOREWORDThe report is a summary of the work performed in a frame of a
Coordinated Research Program organized by the IAEA and carried outfrom 1981 till 1986. It consist of a survey on our knowledge oncoolant-cladding interaction: the basic phenomena, the relevantparameters, their control and the modelling techniques implementedfor their assessment.
Based upon the results of this Coordinated Research Program,the following topics are reviewed on the report:
- role of water chemistry in reliable operation of nuclearpower plants
- water chemistry specifications and their control- behaviour of fuel cladding materials- corrosion product behaviour and crud build-up in reactor
circuits- modelling of corrosion product behaviour
This report should be of interest to water chemistrysupervisors at the power plants, to experts in utitility engineeringdepartments, to fuel designers, to R&O institutes active in thefield and to the consultants of these organizations.
EDITORIAL NOTE
In preparing this material for the press, staff of the International Atomic Energy Agencyhave mounted and paginated the original manuscripts as submitted by the authors and givensome attention to the presentation.
The views expressed in the papers, the statements made and the general style adopted arethe responsibility of the named authors. The views do not necessarily reflect those of the govern-ments of the Member States or organizations under whose auspices the manuscripts were produced.
The use in this book of particular designations of countries or territories does not imply anyjudgement by the publisher, the IAEA, as to the legal status of such countries or territories, oftheir authorities and institutions or of the delimitation of their boundaries.
The mention of specific companies or of their products or brand names does not imply anyendorsement or recommendation on the part of the IAEA.
Authors are themselves responsible for obtaining the necessary permission to reproducecopyright material from other sources.
CONTENTS
1. INTRODUCTION ...................................................................................... 7
1.1. History of the CCI program ................................................................... 71.2. Role of water chemistry in reliable operation of nuclear power plants ............... 9
2. WATER CHEMISTRY SPECIFICATIONS AND THEIR CONTROL ..................... 15
2.1. PWR primary water chemistry specifications .............................................. 152.2. BWR reactor and feed water chemistry specifications .................................... 262.3. Water cleaning control systems ............................................................... 332.4. Monitoring water chemistry ................................................................... 39
3. BEHAVIOUR OF FUEL CLADDING MATERIALS .......................................... 42
3.1. Waterside corrosion of fuel cladding ........................................................ 423.2. Hydrogen absorption ............................................................................ 503.3. Accident conditions .............................................................................. 51
4. CORROSION PRODUCT BEHAVIOUR AND CRUD BUILD-UP INREACTOR CIRCUITS ................................................................................ 57
4.1. Migration and deposition of corrosion products and impurities ........................ 574.2. Modelling of corrosion product behaviour and crud build-up in
PWR and BWR circuits ........................................................................ 66
5. CONCLUSIONS ........................................................................................ 79
ANNEX I: ANALYTICAL TECHNIQUES FOR MONITORING WATERCHEMISTRY ................................................................................. 81
ANNEX II: MODELLING THE BEHAVIOUR OF CORROSION PRODUCTS INTHE PRIMARY HEAT TRANSFER CIRCUITS OF PRESSURIZEDWATER REACTORS - A REVIEW OF PRINCIPLES ............................. 105
ANNEX III: A REVIEW OF MODELS DESCRIBING THE BEHAVIOUR OFCORROSION PRODUCTS IN PRIMARY HEAT TRANSFERCIRCUITS OF BWRs ...................................................................... 165
1. INTRODUCTION
1.1 History of the CCI Program
The CCI Program is a Coordinated Research Program which hasbeen pursued over the period 1981 to 1986; its official title being"Investigation of Fuel Element Cladding Interaction with WaterCoolant in Power Reactors". This program was not defined to satisfya precipitant decision by technocrats: it was the response ofconcerned experts to the growing perception of the challengesarising from improved fuel utilization.
The IAEA initiatives are guided, in the field of Water ReactorFuels by an advisory body, the International Working Group on FuelPerformance and Technology ("IWGFPT"). This IWGFPT is made up ofexperts representing an IAEA Member State or an internationalorganization. The discussions during the IWGFPT sessions and theconferences, symposia, specialists meetings, technical committeesand consultant meetings organized by the IAEA in line with IWGFPT'srecommendations shed light on fuel related technical areas where aspecific IAEA initiative would be welcome.
Such a sequence of events was operative in the case of the CCIProgram. In the 70's, the thermal hydraulic operating conditions ofthe coolant were stretched in the most recent power plants toimprove their efficiency. Simultaneously, the economic advantagesof improving fuel utilization and the inherent margins of commercialfuel fostered a progressive increase of discharge burn-ups as wellas possibilities of load following in most power plants. Theresulting longer fuel residence times and the more aggressivecoolant conditions highlighted life limitation features resultingfrom coolant-cladding interaction: corrosion (uniform and nodular)and crud deposits (capable of influencing cladding corrosion).
The awareness of these life limitation phenomena led interestedmembers of the IWGFPT to suggest, in 1979, that the IAEA shouldsurvey the field, investigate the involved phenomenology and control
systems, and report the state of the art for the benefit of theMember States. The CCI Coordinated Research Program ("CRP") waslaunched in 1980, started in 1981 and was pursued until 1986. Thepresent report is the final summary of this action.
The CCI program was sub-divided into 4 tasks implementedsuccessively as the work progressed and the following four areaswere defined:
Task 1 "Survey of fuel element cladding interaction with watercoolant in power reactors" included the formulation of a surveyquestionnaire, the evaluation of the responses by experts, thesummarization of the information into a survey report "Fuel CladdingInteraction with Water Coolant in Power Reactors" and itspublication as IAEA-TECDOC-356 in 1985. The results of this Task 1are reflected in sections 1.2, 2.1, 2.2, 2.3, 3.1, 3.2, 3.3 and 4.1of the present report.
Task 2 "Analytical techniques for monitoring Water Chemistryin Power Reactors", was launched in 1982, as a result of thequestion marks appearing during the progress of Task 1. Itconsisted of the formulation of a questionnaire and summarizing theresponses. The outcome is reflected in section 2.4 andAnnex I of the present report.
Task 3 "Modelling of corrosion products and crud build-up inwater-cooled reactor circuits" was launched in 1983 as it becameapparent that powerful models were developed or being developed inseveral Member States to deal with the prediction and understandingof the phenomena. The review of PWR and BWR modelling approaches isreflected in Chapter 4 and Annexes II and III of the present report.
Task 4 "Summary of the results of the CRP" was launched in1984, as it became apparent that the findings of this CCI should beorganized in an orderly manner for the benefit of the Member Statesand the Institutes which have not been directly connected to theCRP. It has resulted in the present report.
Being a generic overview of the situation by the participantsbased on their own expertize and on technical reports publishedearlier (IAEA TECDOC - 356) or incorporated as Annexes 2 and 3, itwas not found appropriate to repeat bibliographical references inthe report itself. The reader is therefore provided in this reportwith a digested view on the topics and not an exhaustive technicalreview.
This 5 year study period of the CRP and the results thereofwere only possible thanks to the collaboration of the participatingMember States and the individuals who have contributed to the work,mainly:
Argentina: A J. G. MarotoBelgium: H. Bairiot, P. Blanpain, M. Lippens and J. Van VlietBulgaria: I. DobrevskyCzechoslovakia: J. KyselaFinland: J. HakalaUnited Kingdom: P. Campion, G. Comley, D. J. Ferrett, M. V. Polley,
R. S. Rodliffe and E. W. ThorntonHungary: A. Elek, F. Molnar, E. Szabo and J. SchunkIndia: M. Das, S. H. Gadiyar, P. K. Mathur, S. V. Narasimhan,
G. Venkaterswaran and K. S. VenkateswarluJapan: K. Ishingure, Y. Mishima, Y. MeguroRomania: G. SemenescuSoviet Union: Y. A. Khitrov, V. Kritsky and I. RybalchenkoIAEA: A. Nechaev, N. Noe, V. Onufriev and I. Rybalchenko.
1.2 Role of Water Chemistry in Reliable Operation of Nuclear PowerPlants
At present and for the near future nuclear power development inthe world is based mainly upon water-cooled reactors. In nuclearpower plants water is used as a neutron moderator and a reactorcoolant, in the secondary circuit (PWRs and PHWRs) and in a numberof auxiliary systems. At elevated temperatures water is anaggressive substance when in contact with structural materials.This means that the reliability of many nuclear power plant systemsis dependent upon water quality which is especially critical when in
contact to fuel cladding and core materials. Normally, reliabilityand safety are achieved by using high integrity cladding andstructural materials together with special measures to preventexcessive corrosion, erosion and other processes. Reliable waterchemistry regimes have been developed for BWRs, PWRs and PHWRs. Inrecent years there has been a growing interest in thisproblem. Water-cooled power reactor experience shows that evenunder normal working conditions some undesirable effects on the fuelelement cladding integrity can occur due to corrosion, erosion,hydriding and deposition on heat transfer surfaces of corrosionproducts as well as other coolant impurities.
Moreover, migration and transport of corrosion products andother impurities followed by activation in the reactor core can leadto formation of highly radioactive deposits in some out-of-coreparts of the primary circuit and to radioactive contamination of theprimary pipes and equipment. This causes difficulties inmaintenance and repair because of the high radiation field and couldrequire decontamination of some equipment and even of the primarycircuit as a whole. More serious problems connected with theinteraction between the water coolant and cladding material mayoccur under abnormal conditions. The need to decrease radiationdoses from the primary circuit equipment and piping has led to newefforts to improve water chemistry. Development of advanced waterreactor concepts to achieve higher burnups and better fuelutilization also requires additional safety measures because oflonger residence time of fuel in reactors, in some cases highercoolant temperatures, with the possibility of nucleate boiling inPWRs, etc.
The main objectives of reactor coolant chemistry are:reduction of water agressivity towards the structural materials;
minimization of in-core impurity ingress and out-of-coreradioactivity build-up;
prevention of fuel surface fouling with solid deposits whichmay result in heat transfer degradation and enhanced corrosionrates ;
10
prevention of parasitic neutron capture by the core deposits;
achievement of the lowest possible occupational radiationexposure;
prevention of radioactivity release to the environment.
The structural materials for the reactor primary circuit mustwithstand severe stresses imposed by intense neutron fluxes [10
15 2to 10 n/(cm . s)] and radiation, high operating temperatures(typically 300 - 400 C) and pressures, strong corrosive attack ofsome coolant impurity anions such as chloride, sulphate etc.
Austenitic steels are the dominant materials for reactorinternals, piping, steam generator tubing, pumps, valves,reheaters. This type of steels has excellent general corrosionresistance and mechanical strength. They can resist operatingtemperatures up to 500 C. Experience indicates however that inthe presence of anions e.g. chlorides, they become susceptible totransgranular stress corrosion cracking.
Low alloy Ferritic steels are used for pressure vessel shells, steamgenerators, heat exchangers, drum separators and the like. Forthese materials the operating temperatures should be limited tobelow 500°C.
The use of mild steels is restricted to feed water linesystems, heat exchanger vessels, saturated steam tubing and so on.The rated operation temperature should be limited to 350 C.Because of their low corrosion and temperature resistance they arenot suitable for the primary components and equipment.
In the current water-cooled reactors zirconium alloys are theusual materials for fuel cladding and fuel channel pressure tubes.Zirconium alloys have low thermal neutron absorption cross-sectionsand show high temperature and corrosion resistance in demineralisedwater up to 350 C.
11
Nickel and Titanium -alloys are finding applications in steamgenerators, reheaters, regenerative heat exchangers and the like.Ni-alloys are especially attractive for their high temperatureresistance, and a good resistance to transgranular corrosion (in thepresence of chloride) but are susceptible to caustic intergranularcracking. The application of titanium alloys is restricted by costand availability considerations.
Copper alloys are employed on the low temperature surfaces ofthe heat transfer systems (turbine condensers, regenerative heaters,auxiliary heat exchangers).
Nuclear power plant systems are composed of a large variety ofmetals and alloys. Corrosion resistance and possible corrosionproduct contamination of the primary system must always be takeninto consideration. From the standpoint of the heat transferflow-sheet, the coolant circuits in water-cooled nuclear reactorsmay be single or double.
In the single (direct cycle) circuit the nuclear boiler iscoupled directly to the turbine system. The turbine condensateprovides the primary contribution to the feed water flow, whichcontinuously enters the reactor. Water chemistry control iscomplicated by a sophisticated circuit design and the necessity forfeed water purification from impurities originating in the turbineand condensate systems. In addition, the in-core accumulation ofthe radioactive contaminants (on the core surfaces and in the liquidcoolant) can lead to serious out-of-core activity buildup.
In the double (indirect cycle) systems, the closed primarycircuit is coupled to the secondary circuit through the steamgenerator surfaces. These systems are characterized by highintegrity and limited branching of the primary circuit. The primarycircuit operates at higher pressures than the secondary circuit (toprovide the necessary thermal gradients in the steam generator).
High water purity is the major requirement for proper coolantquality control. Reactor cooling water is treated with differentchemicals (such as hydrogen, alkalis, boric acid) to prevent
12
radiolytic water decomposition, minimize corrosion of reactorstructural materials and provide some reactivity control. To meetthe purity requirements strict limitations are placed on waterchemical composition and impurity content for primary and feedwater, steam, turbine condensate, etc. (Table 1.1).
Table 1.1.Impurities in coolant circuits
Source of impurities Chemical compositionof impurities
Measures to prevent ordecrease impurityaccumulation
Initial feed water forcircuits and make upwater
Soluble salts and gases(Na+, K+, Mg*+,
2+, Cl~, N0~ Si02Ca
Efficient purificationof water
Cladding materials(other than Zircaloys)
Corrosion products Minimizing corrosionby water treatment
Uranium and fissionproducts due to surfacecontamination duringfuel fabrication
Structural materials Corrosion products:Fe,(heat exchangers, Co, Cr, Ni, Mn, Cu,vessels, valve seats Zn etc.etc.)Pressurizers (for PWR) 0-, N_
Surface cleaning afterfuel fabrication
Coolant and condensatepurification systemsCooling water and airejection in vacuumpart of turbineChemical additives forconservation, decon-tamination, etc.
Ion exchange resins
Salts, particulatematter, air
Acids, alkalis, saltschelating agents etc.
-Selection of low inputsmaterials-Minimizing corrosionby water treatmentUse of pure helium orsteamUse of filters
Improvements in sealingof turbine and condenser
Proper choice ofchemicals,Efficient clean-up
13
Selection of coolant quality specifications depends on the modeof reactor operation and the type and composition on the structuralmaterials. Commonly specified parameters are: conductivity, pH,hardness, dissolved oxygen, chlorides, fluorides, corrosion productsand insolubles.
14
2. WATER CHEMISTRY SPECIFICATIONS AND THEIR CONTROL
2.1 PWR Primary Water Chemistry Specifications
A specific feature of the PWR nuclear power plants is the presence oftwo separate circuits - primary circuit and secondary circuit(Fig. 2.1, 2.2).
HP t u rb ine Mois ture separa tor and r e h e a t e r
Circu la t ing pump
To reactorc lean-up
Fig. 2.1 PWR - Flow diagram
1 - reactor; 2 - presaurizer; 3 - steam generator;4 - recirculation pump ; 5 - HP-turbine; 6 - moistureeeparator; 7 - reheater; 8 - LP-turbinej 9 - conden-ser; 10 - condensate pump; 11 - coudeoaate polishing!12 - low pressure reheater; 13 - feed water pump; 14 -high pressure reheater.
Fig. 2.2 Coolant system f low diagram - WWER-440 type
15
The primary circuit includes a reactor (the reactor vessel houses fuelelement assemblies which form a reactor core) and cooling loops withcirculation pumps. The pipes of the cooling loops connect the reactor witha steam generator. In PWRs normal (light) water flowing through the coreserves both as a neutron moderator and as a coolant which removes the heatgenerated during the reactor operation and transfers it to the heattransfer surface of the steam generator.
Bulk boiling is prevented by the use of high pressure, normally15-16 MPa.
It is noteworthy that the cladding of the fuel elements and the wallsof the assemblies are made of zirconium alloys in nearly all nuclear powerplants throughout the world. The main construction material in the primaryciruit is stainless steel. The tubes of the steam generators are mostly ofInconel, Incoloy or stainless steel. Normally, the reactor vessel ismanufactured of pearlite steel with internal surfaces clad with stainlesssteel.
Information concerning the operating parameters of these reactors isgiven in Table 2.1. and Figure 2.1.
Table 2.1
LIGHT-WATER PWR : OPERATING PARAMETERS OF MAINREACTOR SYSTEM AND EQUIPMENT
R e a c t o rParameter Unit
1 2
Thermal output MWElectrical output MW
Kozlodu]Bulgaria
3
1 375440
Mihama-2Japan
4
1 456500
Takahama-1Japan
5
2 440626
Otu-1Japan
6
3 4231175
No. of loopsPressure vessel:
Diameter mHeight m
hater volume inreactor circuit m-*Primary coolantcirculation flowrate t/hType ot circulationPressure MPa
3.8411.60
256
39 000forced
12.5
3.4011.50
165
4.0012.40
260
30 000 45 000forced forced
15.4 15.4
4.4012.90
340
60 000forced
15.4
16
Table 2 . 1 ( c o n t i n u e d )
1 2
Temperature CSteam generator:
Diameter mHeight (length) m
Heat transfer area mSteam generator:Tubes Type
No. of tubes pcs
Type/fuelFuel rod diameter mmFuel rod length m
3
285Horiz.
3.211.9
2 500
I5 146
rod/UO9.1
2.57
4
300vertical
3.119.3
4 100
U3 300
rod/U0210.73.9
5
310vertical
3.320.6
4 BOO
U3 400
rod/UO10.73.9
6
310vertical
3.320.6
4 800
U3 400
rod/UO9.53.5
Number of fuel rodsper assembly -CoreCoolant inlet Qtemperature CCoolant outlettemperature °CCoolant velocity m/sHeat flux (mean) kW/mburnup MWd/kgMate^i^ls nie£ci°£ £lJLcu^iReactor vesselVessel inner claddingCircuit pipesFuel elementcladding
126
268
3013.5
45028.6
179
289
3204.5
54039.0
204
287
3224.4
60039.0
264
289
3254.8
60039.0
12x2MF-A C-steel C-steel C-steel
CrNiTl.18.12V SS 304 SS 304 SS 304
CrNiTi.lB.10 SS 306 SS 306 SS 306
ZrNbl(H-l ) Zircaloy-4 Zirca- Zircaloy-4loy-4
Steam generatortubes
Keactor coolantby£a ss_pu£ i f i£a -
Pressure MPaTemperature C
M a x i m u m f low rate m^/h
CrNil i .18.9
12.5
40
20
Inconel Inconel Inconel
600 600 600
1.4 1.4 1.4
53 46 46
20 .4 27 .2 27 .2
V Some PWRs of the WWER-440 type have no vessel cladding.
Data contained in Table 2.1 reflect the nain differences between PWEreactors, viz:
- The WWER-440 reactors are provided with horizontal steam generatorswith stainless steel straight tubing while most other PWRs have
17
vertical steam generators with U-shaped tubes made of Inconel 600 orIncoloy 800,
- Fuel element cladding of the WWER-440 is made of zirconium-niobiumalloy H-l containing 1% niobium while the fuel elements of the otherreactors are clad mainly with Zircaloy-4;
- temperature and pressure as well as heat load are lower in the caseof the WWER-400 reactors;
- The reactor coolant by-pass purification systems of the WWER-440reactors are operated at the reactor circuit pressures (12.5 MPa)while the purification systems in the other reactors are operated ata lower pressure (1.5 to 2.5 MPa).
A specific feature of PWR coolant water chemistry is the fact thatboric acid* (H BO ) is added to the reactor water. Under operatingconditions, boric acid concentration varies between 0 and 1200 ppm ofboron, depending on the mode of power operation an burnup conditions in thecore. On the shutdown conditions, Boron concentrations up to 4000/pm canbe encountered.
One of the main requirements of reactor water chemistry is sustainingthe optimum range of pH values in the reactor water for the operatingtemperatures**. (Corrosion rates of the constructional materials andcorrosion product solubility, as well as corrosion product transport in theprimary circuit depend on pH at the operating temperatures). Since boricacid causes a decrease of pH of pure water this requires the use ofalkalizing agents for pH control. Two models of pH control have beendeveloped today.
*H BO is added as neutron poison to control reactivity in the nuclearreactors.
**pH of water is dependent on temperature; pH can usually not be measuredrdirectly at operation temperatures (260-329 C) but must be calculated.
18
PWRs use lithium-7 hydroxide ( LiOH) as an alkalizing agent:
Li has a small cross-section for the neutrons, it producesneither radioactive isotopes as observed with Na and K, norbreakdown products derived from NH,;Li is used instead of natural lithium to minimize the production
of tritium in the primary coolant through the nuclear reactionLi(n,p), H;
- Neutron absorption results in one boron's isotope ( 6} producingLi, i.e. about 5-6 Kg per fuel cycle of a 1200 MW(e) reactor. Insome circumstances excess Li must be removed from the reactorcoolant.
WWER-440 type reactors use KOH and NH_ to alkalize the reactorwater(Table 2.2). The specifications of water quality of WWER- reactorsallow for rather small variations of reactor water pH values, through acorrelation between the concentration the alkali and boric acid in order tomaintain the specified pH (see Fig. 2.3).
The Westinghouse water chemistry of PWR systems allowed for pHvariation in a wider range.
The specifications for the primary coolant in KWU PWRs allow pHvariations in the range pH o 5-10.5. Coordination of LiOH and* I) C
Table 2.2Specifications of reactor water quality for PWR - type WWER-440
Indicator (with reactor "on-load") Value
pH <25°C)........................................ > 6,0(KOH as K+ (depending on H BO concentration,..)
«g/kg............................................2.0 - 16.5NH3, mg/kg....................................... 5Hydrogen, ml/kg...................................... 30 - 60Chlorides, ug/kg................................... < 100H3B03, g/kg.................................... 0 - 8.0Oxygen, ug/kg...................................... < 10
19
Fig. 2.3 KOH concentration change as K+ as function of 113803concentration in the range of pH260° ~ 7.1-7.3 •
H Bo has also been employed since 1979 in the Biblis nuclear powerstation, FRG. (Fig 2.4).
More recent worldwide practice is to follow a coordinated boron tolithium ratio so as to achieve a minimum PH300°C=6•9, as believed toprovide a dissolving chemistry for the corrosion products in the reactorcore.
These differences in concept are also reflected in the specificationsfor reactor water quality (Tables 2.2 and 2.3).
«rt-icwu
Fig. 2.4 Lithium concentration as function of boric acidconcentration in the reactor water of NFP Biblis
For comparison purposes Table 2.4 contains the specified qualities ofthe reactor water in the Japanese, Babcock and Wilcox and German PWRs understeady state conditions.
20
Table 2.3
THE WESTINGHOUSE CO. REACTOR WATER QUALITY SPECIFICATIONS
Indicator Unit Standardvalues Remarks
pH at 25°C 4.2-10.5
pH at 300°C, Fig. 9 5.4-7.6Conductivity at 25°C S/cm 1-40Boric acid as boron mg/1 0-40007LiOtiV as 7Li + mg/1 0.22-2.2Ü/
Hydrogen cm3/Kg 25-35Chlorides, max. ppm 0.15Fluorides, max. ppm 0.15Total suspended solids ppm 1.0Oxygen, max. ppm 0.1
Oxygen, average ppm 0.005
Depends on HjBOjand 7LiOH concen-tration
As for pH 25°C
0.03-3.2 xID'4 mol
For operatingtemperatures inexcess of 250°FDuring power oper-ation when «2 ismaintained in thecoolant
jV Irrespective of 83803 concentration in coolant.**/ The recent data are "0.7-2.2" (Private communication, 1962).
Table 2.4
CHEMISTRY
Indicator
pH at 25°C7L1OH as 7Li+
Boric acid as B
HydrogenOxygen, max.
Chlorides, max.
Fluorides, max.
Total suspendedsolids, max.I ron, max.
Silicates, max.
T u r b i d i t y , max.
SPECIFICATIONS FOR
Unit
ppraPPm
cc/kgPPm
PPmppm
ppm
ppm
ppm
ppm
JapanesePWR
4.2-10.50.2-2.2
0-4000
25-350.005
0.05
0.05
-
0.05
0.5
1.0
REACTOR WATER OF PWR
Bibcock tWilcox
4.6-8.5
0.2-2.00-2100
15-40-
0.10
0.10
0.1
---
VGB , FRG
_
0.2-2.0-
2-4l/0.05
0.20
-
_
-
-
-
Measured results are in ppb.
21
Worthy of mention are the considerably lower limits of the chlorideand the fluoride concentrations in the Japanese specifications for thecoolant quality. This is associated with the well-known sensitivity ofstainless steel even to low levels of anionic impurities.
There is a general consensus on the role of the following watercoolant impurities and additives:
- Hydrogen is added, to suppress water radiolysis (oxygen formation);
- Oxygen contamination results in an increase of primary circuitcorrosion rates and crud concentration in the reactor water and on the fuelcladding. The presence of both oxygen and halogens can causestress-corrosion of stainless steel;
- pH control agents are used for corrosion protection and as a meansof limiting corrosion product transport to the core and crud deposition onfuel cladding;
- The limitations for chloride and fluorides have been established tominimize the corrosion of the coolant circuit construction materials andparticularly of stainless steel. The control of fluoride concentration isalso linked with the fact that Zircaloy undergoes accelerated corrosion atfluoride concentrations above 2 ppm in the presence of lithium hydroxide.
Data contained in Table 2.5 indicate that the reactor water quality ina number of PWRs in Europe, Japan and the United States of Americapractically corresponds to the specifications. This is an indication ofthe realistic approach adopted when establishing the specifications and ofthe successful control of reactor water quality in the various nuclearpower stations.
Water chemistry as applied today in PWRs does guarantee the integrityand reliability of the primary circuit including the fuel elements. Thegenerally low level of corrosion product total activity in the primarycoolant can be regarded as an indicator of acceptable operation.
However, despite this optimistic conclusion certain facts relating tomore precise control should not be overlooked. In this connection the
22
Table 2.5
OBSERVED CONCENTRATION OF IMPURITIES AND ADDITIVES IN PWR REACTOR WATERUNDER STEADY-STATE CONDITIONS
Nuclear power Chloride,station (PHR) ppm
Takahamà 1, Japan
Hihama 2, Japan
Kozlodu], B u l g a r i a
Obngheira, ffCNeckerwes the im, FHG
Biblis A 4 B, FHGStade, FRG
Ringhals , Sweden
Doel 1 t 2, Belgium
Tihange, France-Belgium
Beznau, Switzerland
Zor i ta , Spain
Novovoronezh, USSR
BR-3, Be lg ium (1979-1980)
Oconee, USA
0
0
0
00
0
0
0
0
0
0
.05
.05
0.1
.06
0.1
.05
.10
.10
.03
.03
.15
.02
0.1
.03
0.05
Fluor ide,P?™
0.05
0.05
0.1
-
-
--
0.05
0.01
0.02
0.05
0.02
0.1
-
0.05
Hydrogen,cc/kg
30
30
40-50
-2-3 ppm
2-3 ppm3.0 ppm
15-50
57-59
23-30
35
35
40
7-9
20
Oxygen,ppm
0.
0.
0.
0
0
0.0.
0.
0.
0.
0.
0.
0.
005
005
005
.03
.01
005010
005
005
010
005
001
OOS
0.03
0. 005
Suspendedsolids, ppm
0.1
0.1
0.1
-
0.1
LIppm
2-17
1
0.7-2
pH
-
-
K
.4
.0
6.(«t
6.(at
7.(at
6.
5-7.0300°C)5-7.0300°C)1-7.4260°C)
-
2-7.5
o.2-2V0.5
0.1
-
0.025
0.3
0.03
0.1
0.01-0.18
0.01
1.1-1
0.5-1
0.4-0
1
0
.5
.0
.6
-
.0
.7
2-17K
0.9-1
0.4-0
.1
.5
7.<at
(at
(at
-
-
-
-
-
-
1-7.4260°C)
6-825°C)
6300°C)
K* and Li concentration at Kozlodu), Novovoronezh and Biblis depends on HjBOj concentration
investigations conducted by Thomazet, Noe and Stora revealed that effectiveprevention of fuel element cladding crud deposition requires themaintenance of practically constant pH values at the operating temperature,thus requiring the continuous decrease of lithium hydroxide during thecycle.
Results obtained by Thomazet et al. correspond with the earlierpractical conclusions drawn at Biblis in 1979 on the need to maintainreactor water pH within a narrow range through a suitable concentration ofH B03 and LiOH, Fig 2.4.
It seems advisable therefore that the future PWR coolant chemistryspecifications should be amended to allow for a practically constant andhigh pH value, which should be maintained throughout fuel cycles. Thiswill involve the coordinated variation of the alkali level with the boronconcentration. Data on pH changes in the reactor coolant of a Japanese PWRfor the period 1975-1981 indicate that such a policy of pH control hasalready been achieved in the past. It is now worldwide common practice.
23
From the standpoint of the behaviour and reliability of the fuelelement cladding it is also necessary to take into account the adverseeffect of any deviations from the reactor coolant specifications.According to Lippens, Haas, Gubel, Van der Velde and Van Loon the failureof the fuel elements in the Belgian reactor BR-3 was due to theconsiderable amount of impurities entering the primary circuit water duringdecontamination operations. These impurities deposited on the fuelelements and accelerated the corrosion of the cladding.
An important circumstance influencing fuel rod failure in the BR-3reactor was significant deviation from the reactor coolant specifications:
- "Hydrogen concentration oscillated between 7 and 9 cm /Kg",i.e. a lower than specified value (25 - 30 cm /Kg)...,
- "Oxygen content in the reactor coolant was 0.03 ppm on average,i.e. a higher than specified value (0.005 ppm)".
Evidently, the low hydrogen content is the reason for the observedhigher oxygen amount (resulting from water radiolysis) on one hand, and thedecrease of the dissolved iron concentration on the other. Thecomparatively low pH of the reactor water during the reactor campaign(pH o = 8) also contributed to this effect.
Under these unfavourable conditions, the comparatively moderateconcentration of undissolved crud between 0.01 and 0.18 ppm had been enoughto form significant crud deposits on the fuel element cladding. As aresult, the process led to corrosion damage.
Pressurized heavy water reactors are presently in operation in Canada,Argentina, India and in some other countries. The high temperature - highpressure primary coolant (DO) system of these reactors uses an alkalinepH regime under controlled dissolved oxygen levels. The philosophy behindthis primary system chemistry control is to minimize the corrosion/releaserates of the major constructional materials, like carbon steel andMonel 400, and also to favour the reduction of corrosion product residencetime in the core. These objectives would reduce:
(a) the deposition of corrosion products on fuel elements and theconcentration of aggressive chemicals within the deposits;
24
(b) the radiation field build-up on out-of-core surfaces.
The specifications of water quality for the reactor coolant of NFSAtucha-1, Argentina* (1179 MW(th) and 335 HW(e» are similar to PWRs.
The permissible concentration ranges of various ions and compounds are:
LiOHD2OxygenChloridepD at 25°CSilicaDissolved ironCrud
1-2 ppm Li6 ppm0.05 ppm0.2 ppm10.5-10.9max 4 ppm SiO0.5 ppm1 ppm
The observed concentration for impurities and additives during normaloperation for PWR Atucha-1 and for RAPS (PWR of Rajastan Atomic PowerStation) are given in the Table. 2.6.
Table 2.6
indicators Unit Atucha-1 RAPS
Conductivity, 25°CChloridesOxygenD2pD at 25°CLiOHBoron
uS/cmppmppb3cm /kg
ppm Lippm
10-150.015-20
1-210.0-11.00.7-1.1
0.5
19-250.1-0.2
103-10
9.5-10.5-—
«Data about Atucha-1 are taken from "Answers to IAEA QuestionnaireNFS Achuta-1, Argentina (1981).
25
2.2 BUR Reactor and Feed Water Chemistry Specifications
In a typical light water BWR, water passes through a core acting asboth the moderator and coolant removing heat from the core. (Fig 2.5) Aportion of the coolant then evaporates and enters the turbine as steam.The condensed steam is collected in a hot-well located below the condensercooling tubes. The full flow of condensate passes through a condensatepolishing unit in order to remove soluble and insoluble impurities. Thishigh-purity water is heated and is injected as feedwater into the reactor.The required reactor water quality is provided by purification of separatebleed in the reactor water polishing unit.
steam line
generator
condenser
recircula ting! fpump I !
• '"' reelrculat ingpump i i i i i i i
prehcater[~v^| feed pump |'^cooler
"UJ
condensate cleanupsystem
li Ml reactor watercleanup system
Fig. 2.5 BWR - Flow diagram
BWRs which use heavy water or graphite as the moderator and lightwater as the coolant and working fluid can be considered from the chemicalviewpoint as a typical light-water BWR. For that reason, this chapter onthe light-water BWRs will include also reference to the Steam GeneratingHeavy Water reactor in Winfrith and the BWR of the type RBMK-1000 in theUSSR. The latter is a light-water BWR of the channel type with thegraphite moderator (Fig 2.6).
Information on the main system characteristics, operational parametersand construction materials is given in Table 2.7.
26
A. specific feature of BWR water chemistry is that water radiolysis inthe core is not suppressed*. Reactor operation at a given capacity leadsto water radiolysis at a constant level. In most cases the 0 level inthe «circulating coolant due to radiolysis is usually between 0.1 and 0.3ppm, and the 0 level in the produced steam ranges between 15 and 30 ppm.
1- reactor; 2 - main recirculation pimp; 3 - mixer;4 - cooler pump; 5 - cooler; 6 - separator; 7 - fcypas«cleanup; 8 - control end safety rod asaeiably cleanup;9 - conde&Bate pump ; 10 - deminerallter; 11 - lowpressure beater; 12 - feedwater punp; 13 - deaerator
Fig. 2.6 Flow diagram of the coolant circuit: BWR ofthe RBMK-1000 type
It is known that even small amounts of oxygen in the coolant inducedeleterious effects on the intergrannular corrosion of the recirculationpiping. It has turned out that the addition of hydrogen reduces the oxygenlevel in the recirculation lines through the radiolytic recombinationreaction in the downcomer. Therefore, hydrogen injection to the feed watersystem of BWRs has been proposed as remedy to piping corrosion. Thiswater chemistry regime is called: "hydrogen water chemistry" and, itsdemonstration is now being conducted in several power plants.
The presence of radiations rules out pH control by hydrazine orammonia because the radiolysis can produce nitric acid which is
* Unlike PWRs, water radiolysis suppression by maintaining certain H,concentrations is difficult because the hydrogen added is removedcontinuously with the steam from the core.
27
Table 2.7
LIGHT WATER BWR. MAIN REACTOR CHARACTERISTICS AND PARAMETERS.CONSTRUCTION MATERIALS
Parameters Unit
1 2
Thermal capacity MWElectrical capacity MW
Coolant system dataFlow diagramPrimary coolant circulationflow rate t/hType of circulationNumber of loops
Steaa production t/h
Pressure MPa
Temperature °CVolume of water m 3 ( t )
Fuel elements
Type
Number of fuel elements in core
Core
Heat f lux , mean kW/n2
Heat f lux, maximum kW/o2
Heat transfer surface m2
Neutron f lux , meanthermal neutrons n/cm2.sfast neutrons n /cm3. s
Coolant velocity m/sec
Inlet temperature °COutlet temperature °COutlet void, mean »Outlet void, maximum »Burnup , mean MWd/t
Mater ia ls in reactor
Reactor vessel typeVessel cladding typePressure tubesFuel cladding
Reactor water clean-upsystem typeRecirculation loops type
Materials in secondary side
Steaa piping typeCondenser tubes typeLP feed heater type
HP feed heater type
Feedwater pip« type
HSCHRUW
3
330
100
7 000
forced2
550
6.4
280
38
104
411
1 087
793.4
5.7 x 1013
1.3 x 1013
s.o274
2809
22.5
27 500
Zircaloy 2
St. 321
St. 304
70/30 brass
90/10 brass90/10 brass
St. 304
Boiling-water
TarapurIndia
4
-
210
10 500
forced2
I - 875II - 362
5.8
285
93.7
6 x 6
284
383
1 277
1 672
3 x 1013
1.8 x 10*2.17
267
285
21 600
St. 302
St. 308
Zircaloy 2
St. 304SS316
St. 304
reactors in nuclear
MuhlebergSwitterland
5
1 000
320
14 000
forced2
1 800
7.1
276
100
8 x 8
240
457
1 003
2 200
0.7
190
276
25 000
St.ASOB
St. 304
Zircaloy 2
St. 304St. 304
St. 45. 8
power stations
TsurugaJapan
6
1 070
357
17 700
forced3
1 935
6.99
285
160
7 x 78 x 8
308
370
1 120
2 793
3.6 x 1013
3.7 x 1013
2.0
189
28559
69
25 000
St. 3028
St. 308
Zircaloy 2
St. 304SA106
A106
Admiral ty
St. 304
AxlOCrNiTi 18/9
St. 35
St. 304
St. 304
A. 106
Hamaoka 2Japan
7
2 436
840
34 900
forced2
4 750
7.2
286
212
8 x 8
560
452
1 105
5 147
4.6 x 1013
1.3 x 10U
2864 2 . 2
27 500
St. 533
St. 371
Zircaloy 2
St. 304St. 304
St. 304
brass
28
corrosive. In BWR coolant pH cannot be controlled even with thenon-volatile KOH or LiOH because of the danger of local concentrationincrease of these alkalis in the core.
Under these circumstances, the minimization of the corrosion processesin the BUR requires both the use of highly corrosion-resistant constructionmaterials and of high-purity water throughout the cycle.
As it is can be seen from Table 2.7 reactor circuit system is made ofcorrosion-resistant materials, mainly stainless steel. However, in thepresence of chlorides and oxygen these materials are susceptible to stresscorrosion cracking. For this reason, the reactor water, and hence thefeedwater of a BUR should have the lowest possible chloride and fluorideconcentration.
Other parts of the plant can use less corrosion-resistant materialssuch as low-alloy steels. The corrosion products released from the largesurface areas of these materials must be removed. Condensate demineralizerinstalled after the turbine condenser before the flow enters the reactorvessel are used for this purpose. However, the corrosion products releaseddownstream of the condensate demineralizer are not removed. The oxygenconcentration is very important for minimizing the corrosion productsrelease in the feedwater system. Therefore the oxygen level must becontrolled in the feedwater. In this way fuel surface deposits areminimized and flow and radioactivation problems reduced.
For that reason, corrosion product control of the water before itenters the core is essential in BUR operation. The corrosion productconcentrations in the reactor and feedwater are therefore subject to strictspecification limits.
Evidently, both the feedwater and the coolant in the BURs should havea high degree of purity. This has been reflected in the specifications forfeedwater and reactor water quality in BURs. Table 2.8, 2.9 and 2.10 givethe feedwater and reactor water qualities in a number of BURs as specifiedby various companies and countries.
A comparison of the data in these Tables leads to the conclusion thatirrespective of some differences in the specified parameters and the
29
Table 2.8FEEDWATER SPECIFICATIONS FOR BWRs IN SOME NUCLEAR POWER STATIONS
Parameters Unit USGHUR Tarapur Mühleberg Tsuruga Hamaoka 2
ConductivityChloridesSilicaCorrosion productsIronCopperCr totalNi totalOxygenpH at 25°C
uS/cmppbppb
:ppbppbppbppbppb
0.22030
205(3)
0.1n . s . * /
2
14
0.1n . s . *_/
5215
15
0.1 0.1n. s.* / n. s.* /
is y 302
20-200 20-2006.5-7.5
V Not specified but should be maintained low enough to satisfy the reactorwater requirements.
**/ Total metal.
Table 2.9REACTOR WATER SPECIFICATIONS FOR BWRs IN SOME NUCLEAR POWER STATIONS
Parameters Unit WSGHWR Tarapur Mühleberg Tsuruga Hamaoka 2
ConductivitypH at 25°CChloridesSilica
us/cmppbppb
1.0
2002000(1000)
1.05.6-8.6
1001000
Iron (below 0.45ura) ppbCopperBoronDissolved oxygen1-131
ppbppbppb
uCi/1
35
100
2004005000
1.05.8-8.6
20010002050
1.0 1.05.6-8.6 5.6-8.6
100 200
Table 2.10FEEDWATER AND REACTOR WATER SPECIFICATIONS FOR BWRs
VGB***Parameters
ConductivitypH at 25°CChloridesSilica as SiC>2IronCopperOxygen
Unitfw
uS/cm 0.15ppb nsVpb nsppb 25ppb 3ppb ns
KWU GeneralWürgassen Electric
rw
1.02004000nsnsns
fw
0.16.5-7.5
105
ns214
rw1.05.6-8.6100nsnsnsns
fw0.1
nsns132
ns
rw1.0
7.0+0.22004000nsnsns
RBMKUSSRfw
0.16
4ns102
ns
rw
1.0.5-8.5100 7100020050ns
Insolubles, (below 0.4 Sum)total crudTotal activity
ppb nsuCi/1 ns
nsns
nsns
nsns
nsns
nsns
nsns
200100
*/ ns = not specified.**/ Total chlorides and fluorides (50 ppb chlorides + 50 ppb fluorides).***/ VGB: Vereinigung der Grosskraftwerks Betreiber.
30
permissible amounts of the impurities in the feedwater and the reactorwater of the BWRs the specified main characteristics of reactor water andfeedwater are essentially the same:
Feedwater is highly demineralized. Specific conductivity variesbetween 0.1 and 0.2 uS/cm. Chloride level in the feedwater mustbe sufficiently low to meet the requirements for chlorideconcentration in the reactor water. (In a number ofspecifications the chloride level in the feedwater is notspecified due to difficulties in the experimental determinationand the extremely low chloride ion concentration.)
- The reactor water is also highly demineralized, the amount of therespective impurities being in such ranges as to guarantee the reliableoperation of the fuel elements and the stainless-steel equipment andpipes as well as the sufficient purity of the produced steam. Thespecifications generally allow for max. 100 or 200 ppb chloride whilethe critical concentration is 350 ppb.
The silica concentrations are specified for two reasons - first, to avoiddeposits on the heat exchange surfaces in the core, and secondly, to avoidimpurities in the steam which could in turn lead to deposition in theturbine. A silica concentration of 1000 ppb is sufficiently low to avoidthis. However, in the case of demineralizing make-up of water (specificconductivity about 0.1 uS/cm) the maximum silica concentration is between 20and 30 ppb. Thus, silica from make-up water is rarely the source of highcoolant levels of SiO . Silicious matter, e.g. lagging and dust frombuilding materials, can however enter reactor systems at shutdown and duringmaintenance and this is the main cause of high levels, particularly afterreactor startup.
As illustrated in Table 2.11 under normal operating conditions of the BWRthe feedwater and the reactor water qualities meet the specifications, or areeven better. Reactor water conductivity is 3 to A times lower than thespecified value. The same is true for the amount of silica in the reactorwater. In most BWRs, corrosion product levels present in the feedwater andthe reactor water are considerably less than the specified values.
31
Table 2.11
OBSERVED QUALITY OF FEEDWATER AND REACTOR WATER DURING NORMAL OPERATION OF BWR(AVERAGE VALUES)
W9GHWR Tarapur Mühleberg Tsuruqa Haraaoka 2Indicator Unit
ConductivitypH at 25°CChloridesSilica as SiOIronCobaltCopperNickelChromiumOxygenPurificationRemarks fw «
M s/cm
ppb2 PPb
ppbppbppbppbppbppb
system typefeedwater} rw
fw
0.076.922a
0.008i
0.3
40Pow
• reactor
rw fw0.3 0.07
200
600.115 0.21360 20Pow DB
water
rw
0.3620200500.4125
250MF/DB
fw
0.1
0.10.0050.050.10.2
Pow
rw0.1
10250
2
30.2
3
Pow
fw
0.07
1.10.0010.10.030.0140DB
rw0.166.620106.30.10.30.070.03300
MF/DB
fw rw
0.06 0.246 8.0
10168
6 11
29DB Pow
Pow - Powdexi DB - deep bed) MF - mechanical filtration.
Any change of operating conditions, startup or shutdown of BWRs can leadto considerable deterioration of the feed and reactor water quality. Theeffect of the transient conditions is best illustrated by reference toTable 2.12.
Table 2.12
METAL IMPURITY CONCENTRATION IN THE FEEDWATER AND THE COOLANT OF THE BWRAT MUHLBERG UNDER NORMAL OPERATING CONDITIONS, WHEN CHANGING THE MODE OF
OPERATION AND DURING STARTUPS
IronChromium
Nickel
CobaltCopper
Unit
ppb
ppbppb
ppbppb
Normal
feedwater
0.1
0.02
0.1
0.005
0.05
operation
coolant
0.32.1
0.1
0.062.0
Startup
feedwatec
1.50.1
0.1
-
0.1
coolant
50
4
0.1
-
3
Change of mode
feedwater
10
0.1
0.2
-
0.4
coolant
12.5
0.3
-
3
32
Many results indicate that during change« of operating conditions orstartup of BWRs the clean-up plant is not always able to adequately removecorrosion products and some ion impurities from the treated water. In thecase of VSGHVR the situation is as follows:
"- Feedwater conductivity is higher than the specified value during thefirst five hours after startup of the reactor and immediately afterrecoating the Powdex-filters (with powdered ion-exchange resin inthe plant) for simultaneous clean-up of the feedwater and thereactor water;
"- Silica levels in the feedwater are higher than the specified valueduring the first five hours after startup and shutdown of thereactor;
"- Considerable increase of iron concentration in the reactor water isobserved for short periods after reactor perturbation but there isno increase in the feedwater."
These points emphasize the importance of the plants for maximum possibleclean-up of the feedwater and some of the reactor water in BWR water chemistry.
It can be concluded therefore that the feedwater and the reactor waterqualities as required by the specifications are realistic and attainable.This is every indication that under normal operating conditions the plants forcondensate (feedwater) and reactor water clean-up can satisfactorily meet therequirements for impurity removal from the processed water.
2.3 Water Cleaning Control Systems
The reactor water purification is the key to adequate coolant chemistrycontrol. Usually the coolant purification flow from the primary circuitconstitutes 0.1 to several percent of the reactor steam generating flow rate(up to tens of tonnes/hour for PWRs and some tens to hundreds of tonnes/hourfor BWRs.)
The normal clean-up flow in BWRs is about 2% of the feedwater flow at100% reactor power. However, in some BWRs the amount of bypass treatedcoolant is larger, e.g. in Tsuruga the plant for coolant clean-up has acapacity 6.7% compared to that for feedwater.
33
The reactor water may be purified in by-pass equipment or by "feed andbleed" method (removal of the system water with simultaneous addition of highpurity make-up water).
The selection of the purification technique is based on the followingconsiderations :
- the primary water directed to the purification plant has thetemperature 250-300 C and pressure 75 to 150 bars
- the water impurity inventory consists of corrosion products (suspendedand dissolved), organic species, dissolved salts and gases;(radioactive species are also unavoidably present);
- with the by-pass purification the decontamination factor (forindividual components) should be about 10; a higher value isunnecessary since it does not affect the impurity balance (except ifsome fraction of the treated water is used for another technologicalaims, eg. for cooling the control rod drives).
There are three basic concepts of the reactor water purificationtechnology:
- depressurizing (with respective water temperature decrease),cleaning-up and returning to the circuit;
- purification at nearly circuit pressure, but at a lower temperature;- purification at the circuit pressure and temperature.
The two former concepts have been preferred.
Water may be cleaned up by filtration, ion exchange, distillation,electrodialysis, electrophoresis, etc. The former two types are proven andmost widely used today.
The demineralization units are generally loaded with mixed beds of cationexchanger resin in H-form and anion exchanger resin in OH-form in ratio1:1 to 2:1.
The ion-exchange resins used have grain size 0.3-1 mm or 0.02-0.06 mm.The first type is used in bulk while the second type forms a thin (0.2-0.8 cm)filtering layer in special pre-coated filters of (eg., Powdex filters at thewinfrith WSGHWR).
34
In a number of clean-up plants the ion-exchanger units are preceded bymechanical filters. These are mostly pre-coated with cellulose, Solcafloc orother filtering materials. The mechanical filters are designed to remove themain portion of the suspended corrosion products. For this purposeelectromagnetic filters have also been recommended.
Powdered resin filters, (and in a number of cases Monobed*/ units) actsimultaneously as mechanical filters and ion-exchange demineralizers.
Typical reactor water clean-up systems (for PWRs and BWRs) are shown inFigs 2.7 and 2.8.
FILTER
COMPONENTCOOLING WATER MIXED BED
DEMINERALIZERS
LETDOWN HEATEXCHANGER TO THERMAL
REGENERATION SYSTEM
TO BORON RECYCLE SYSTEMOR WASTE PROCESSING SYSTEM
COMPONENTCOOLINGWATER
FROM REMINERALIZED MAKEUP WATER SUPPLY
I —— FROM BORON PROCESSING SYSTEMCHEMICALMIXING TANK BATCHING TANK
AGITATORPOSITIVEDISPLACEMENTCHARGING PUMP
BORICACIDFILTER
CENTRIFUGALCHARGINGPUMP
REACTOR COOLANT SYSTEMBORON RECYCLE SYSTEMWASTE PROCESSING SYSTEMSAFETY INJECTION ACTUATION SIGNALVENT HEATERSAMPLING SYSTEMCOMPONENT COOLING WATERREACTOR MAKEUP WATER
RMW
STEAM
Fig. 2.7 Chemical and volume control system
* Monobed is a commercial name for single stage unit.
35
1 - reactor; 2 - drum separator; 3 - recirculation pump;4 - regenerative heat exchanger; 5 - cooler; 6 - pre-coated filter; 7 & 8 - déminéralisera; 9 - trap-filter;10 - slurry tank; 11 - tank for ponder filter precoat-
Fig. 2.8 Bypass reactor water cleanup sys tem for BWRof the RBMK-1000 type
Removal ef f ic iency of impurities from treated coolant is about 90% whenthe plant comprises only ion-exchange demineralizers of the Monobed type.
The mechanical filters (pre-coat type) preceding the Monobed remove96-99% of the particulate corrosion products (size larger than 0.45 urn)
Table 2.13 illustrates the decontamination capacities of Monobedion-exchange demineralizers (installed after the mechanical filters) in thecase of Tarapur NFS.
Table 2.13
Decontamination factors in theion-exchange units of the coolant
Nuclides
13l!132X133X239«D"V99Tc137CsL*S134Cs60Co103Ru95Zr
clean-up plantInitial phase ofoperating cycle
80003.1 x 1045.2 x 1037.1 x 104
_5.8 x 1035.3 x 103
over 1.5 x 102-"
at TarapurEnd ofcycled
162.6 x 1031.3 x 103
6706.5 x 1031.2 x 1021.4 x 102
1.75.91.52
*/ This cycle ranges between 8 and 12 months.
36
For comparison purposes, Table 2.14 illustrates the overall effect ofdecontamination at Tsuruga NFS whose coolant clean-up consists of:
- Unit for mechanical filtration: pre-coated filter with Solcafloc- Unit for ion-exchange demineralization, Monobed type.
The usual life of the ion-exchanger is between 8 and 10 months afterwhich it is replaced or regenerated.
Table 2.14
Impur i t ies , nuclides
Conductivity at 25°C
Chlorides
Ironi
Copper :
Nickel:
Cobalt»
1-131
1-133
Cs-137
Co-60
a/ > 0.45 pro
b/ < 0.45 \i m
a/ > 0.45 p m
b/ < 0 .45 p m
a/ > 0.45 pra
b/ < 0.45 ma
a/ > 0.45 Mm
b/ < 0.45 Mm
Unit
ps/cm
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
wCi /ml
M C i / m l
pCi/ral
vCi /ml
Inletmechanical f i l ter
0.10-0.16
20
6.3
0.04
0.1
0.32
0.02
0.05
1.0
8.3
2.5 x 10~*
3.5 x 10"3
9.0 x 10~7
3.6 x 10~5
Outletmixed bed
demineralizer <
o.os20
0.02
0.01
0.001
0.005
0.005
0.007
0.1
1.0
2.7 x 10"8
5.2 x 10~7
1.1 X 10~7
4.6 x 10~6
Removaleff iciency
inlet concentr .outlet concentr.
315
4
10
64
4
7
10
8.3
9360 » DF
6730 " DF
8.2 - DF
9 ° DF
The efficiency of the coolant clean-up plants consisting only ofPowdex-fliters is usually higher than that with Monobed. The life of thePowdez-fliters is normally about a month. The experience of Mühleberg NFSreveals that the coolant clean-up plant comprising only Powdex-fliters treats
3about 220 000m monthly, and powdered ion-exchange consumption is about30.25g per 1m treated water.
The condensate clean-up plants consisting only of Monobed demineralizers,when treating the whole amount of turbine condensate, guarantee an efficiencyclose to that illustrated in Table 2.15.
37
Table 2.15
Indicator
Conductivity at 25°C
Iron ( > 0.45 y m
< 0.45 p ra)
Copper ( > 0.45 w m
< 0.45 um)
Nickel ( > 0.45 um
< 0.45 pm)
Chromium (> 0.45 um
< 0.45 u m)
Cobalt ( > 0.45 um
< 0.45 um)
1-131
1-133
Co-60
Unit
M s /cm
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
uCi/ml
pCi/ral
PCi/ml
Inlet CCU
concentration
0.07
26.6
0.7
1.56
0.50
0.03
0.05
0.05
0.004
2.6
12.1
8.1 x 10~6
8.5 x 10~5
1.1 x 10~8
Outlet
0.06
1.6
0.4
0.03
0.03
0.004
0.007
0.004
0.002
0.3
0.9
1.1 x 10~8
1.3 x 10~7
1.3 x 10-10
Removal efficiency:inlet concentr.outlet concentr.
-
17
1.8
52
17
7.5
7.1
13
2
8.7
13
736 - DF
654 - DF
85 « DF
The efficiency of turbine condensate clean-up is usually higher in theplants with Powdex-filters. However, a disadvantage is the short life incases when large amounts of salt enter the condensate as the result of leakagein the turbine condenser. (This disadvantage is particularly important whenthe turbine condensers are cooled with highly saline water, e.g. sea water).
In order to guarantee operation under condenser leak conditions in someBWRs (e.g. Ringhals, Sweden) the Powdex-filters are complemented with deep-beddemineralizers. Under normal operating conditions only the Powdex-filters arein operation. When turbine condensate salinity (conductivity) increases thePowdex-filters are bypassed, i.e. the deep beds are operated.
The use of magnetic filters for removing insoluble corrosion productsremains of potential interest. This could be of importance in purifyinghigh-temperature condensate which comes from the feedwater heaters beforebeing mixed with the turbine condensate or the feedwater.
38
2.4 Monitoring Water Chemistry
Sampling and control of primary coolant is performed on a regular basisto confirm that the water chemistry is within specified values. Thesespecifications have been selected to meet the two following objectives:
1. Keep the integrity of fuel cladding and related core materials;
2. Minimize the radiation buildup on the out-of-core surfaces, so as toreduce personal exposure.
A questionnaire was issued in 1982 to collect information on the samplingand analytical procedures and methods which are used to qualify the primarycoolant and corrosion product behaviour in the primary circuits at theoperating nuclear power plants. Responses were received from 42 PWR, 12 PHWR,14 BWR reactor type units and from one WSGHWR and one RBMK-1000 type plant.The list of responding countries along with their code number is given inAnnex I, which also shows the plant capacity and year of commissioning.
The responses were compiled in the form of condensed tabulations(Annex I) according to the main points of the questionnaire:
1. Code listing of nuclear power plants2. Overview of NPPs3. General concepts of sampling, control of sampling and parameters to
be measured4. Chemical analytical techniques5. Radiochemical and physical measurements
The answers from the same reactor types are grouped, so as to give acomparative view. For clarity, the flow-sheets of the primary circuitsprovided by the plants are not included.
Chemical parameters measured in the primary circuit are different fordifferent reactor types. Annexe I, Table 1.3) For example dissolved H. ismeasured only in PWRs, and D in PHWRs. In some PWRs nitrate or NO whichis formed under nucleate boiling conditions on the fuel element surfaces isalso measured. Among halogens, chloride is measured by all the plants, whilefluoride is only measured in certain PWRs. Fe and total insolubles are
39
measured in all plants while other corrosion products are measured only insome of them. At certain units continuous monitoring and automated controlare employed. PWRs of advanced generation which can undergo some nucleateboiling conditions on the surface of the hot assemblies are submitted tostringent specifications on mineral impurities, susceptible to form zeolitetype deposits (Ca, Mg, Al, Si). These specifications must be derived bothfrom loop experiments and feedback from PIE bearing in mind practical limitson the purification systems.
The control of these chemical parameters is not an e-asy operation and,representative values are not always obtained; despite an increasingcapability to measure levels in the ppb range. Account must be taken of thecooling of the sample and the potential pollution and/or deposition ofparticles in the sampling lines.
The sampling systems are, without exception, designed for low temperaturesampling. At units JAPW 32-42, ASTM standards are applied, while at unitsFIBW 56 and 57, UNIPEDE recommendations are used.
Stainless steel is mainly used as the material of construction for thesampling system, except in UKSG 55 and in units JABW 60 and 61, where Inconeland Ti lines respectively are used.
Isokinetic sampling is carried out at units JABW 60 and 61, FIBW 56, and57 along with capillary lines at UKSG-55. Continuous flow is maintained insampling lines in units, CAPH 44-51 and FRPW 5 to 30. This specialized designfacilitates the collection of representative samples of the coolant at thesampling point.
Filtration is used widely as a method for crud separation andpreconcentration of soluble impurities. For this purpose, membrane filters(in unit JABW 60-70; FIBW 56 and 57; UKSG 55; BEPW 1; FRPW 5 to 30 except 4and CAPH 44 to 51) and ion exchange filters (in units JABW 62-70;FIBW 56 and 57; UKSG 55; FRPW 5 to 30 except 4 and CAPH 44 to 51) are used.
In reactors where deposits sampling is undertaken (FRPW 5-30 except 4,JAPW 32-39 and 41 and 42, FIBW 56 and 57, INPH 52-54 and JABW 60-62) thesurface of fuel elements is mechanically treated in a pool and the releaseddeposits are collected by filtration.
40
As already mentioned, metallic impurities are preconcentrated mainly byfiltration. In this way the sensitivity of 0.1 ppb is often achieved, thesensitivity of cobalt determination by UKSG 55 is 0.02 ppb.
It is known that dissolved oxygen, chloride and fluoride have deleteriouscorrosion effects. A sensitivity of oxygen determination of 1 to SO ppb cannow be achieved using a diaphragm type electrode.
In PWRs, oxygen pollution can generally not be detected in the primarycoolant under steady state operation because of fast interactions with thecircuit surfaces and eventually recombination with dissolved hydrogen underirradiation. However, oxygen levels should be carefully controled in thefeedwater to avoid crud formation.
For chloride determination the mercury-thiocyanate method was mainly usedand the chloride sensitivities were 5 to 50 ppb. For fluoride determinationion-selective electrodes were used (Code No. of unitsJAPW 32 to 40) with a sensitivity of 50 ppb.
From the analysis of the responses the following conclusions can be drawn:
- a great variety of analytical techniques have been used at thedifferent NPPS;
- to achieve representative samples, the sampling systems in some plantsare designed with continuous and isolcinetic flows; sample lines beingstainless steel, Inconel or titanium;
- in many plants, the material and radioactivity balance of impurities isevaluated by using membrane and ion-exchange filters;
- the filtration method is also used for the evaluation ofsoluble/insoluble impurity balance;
- in certain plants, parameters like pH, specific conductivity, dissolvedoxygen, chloride and sodium concentration are continuously monitored byautomated techniques.
41
3. BEHAVIOUR OF FUEL CLADDING MATERIALS
From considerations of neutron economy the suitability of materials forwater-cooled thermal power reactors is restricted to metals like aluminium,beryllium, magnesium and zirconium or their alloys. While pure aluminium (25)has been used for cladding and structural components in research reactorswhere operational temperatures are low (100 C or less), severe blisteringand accelerated corrosion has precluded its use for power reactor applicationsat temperatures above 150 C. Beryllium, magnesium and their alloys,although possessing a low neutron absorption cross-section, were notconsidered for cladding because of their failure to meet the mechanicalproperty and corrosion requirements at water cooled reactor operatingtemperatures (200-300 C). The potential of zirconium alloys for use ascladding was thus recognized, and this was as early as 1946-47, when it wasrealized that hafnium- free zirconium absorbs only a few neutrons and has allthe qualities required for cladding. The first use of pure zirconium forcontaining the fuel was for the MARK 1 PWR. Research on zirconium-basedalloys has subsequently led to the general use of these alloys for cladding inwater-cooled reactors.
The alloy development work was aimed primarily at: (1) overcoming thedeleterious effects of impurities, (2) achieving the desired mechanicalstrength and (3) strengthening the oxide against cracking. The choice and theproportions of the alloying elements were dictated by the following: (1) thealloying elements should not unduly increase the absorption of thermalneutrons; (2) they should stabilize the corrosion resistance and reduce thetendency to pick-up hydrogen; (3) they should have an out-of-pile
2post-transition corrosion rate of Img/dm /day or less and the oxide shouldbe adherent; (4) they should improve the mechanical properties; and (5) theyshould not make the metal more difficult to process.
Table 3.1 summarizes the alloys currently acceptable under reactorconditions and the experimental ones being investigated.
3.1. WATERSIDE CORROSION OF FUEL CLADDING
Experience with water reactor fuels has been reviewed extensivelyelsewhere. The main operational requirements for LWR fuel are summarized in
42
Table 3.1
ZIRCONIUM ALLOYS FOR WATER- COOLED REACTOR SERVICE(ALLOYING ADDITIONS, WEIGHT PERCENT)
A. Commercial Alloys:Zircaloy-2; Zircaloy-4; Nickel-free Zircaloy-2;
Zr-lNb; Zr-2.5Nb; Zr-3Nb-lSn.B. Experimental Alloys;
Ozhennite 0.5 (Sn 0.2, Fe 0.1, Nb 0.1).Scanuk-4 (Nb 0.91, Fe-0.026, Cr - 0.01, 0-0.1, UTS (25°C);
[47, kg/mm2].Scanuk-4 (Nb-0.52, Fe-0.036, Sn-0.06, Cr-0.49, 0-0.134,[UTS 47 kg/rain2] .
Zr-lCr-0.1Fe (Valloy)Zr-lCu-0.1 to 0.3FeZr-1.OSn-1-ONb-O.5FeZr-lNb-lCuZr-o.5Nb-lCr
Table 3.2. With regard to the status of zirconium alloy corrosion, it can bestated that most of the problem areas relating to hydriding, pellet cladinteraction, wear and fretting have been identified, and wherever needed, thecorrective measures have been taken either during fabrication or duringoperation. Experience with UO fuel performance has shown very low(<0.2%) failure incidence, which has demonstrated that reactor operation isnot much affected by fuel failure. Good results are also reported fromhigh-rated PHWR plants with a <0.03X bundle failure in CANDU reactors and witha <0.01% rod failure in PHWRs delivered by KWU.
It becomes apparent then that under maximum burnup conditions for currentfuel generations, waterside corrosion has not been a substantial operationalproblem. However, delays in fuel reprocessing have motivated severalprogrammes to demonstrate whether LWR fuel burnups can be extended, forexample, to 35 000-40 000 MWD/te for BWR fuel and to 50 000-55 000 MWD/te forPWR fuel. In this context, it has been found necessary to minimize corrosion
43
Table 3.2
OPERATIONAL REQUIREMENTS FOR LWR FUEL (TYPICAL VALUES)
Parameter PWR BWR
Average linear heat generationrate (W/cm)Residence time (years)Hot channel factorSteady stateTransientNeutron flux:thermal(cm-Zs-1)fast (cur2 S-1)Burnup(assembly average)(GW.d/t(U)Coolant pressure (bar)Coolant temperature (°C)
155-2553
1.5-2.12.3-2.5
4-6 x 10136-9 x 1013
28-34145-158303-316
155-2304
1.8-2.22.3-2.5
3-5 x4-6 x
22-2872287
for the high burnup fuel. The dissolved oxygen concentration of the water hasbeen shown to be of major importance with respect to fuel cladding corrosionduring irradiation. At dissolved oxygen concentration above 15 ppb,accelerated corrosion has been observed and is often characterized as nodularcorrosion. When hydrogen is added to the coolant to maintain reducingconditions in the system, corrosion is uniform and irradiation-inducedaccelerated corrosion is small.
Reported results with respect to influence of sub-cooled nucleate boilingare inconclusive . Although work reported from experimental CANDU-BLWirradiations suggested that corrosion may be accelerated under sub-cooledboiling conditions, examinations of higher rated German PWR cladding did notindicate that it is of major influence.
Since the PWR coolant temperatures generally exceed those of BWR coolant(by 30-50°C) the PWR thermal corrosion rates are higher. However, hydrogenis maintained in the PWR coolant to suppress oxygen, and hence theradiation-enhanced corrosion of cladding is minimized. The corrosion patternin PWRs in general follows the out-of-pile rates and is dependent on thetemperature. At the temperatures above 425 C, the oxide growth is quiterapid and spalling of the oxide is initiated in a relatively short time; thisbecomes more prevalent with further increase in temperature.
44
Under normal operating conditions, maximum corrosion (oxide growth)occurs over the upper third level of the fuel rods. The cladding corrosiondata for PUR fuel rods at this axial position summarized by Garzarolli ispresented in Fig. 3.1. The measured values were higher compared to thosetheoretically predicted from the out-of-pile rate constants. This deviationto higher values has been attributed to irradiation enhancement of corrosion.
80
f
*•' 40£30
ee 20
I*K0 jo
L* WIKM1NGPOR7b SAN ONOFRE . 10 Ht «OBJNSOH.2V MAINE VANlCCO UtINAU. 1O JOil CABHIBAÛ 1A« to» COM II0 JAY70N CORE IIIO REACTOR A ,KWU<> RCACTORB , HWUD M ACTOR C . HWUA RE ACTOR O , »WUO REACTOR E ,KWU
NO NUM6F.R OF NOK
I )
I I
( I
É ^-PSfOICTKJN FOR •
IO.OOO IftOOO 30,000 4°.«00«VIDA« BURNW (HkkVMTU;
».0X5
Fig. 3.1 Maximum oxide layer thicknessof PWR fuel rods versus burnup
As seen on fig. 3.1 corrosion rates from reactors are usually higher thanthose expected from autoclave tests, and models in which the thermal barrierof the oxide to the heat transfer is calculated.
To account for these deviations, some models include an "enhancement" orfitting factor, F, introduced to account for reactor effects on the corrosionrate. Typical values of F range from 1.7 to 2.5 or according to a more recentassessment done by Garzarolli et al., 0.8 to 2.8 (data from 13 reactors, withburnups up to 44 GW/d/t). It should be noted that the fitting factor and itslarge scatter from reactor to reactor and from cycle to cycle could beattributed to specific physico-chemical or thermo hydraulicalconditions existing in reactors, and not accurately described in the models(variations in zirconium oxide thermal conductivity, existence of additionalthermal barriers due to crud, variable power histories, occurence of nucleateboiling heat transfer, and eventual irradiation effects.)
45
Circumferential variations in the oxide thickness, up to a factor of 2,have also been currently observed, which could probably be attributed to localvariations in the thermal-hydraulic conditions at different locations of themetal-to-oxide interface.
Thus, the contribution of irradiation to corrosion remains uncertain. Itwas concluded that there is not a sufficient basis to extrapolate currentcladding corrosion data to burnups of 50 000 MWD/te. The major uncertainty inthe calculated models is the variation in oxide thermal conductivity over fuelrod life. The uncertainty could only be resolved by actual corrosionmeasurements on fuel rods with burnups exceeding 50 000 MWD/te.
It is worth noting that, in contrast to flux-enhanced corrosion ofzircaloy, corrosion of Zr-2.5Nb coupons, has been consistently mildlysuppressed in PWR-like environments, compared to unirradiated controls. Theexplanation appears to involve radiation-enhanced ageing of the 2.5Nb alloy,which improves the corrosion resistance by precipitating some niobium from thesolid solution.
Hillner has examined Zircaloy-clad fuel irradiated in the Shippingportreactor (PWR) for 17 years (12.3 y at reactor operating conditions). Maximumfuel burnups were 41 000 MWD/te, reaching a maximum neutron fluence
21 2of 7.8 x 10 n/cm , > 0.8 MeV. Corrosion films were light grey andtightly adherent. Pre-transition in-reactor corrosion rates appeared to besimilar to corresponding out-of-reactor rates. However, post-transitionin-reactor rates were consistently accelerated compared to the correspondingout-of-reactor rates. (The mean post-transition in-reactor corrosion rate was
2 20.093 mg/dm d; the corresponding out-of-reactor rate was 0.024 mg/dm d.The range of the clad temperature was estimated to be 260-280 C).
Two major types of flux-related corrosion have developed in theoxygenated BWR coolants: Figure 3.2 shows the uniform and local oxide growthon Zircaloy-2 cladding in BWRs. There is continuing evidence from BWR fuelcladding investigations that uniform Zircaloy corrosion is accelerated by theBWR flux. This observation correlates the earlier loop tests whichdemonstrated that oxygen and radiation interact to cause accelerated uniformcorrosion of zirconium alloys. While nodular corrosion is suspected to havebeen a factor in fuel failures at one or two reactors, it has not appeared tobe a substantial cause of cladding failures for the current generation of BWR
46
fuel. Some cases of oxide coalescence on fuel rods have been observed,leading to areas of contiguous oxide. Oxide spallation has occurred due toadvanced nodular corrosion of BWR fuel channels.
BRPVAX 1 Cpr«VA* ReloadKSBlCof«KWL1C««JOHWR
1000 Dttyj
Fig. 3.2 Uniform and local oxide growthof Zircaloy-2 cladding in BWRs,Most data collected by AEG
Since heavy nodular corrosion is accompanied by considerable wallthinning, efforts are required to control this type of corrosion, particularlyat higher burnups. Considerable effort has developed to mitigate the nodularcorrosion of Zircaloys. It was demonstrated that anneals in the beta or alphaplus beta regions followed by rapid cooling resulted in complete eliminationoof nodules on subsequent autoclaving at 475-500 C in steam. Subsequently,Urguhart and Vermilyea investigated the relationship between noduledevelopment in a selection of out-of-reactor tests and nodule developmentunder irradiation in a BWR. They concluded that the 500 C steam testprovided a reasonable simulation of BWR nodular corrosion phenomena. Morerecently, Swedish workers also reported that the 475-500 C autoclavetreatment forecasts in-reactor resistance to nodular attack. They found thatbeta treatments prior to final rolling followed by an alpha anneal were almostas effective as the final beta anneal to suppress nodular corrosion ofZircaloy.
A third type of increased uniform oxide layer has been observed at (a)the contact area between the flow channel and the upper core grid, (b) atoxygen-contaminated heat-affected zones and (c) at flow channels made frompoorly heat-treated samples. This oxide has shown spalling above thethickness of about 70 urn.
47
The corrosion rate of Zircaloy under oxygenated conditions at about280°C as a function of fast flux is summarized in Fig. 3.3. The increase incorrosion with flux can clearly be seen.
Ir
o.
0*1
(TRAT«
DHTK
•vow*
SGriW«
10" KT 10.1*
Flux (ivton2 t«c. > I Mev)
Fig. 3.3 The corrosion rate of Zircaloyunder oxygenated conditions asa function of fast flux
Fretting corrosion, a type of accelerated attack caused by the relativemovement of two surfaces in contact, occurs mainly at the spacer contacts.This is concerned more with design aspects rather than with corrosion, e.g. bycareful spacer design, particularly with respect to construction and selectionof materials, this type of attack can be prevented.
Under normal operating conditions of LWRs, the external corrosion ofZircaloys is not considered to be life-limiting, which is evident from thefact that a large number of fuel rods have been brought to full design burnupwithout corrosion failures. In the context of minimizing clad corrosion, inaddition to water chemistry aspects, the following three options were alsosuggested: (i) to improve upon the current fabrication procedures for Zircaloycladding; (ii> to modify the chemical composition of Zircaloy with respect toimpurity levels; (iii) to improve surface treatment; (iv) to select analternative zirconium-based alloy. The alloy development did not get largeimpetus, in the 1970s, because of fairly adequate in-reactor performance ofZircaloys. Improved clad performance was achieved in Japan on modifiedfabrication sequences and better control of chemical composition both withrespect to alloying elements and impurities. As a result the number of failedfuel rods in Japanese LWRs was far below that assumed in the reactor design.
48
The advent of programmes to investigate extending fuel burnups prompted astudy of alloys which might have better long-term in-reactor corrosionresistance than the Zircaloys, particularly regarding nodular corrosion.Alloys Zr-2.5Nb and Zr-INb-lSn-O.5Fe resisted nodular corrosion underconditions which produce nodules on Zircaloy-2. Zr-2.5Nb was also reported toresist nodular attack in the SGHWR. To date, the corrosion performance ofZircaloy-clad fuel with higher burnups appears to be acceptable, but fewdetailed examinations are yet available on fuel with the highest burnups.
Early alloy development in the Soviet Union centered on alloys containingniobium. The Zr-lNb alloy has been the principal cladding alloy in Sovietreactors, both BWRs and PWRs. its performance appears to be satisfactory.
The Zr-2.5Nb alloy has shown better corrosion resistance, at least in someenvironments, but the neutron economy is not as good as with Zr-lNb. In thestress-relieved condition, the alloy can be considered for fuel cladding forprolonged high-temperature operation under normal PUR conditions. Welding maydisturb the optimum structure attained by the heat treatment, but as mentionedearlier, these problems can be overcome by adopting resistance weldingtechniques. Valloy, scanuk and ozhenniter-alloys have merits at and above400 C, but their advantages over Zircaloys or Zr-Nb alloys have not clearlybeen established at the current operating temperatures.
Two factors which have been shown to be important in determining theuniform oxidation rate in LURs are fast neutron flux and water chemistry. Itis also important that one encounters post-transition regimes of oxidationduring the residence time of the fuel elements. Expressions have been derivedby combining the contributions due to thermal parameters, flux parameters andpre-film weight gain. At high fluxes in oxygenated coolants, linear,flux-dependent kinetics were rapidly established for zirconium and most of thealloys.
BWR environments contain 100-300 ppb of dissolved oxygen in the reactorwater. Studies of oxide film grown in irradiated oxygenated water suggest anincrease in porosity, and also have given evidence of enhanced porosity in theoxide which would reflect in the enhanced oxide growth in these environments.Another factor which is of importance is the creep of zirconia in the presenceof oxygen and irradiation which would affect the stoechiometry of the surfacelayers giving rise to an increased number of pores. Thus, there is
49
considerable evidence that radiation nucleates local oxide breakdown andporosity, which also includes nodule nucleation and growth. Segregation ofintermetallics enhances the local oxide growth, giving rise to initiation ofnodular growth. Alloy composition and heat treatmentmodify the corrosion response possibly because of the different sites createdby these factors. At elevated temperatures (400-450°C), the slowdisappearance of the effect of irradiation could be as a result of slowthermal healing of the damage in the oxide.
In conclusion:
1. it becomes evident that the oxide build-up due to uniform corrosion maynot be a serious problem under current operating conditions.
2. If it is found necessary to minimize corrosion for high-burnup fuel, inaddition to careful control of water chemistry, the following threeoptions have been suggested: (i) to improve the current fabricationprocedures for Zircaloy cladding, (ii) to modify the chemical compositionof Zircaloy with respect to impurity levels, and (iii) to select analternative zirconium-based alloy.
3.2 Hydrogen absorption
It is known that the hydrogen formed during corrosion can either escapeto the environment or can get absorbed by the metal or the alloy, forminghydride precipitates. The pick-up depends upon the nature of the oxide scalepresent on the metal or alloy. During the pre-transition period, the hydrogenpick-up by the metal is expected to be very low due to the protective natureof the oxide. During the post-transition, the hydrogen uptake depends uponthe temperature, the environment and the alloying additions. On Zircaloy-2for example, about 40% of corrosion hydrogen can be absorbed in PWRenvironments, while in oxygenated coolants of BWR this can be only as much as5-10V The beneficial effects of Fe, Cr and Nb as alloying additions to Zragainst hydrogen pick-up have been attributed to the cathodic nature of therespective intermetallic compound particles existing in the structure. Theintermetallics depolarize hydrogen and decrease the possibility of its passageinto the film. The functions of Zr-Ni intermetallics are not clearlyunderstood, but these enhance the ability of H„ to enter the film,eventually to end up in the metal or alloy substrate.
50
The pick-up rates on the cladding material in the oxygenated environmentsencountered in BWRs, are well within the design limits. In PWRs, the hydrogenabsorption rates have been relatively higher, but they still lie below thedesign limits. For pressure tubes and core internals, Zircaloy-4 or theZr-2.5Nb alloy are superior materials from the hydrogen pick-up point ofview. The latter alloy, after proper heat treatment, possesses superiormechanical properties compared to Zircaloys.
The more disturbing factors on cladding failures have been attributed tostress corrosion cracking by fission products (mainly iodine) and subsequentcorrosion and hydriding of the defected cladding. Design and operationalparameters are defined to overcome this serious phenomenon. Internalhydriding of the cladding, which is attributed to residual moisture present inthe fuel, is another occurrence observed during the initial stages of reactoroperation. Adopting proper moisture control in the fuel has alleviated thistype of hydriding.
The resistance of the cladding to hydrogen embrittlement is not onlyrelated to the composition of the alloy, but also to the last annealing stepsand the extent of cold work left in the material, which creates preferentialorientations of the hydride platelets.
Several arguments have been put forward to explain the various aspects ofzirconium alloys' corrosion, particularly on the transition and the behaviourunder actual service conditions. It still appears that more work is to bedone in fields such as the effect of microstructure, second phase and heattreatment, environmental chemistry variation, fast-neutron flux, nodularcorrosion and surface effects. Quantifying the pore formation in the oxidelayers in the presence of dissolved oxygen and fast flux as a function oftemperature would be a contribution needed to understand more on corrosion andhydrogen pick-up under BWR environments. Though, at this stage, a lot ofinformation is available on the zirconium alloy performance, it is yet feltthat the post-irradiation evaluation data can be compiled and compared for averification of the design codes.
3.3 Accident conditions
The behaviour of the fuel cladding during accidental transients is a keyphenomenon which determines whether an actual or a design basis accident will
51
or will not lead to activity release to the coolant or to the environment; itdetermines also the extent of core damage which can possibly penalize furtherpower plant operation.
In off-normal and accidental situations, a strong chemical interactionmay arise between Zircaloy and the coolant. This results essentially from thechemical affinity of Zircaloy for oxygen, together with the thermodynamicconditions of the interaction.
Fuel rod behaviour during any accident is significantly affected by somerod design parameters and by the rod status before the accident. Host of theparameters, including CCI, are interlinked; they are therefore brieflysummarized hereafter.
A necessary condition for significant interaction between cladding andthe coolant is that film boiling takes place. This condition may be satisfiedwith most types of accidents, even with RIAs* because of the thermal timeconstant of the rod which is usually about 10 seconds: in case of sufficientenergy deposition, boiling may take place during 10 to 20 seconds.
The temperature transient to which cladding is subject during a LOCA is avariable of reactor design and the particular form of accident underconsideration. Studies on Zircaloy cladding ballooning in a LOCA situationand results of burst tests with indirectly heated fuel rod simulators arebeyond the scope of this review.
Tests carried out to investigate the ballooning behaviour of Zircaloycladding (LWR) have shown that the deformation mechanism of the cladding tubesis governed mainly by the distribution of temperature in the cladding. In thecase of non-uniform temperature distributions, maximum cladding deformation isconcentrated on relatively small hot zones on the cladding which preventslarge circumferential and axially extended strains. The two-phase coolingduring the reflooding phase enhances axial and azimuthal temperaturedifferences on the cladding tubes and contributes in this way to a limitationof the circumferential strains and the resulting coolant channel blockage in arod bundle.
* Reactivity Initiated Accidents.
52
In most design basis accidents, the initiation phase is followed by atransition from normal cooling to film boiling, resulting in a rapid decrease
2 oof the film heat transfer coefficient, which drops from about 2W/cm Cdown to values ten or a hundred times smaller. Experiments show that, in avery short time, cladding temperatures rise until the clad either melts(severe accident simultations) or stabilizes at a given level, usually 700 to1000 C in PCM* or LOCA situations. In these latter cases, which arerepresentative of power plant accident sequences, the temperature results froman equilibrium between the heat produced and the heat transferred to thecoolant, which may be superheated steam with sub-cooled droplets.
Due to the low heat transfer capability, clad temperatures in that periodare extremely sensitive to heat production and this explains why exothermicclad oxidation may play the dominant role in the temperature evolution. Therise in the clad temperature and also in the fuel surface temperatures quicklyinduces an increase of the inner gas pressure, which is important for thebalance of the forces acting on the clad.
If the resultant inner pressure is less than the outer pressure, thecladding collapses onto the fuel. Its high creep rate and ductility may leadto extrusion of the cladding into pellet cracks. However, as a result of therod pre-pressurization, the clad usually moves away from the fuel, and ifconditions are fulfilled, ballooning may start. At that moment, threeparameters are important:
(1) The azimuthal temperature gradient around the rod;
(2) The extent of clad oxidation;
(3) The extent of alpha-beta transformation.
These three parameters determine by which mechanism the rod might fail.The absolute value of the pressure difference across the clad is of coursealso important: in that respect, one has to consider fission gas releaseduring film boiling as a possible additional contributor. Different failuremodes are, with increasing clad temperature:
* Power Coolant Mismatch
53
- Stress-corrosion cracking due to PCI;
- Stress-corrosion cracking due to gas pressure;
- Ductility exhaustion of oxidized material;
- Clad melt-through.
Other possibilities for clad failures may arise from the contact betweenneighbouring rods. One important point is that at high temperature, brittlefailure is difficult due to the intrinsic material behaviour, and also due tothe follwoing fact: as the heat transfer capability from the clad is very low,the thermal time constant of the rod is significantly higher than the10 seconds previously mentioned; the temperature variations are thereforecomparatively slow.
In the case of ballooning, failure depends on many parameters which havebeen previously examined, i.e. oxidation rate, oxygen diffusion rate,mechanical properties of the oxidized layer, phase changes; the azimuthaltemperature gradient determines whether the overall rod failure strain will behigh or low, and this is the key parameter controlling sub-channelcross-section reduction and potential blockage inside the bundle.
Once failure has taken place, eliminating the pressure difference acrossthe cladding, steam is brougt into contact with the fuel and with the stillunoxidized (or only weakly oxidized) cladding inner surface. Theprocesses that may take place are, for example, the following:
- Fuel oxidation;
- Gross fuel swelling;
- Fission product release;
- Failure propagation.
Fuel oxidation results in an extensive modification of the internal fuelrod chemistry and an enhancement of all the diffusion processes.
54
Gross fuel swelling is mainly observed in RIA tested fuel at highburnup. The high temperatures experienced in such a situation lead to anincrease of fission product inventory on grain boundaries; fission gases orvolatile fission products precipitate into intergranular bubbles which caninduce significant swelling of the fuel; the latter is most important whenfuel, pre-irradiated at low temperatures, is brought up to the melting point.
High temperatures and high diffusion properties of the fuel lead to anenhancement of fission product release: grain growth is accompanied by grainboundary sweeping, and simultaneously, fission products diffuse more easilyout of the grain. This results in a fission product release from the fuelwhich is large with respect to the release under normal operating conditions.
Clad failure propagation may affect the release, if it takes place alongone single rod, or if the propagation takes place between neighbouring rods.Such a propagation has not yet been clearly demonstrated, and it remains anopen question.
Accidents terminate by the return to normal cooling conditions, withdecay heat as the only power production. The key phenomenon at this stage isthe quenching of the fuel rod.
The temperature decrease at the end of the accident results in thebeta-alpha transformation of oxidized Zircaloy if it has been into the betaphase. Depending on the velocity of the quench front, the quenching mayresult in two main phenomena:
- Fragmentation of the oxide, i.e. "chunk" break-up, with liberation ofthe .gases present in the affected grain boundaries; in extreme cases,this may completely destroy the pellet stack geometry;
- Fragmentation of the oxygen-embrittled cladding under the thermal shock.
These phenomena are extremely important because they lead to fueldispersal into the core.
We shall not discuss here all licensing aspects of dad-coolantinteraction during accidents. The main questions connected directly to fuelelement licensing are clad temperatures, clad oxidation, deposited energyfailure threshold, together with the basic demand for core coolability.
55
According to the US-NRC licensing rules, the clad temperature limit forLOCA situations is 2200 F (1204 C) , and the fraction of oxidized clad islimited to 17%. The US licensing procedure has to demonstrate that theseregulatory limits are never exceeded. The discussion of these figures isbeyond the scope of this paper. However, one can remark that these limits areto be supported by an adequate experimental data base.
Experimental evidence supporting the RIA- related deposited energy failurethreshold has been recently gained through tests in PBF* and NSRR**. Theresults of the previous SPERT*** tests were generally confirmed, with howevera trend to reduce the failure threshold from 280 cal/g UO down to 265 cal/g
Coolability of a partially blocked bundle appears to be better than isconservatively assumed in licensing calculations, i.e. clad temperatures donot appear to be significantly increased by local ballooning.
It is clear that coolant-cladding interation plays a dominant role inaccident situations; and determines to a large extent the failure mode of therods and the subsequent activity release and ultimately the loss of geometrywith dispersion of fuel within the primary circuit.
The dad-coolant interaction arises mainly from Zircaloy oxidationthrough reaction with superheated steam. This oxidation reduces the Zircaloycreep rate and induces embrittlement. Furthermore, the exothermic characterof the reaction, together with the reduced heat transfer capability of adried-out rod bundle, shows that, in accident situations, CCI must beconsidered in the global frame of core thermal-chemical-hydraulic behaviour.
* Power Burst Facility** Nuclear Safety Research Reactor (Japan)*** Idaho site facility
56
4. CORROSION PRODUCT BEHAVIOUR AND CRUD BUILD-UP INREACTOR CIRCUITS
4.1 Migration and Deposition of Corrosion Products and Impurities
Reactor experience with the migration and deposition of corrosionproducts and impurities is considered in detail in chapter 3 of IAEA TECDOC356(1985). The following is a brief summary of the key-points:
4.1.1 Transport Processes and Mechanisms
In the foregoing sections a background has been given to the source ofcorrosion products in water reactor coolants, the chemical environment intowhich they are released and the major forms of metal species present. It isuseful to put the transport process into quantitative perspective. Afterabout a year's steady operation the coolant circuit inventory of releasedcorrosion products is likely to be several tens of kilogrammes. It is alsoclear from considerable operating experience that under steady-stateconditions only a few tens of ppbs of corrosion products are carried by WRcoolants. This means that less than 20g or < 0.1% of corrosion productinventory is being transported at any instant in time. In non-radioactiveenvironment, e.g. a conventional boiler, this might be acceptable and not giverise to plant problems but in WRs the situation is entirely different.Because of the large fuel surface area and the internal reactor neutron flux,even slight deposition there produces radioactivation products which become asignificant problem to station personnel when they are in turn transported toout-of-core surfaces. In general fuel surface crud deposition is heavier inBWRs than in PWRs by up to an order of magnitude and in PWRs it is more
_2irregular. Even if the lower PWR crudding (e.g. 0.05mg cm ) levels aretaken as applying to entire cores this can account for over 3 kg in-flux aftera year's operation in a 1000 MW(e) plant. Swedish BWRs have perhaps the
_2lowest level of fuel surface fouling (< Img cm ) but in other BWRs fuel_2deposits of up to 5mg cm are typical, indicating in-core depositioninvolving several tens of kilogrammes. Fortunately the open-texturediron-based deposit in BWRs rarely leads to heat transfer problems and in factit can assist the free boiling of coolant. In PWRs the incidence of fuel
_2cladding fouling is also rare but deposits of up to 9mg cm have beenreported. In the worst cases this can lead to increases in the shear stressand pressure drop across the core resulting in a power reduction due to the
57
Doppler effect. It is possible for over a thousand curies (>3.7 x 10 GBq)of radioactivation products to be present in fuel deposits when current WRsshut down for refuelling and overhauls.
The above data illustrate the basic problem brought about by a complexseries of transport processes occurring in the coolant circuit. As well asbeing detached from corroding surfaces, corrosion products are also releasedfrom deposits on in-core and out-of-core surfaces. Both particulate andsoluble species are involved. In PWRs differential solubility effects areimportant because of the range of temperature and chemistry changes involvedand in BWRs evaporative effects and metal oxide transformations can lead tomigration of corrosion products. The overall scheme is best appreciated bydiagramatic representation as shown in Fig. 4.1 which is widely used as thebasis for transport process modelling in water-cooled reactors. The figuredoes not attempt to differentiate between soluble, insoluble and colloidalmaterial transport. As well as chemical and thermodynamic effects,hydrodynamic processes are also involved because of the high fluid flow ratesthroughout the coolant circuits. In the first cycle of reactor operation,high corrosion product release are generally observed, in both PWRs and BWRs.After several months of steady operation, the in-core situation can tendtowards a steady state. During the initial period of operation protectivefilms are still in the process of stabilizing to the physicochemical
ln-corc region
Ouf-d-coft r«g«xi
FIGURE 4.1SCHD4ATIC DIAGRAM FOR CORROSION PRODUCT TRANSPORT P< VR'a
58
conditions of the operating regime. The preconditioning of system surfaces istherefore important in relation to initial crud input rates especially indirect cycle systems where large areas of ferrous material outside the coolantcircuit are involved.
In order to more fully understand or mathematically model the overallprocess of corrosion product (and activity) migration in coolant circuits itis necessary to define the component stages and superimpose upon these thephysical parameters for a specific reactor design. The stages or phenomena ofthe transport process have been defined for BWRs and PWRs following extensivelaboratory investigations and field studies on operating reactors during the1970s. Their prime aim is the forward prediction of activity formation anddeposition, and isolation of key variables for future design purposes. It isclear that Co is the major cause of dose to operators of all WRs and that
58in PWRs Co is important in the first few years of operation. While someof the models developed are semi-empirical, being based partly on hypothesisand observation, they are all ultimately concerned with the fate of activityproduced by activation of corrosion products more specifically with respect tothe cobalt isotopes. It should be stressed that while cobalt is only a minorimpurity (<0.1 w/o) of most coolant circuit materials and is thus a smallcontributor to corrosion product inventory, it nevertheless plays a
60disproportionate role in activity transport. Co is usually the nuclideresposible for most dose in WRs in the longer term of reactor operation.
Detailed description of the phenomena involved in the formation,transport and deposition of radioactive corrosion products are included inIAEA-TECDOC-3S6 and Annexes 2 and 3.
4.1.2 Corrosion Products in Operating; Reactors
Recent data from operating BWRs have illustrated the significantimprovements that have been achieved in the last decade. In this case theproblem can readily be monitored by measuring final feedwater corrosionproduct levels. In older reactors (e.g. WSGHWR and early operation ofTsuruga) levels in feedwater reached and exceeded 10 ppb on occasions. Beforeremedial actions were taken at Tsuruga iron input into the coolant circuit hadreached several hundred kilogrammes per cycle. The improvements adoptedincluded improved condensate purification as well as oxygen injection to thefeedwater system at 40 ppb during normal operation and dry storage under
59
shutdown conditions. Very large reductions in iron transport, especially ofparticulate material in the feedsystem, resulted in subsequent improvements inactivated corrosion product transport and some reduction and stabilization ofout-of-core radiation fields. It should be noted, however, that there was
60 -1little change in the "soluble"* Co level of about 0.05 uCi/1 in thecoolant. Similar improvements were brought about at the Shimane BWR which nowoperates with less than 1 ppb of total iron (75% insoluble) in feedwater dosedwith 15 to 25 ppb of oxygen.
Good corrosion product transport performance is reported from the SwissMühleberg BWR, where figures for feedwater and coolant are similar to thosereported for Swedish BWRs. It might therefore be predicted that average fuelelement deposition values would be similarly low. The reported "normal"ranges for the minor constituents in Mühleberg fuel crud are, however, verysimilar to those found on WSGHWR fuel. It is important in such comparisonsthat the feedwater, coolant and fuel element deposition data represent thesame operational periods. This is uncertain for the Mühleberg data.
The complexity of the transport picture is emphasized by the differenttime dependences of Co deposition on out-of-core surfaces in the RAPS-1(CANDU-PHW) and in the WSGHWR (BWR). In RAPS-1 the relative increase of 6°Coon two identical boiler banks differed by a factor of 6 over 360 EFPDs. InWSGHWR, feeder pipe contamination levels in the two identical coolant circuitsdiffered by a factor of 2 to 3 after 1200 EFPDs. In both cases the circuitswere exposed to the same chemistries. This illustration suggests thatpre-operational plant history or some early operational feature could beresponsible for the different behaviour. We cannot therefore assume thatdesign, materials or chemistry are the only governing variables in theretention of activated corrosion species.
In the PWRs and PWPWRs reported upon recently, the total corrosion productconcentration in coolants under steady operational conditions is variable butrarely exceeds a few tens of ppbs. For example, in French PWRs iron is themajor component, generally at 10 to 30 ppb, with smaller contributions fromnickel (< 10 ppb), Cu, Mn and Co (< 2 ppb). Somewhat higher iron levels atKozodluj (50-80 ppb Fe) may be due to the stainless steel steam generators or
This term is commonly utilized to denote the fraction not retained byfilters (see Section 4.2.3).
60
the chemistry regime. The Japanese data shows that most of the corrosionproducts are present as dissolved species (i.e. 98% of 21 ppb). Enhancedcorrosion product release is apparent at shutdown. It is significant that theestimate of total annual crud production (13-27 kg) in the 275 MW(e) Atucha(PHWR) reactor is similar to that estimated for other water reactors ofcomparable size. The operational level of corrosion products in bulk PWR andsimilar reactor coolants therefore corresponds to about 10 to 20g incirculation at any time, a very small fraction of the corrosion productinventory.
4.1.3 Reactor Transient Effects
It is an established fact that coolant transients which occur underabnormal operating conditions give rise to changes in coolant-borne corrosionand activation products. In general, because of solubility considerations,these transients are accompanied by significant increases in particulatespecies in the coolant. The initiating factor is a variation in thermal,chemical, physicochemical or hydrodynamic properties of the bulk coolant.
Thermal transients accompany power changes, reactor startup, controlledshutdown or trip situations. The thermal gradient is highest at the fuelsurface, hence the crud release will favour in-core deposits. Coolanttemperature reduction is also accompanied by an increase in the viscosity ofwater with consequential increase in the forces of erosion and entrainment.Under reactor trip conditions suspended crud levels in WSGHWR, for example,have increased by over two orders of magnitude from tens of ppb to severalppm. In PWRs some redissolution of soluble compounds also occurs duringcooling due to the increased acidity of the borated coolant. Reactor evidencealso confirms that such transport transients are reversed on reheating orreturn to power when rapid redeposition occurs. Observations at WSGHWR,suggest that activity levels on out-of-core pipework are not largely affectedby such "crud bursts". It is possible, however, for insoluble speciesreleased in transients to be retained preferentially in low-velocity zones ofcoolant circuits or in dead-legs. The time taken to restore normal operatingconditions is important in this respect.
Chemical transients can occur due to short-term coolant contamination.In direct cycle systems (BWRs) this can follow failure of feedwaterpurification systems or the transport of ion-exchange resins to the coolant
61
circuit. Coolant contamination from IX purifiers in PWR systems has alsooccurred. Release of activity also follows the addition of boric acid in PWRsproducing a similar effect to that of reduced coolant temperature.Oxygénation of PWR coolant, either deliberately by addition of hydrogenperoxide, or by air under cool shutdown conditions causes a more pronouncedrelease and transport of radioactive species. This possible redox effect isassociated with the preferential solubilization or precipitation of somespecies. Again the materials released in this way originate mainly fromin-core deposits where short-lived radiolytic species will also assist bybringing about chemical changes in the absence of excess hydrogen.
The separate effects of reducing temperature (240 - 100 C) and theaddition of hydrogen peroxide in PWR circuits have been studied. Nuclidessuch as Mn, Co, Co and Fe, transported mainly in solublecationic forms , were found to be released to a greater extent by cooling thanas a result of adding 200 ppm of HO The opposite effect was noted in
51 99 124the case of Cr, Ho and Sb, which tend to be present in insolubleor anionic form. The peroxide therefore promotes the formation of solublespecies from nuclides capable of anionic combinations, e.g. Cr. The shutdownoption of deliberately adding HO in order to achieve a partialdecontamination of the PWR coolant circuit (or fuel surfaces) is stillpractised by some stations and rejected by others as having only a short-termeffect. More recent investigations at Doel PWR have indicated that HOhas only limited influence on activity released at shutdown.
4.1.4 Water Reactor Fuel Surface Crudding Levels
The thickness and composition of the deposited crud layer contribute notonly to the problem of radioactivation but also to the extent of interferencewith core heat transfer and pressure drop. Porous magnetite deposits onsimulated fuel rods do not cause large increases in cladding temperature.Laboratory tests on open-textured magnetite of less than 100 microns thick at
« rta heat flux of 100 W cm" showed less than a 10 C alteration of claddingtemperature under BWR conditions; a similar deposit but impregnated with thesalts normally associated with hard water showed a cladding temperatureincrease of up to 92 C. The surface friction factor of a crudded rod inthese tests exceeded that of a clean rod by a factor of 3.3.
Both of these observations have parallels in early BWR experience whencrudding levels on fuel surfaces were high. In the first year of WSGHWR
62
experience, for example, copper infilling of the Fe 0 -based depositoccurred leading in some cases to fuel cladding failures due to steamblanketing and overheating. Subsequent operation of WSGHWR with considerablyreduced Cu and Ni levels in feedwater and lower overall crud input rates haveled to no further fuel cladding failures, although the fuel crud thickness
_2peaks at about 25 urn corresponding to 1.5 to 2.0 mg cm of deposit. Theporous deposits now formed have been found to assist heat transfer by what hasbeen termed "the wick boiling mechanism".
In second-generation BWRs with reduced iron levels in feedwater(< 10 ppb) and with little or no use of non-ferrous alloys, few fuel defectshave occurred due to crudding. Deposition levels still vary considerablydepending on design, materials and operating factors, e.g. the type offeedwater purification plant and its location or the addition of oxygen to thefeedwater. An indication of the improvement in deposition levels in the morerecently commissioned BWRs is illustrated by comparing data from WSGHWR andthe Swedish reactors. A summary of fuel crud deposition levels for variousBWRs is given below:
BW Reactor Typical Fuel Crud DepositionLevels, mg cm-2
WSGHWR 1 to 2Swedish BWRs 0.1 to 0.5Nine Mile Point < 1 to 10German BWRs:VAK (Kahl) 0.5 to 1.0KWL (Lingen) 0.2 to 0.4KRB (Gundremmingen) 3.3 to 7.6
Garigliano 1 to 4Tsuruga 0.5 to 3
In general, fuel deposition in PWR and CANDU-PHWR cores is less than inBWRs by up to an order of magnitude and is explained by differences in thesources of crud, its chemical composition, the non-boiling coolant and thecoolant chemistry itself.
It is evident from the available data and from recent discussionsconvened by the IAEA leading to this review that no fuel failures have occuredin commercial PWRs which have been attributed to fuel surface crudding. Thereare currently no reports of fluid dynamics problems related to core crudding.
63
There is, however, some recent concern in Europe that localized hard crudformation due to elements other than associated with dominant corrosionproducts (e.g. Si, AI, Ca, Mg) could be a problem.
A summary of fuel crud deposition levels for various PWRs is given below:
PW Reactor Typical Fuel Crud DepositionLevel, mg cm-2
Beznau 0.5 to 4.7 (1st cycle)Beznau < 0.01 to 0.48 (3rd cycle)Point Beach < 0.01 to 0.46 (1st cycle)Sazton 0.5 to 0.8 (1st cycle)Yankee Rowe 0.7 to 1.5 (1st cycle)Yankee Rowe 10 to 14 (2nd/3rd cycle)Obrigheim max 8 (1st cycle)Stade negligibleDouglas Point 0.1 to 0.5 (1st cycle)Douglas Point 0.05 to 0.07 (subsequently)Pickering negligible
Crud thicknesses associated with the above deposition levels vary from_2< 0.5 um with minimum deposits of < 0.05mg cm to 100 urn for peak levels
_2of 10mg cm
4.1.5 Radio Nuclides Formed From Corrosion Products
Despite the variations in system chemistries the general nature andcomposition of the corrosion products are essentially the same. They aretransported to the core where following deposition and residence for a finitetime in fluz they become activated either by thermal, epithermal or fastneutrons. The activation nuclides generally encountered in order of half-life
51 59 58 65 54 60are Cr, Fe, Co, Zn, Mn and Co. Other activation95 95products found regularly in water reactor coolants include Zr and Nb
124 110 99(which are also fission products), Sb, Ag and Tc, but these arenot responsible for a significant proportion of dose around the circuit,except in some plants.
A summary of the principal nuclides formed from water reactor corrosionproducts under neutron flux is given in Table 4.1.
64
Table 4.1
PRINCIPAL GAMMA-EMITTING NUCLIDES FORMED FROM CORROSION PRODUCTSIN WATER REACTORS
Radio-nucllde
5 1er
59Fe
5 «Co
"in
5'Mn
60C3
GarnaaParent Fornation Half- * Main source ofnuclide mechanism life parent nuclide
MeV
s°Cr n, r 28 d 0.3 Stainless iron»stainlesssteels;Inconel-600|Incoloy-800
5aFe n, T 45 d 1.1 All steel»1.3 Inconel-600iIncoloy-800
58Ni n, p 71 d 0.8 Stainlesssteels» MoneljInconel-600iIncoloy-800 iCupro nickelalloys
64Zn n, r 244 d 1.1 Brass (directcyclecondensers)
54 Fa n, p 313 d 0.8 Host steels
59Co n, Y 5.28 y 1.2 Cobalt im-1.3 purity level*in carbonsteel, stain-less steels,nickel alloys(0.02-0.3%)Hard-facingalloys -cobalt -based(50-701)
Abundanceof parentnuclldesin naturalMetal
4.3% 50Crinencomium
0.33% 58Fein iron
68% 58Niin nickel
49% 64Znin cine
5.8% S4Fein Icon
100% 59Coin cobalt
4.1.6 Transport of Coolant Impurities other than Corrosion Products
While much R and D effort has been allocated to the problems associatedwith corrosion product transport, little published information exists on therole of other elements capable of fuel-surface deposition in water reactors.The elements referred to in this context are normally associated with the mainstation water supply, water treatment plant materials or as impurities inchemicals used for water conditioning. They include calcium, magnesium,aluminium silicon and possibly carbon and barium which are capable of forming
65
compounds of low solubility in combination with each other or with the metaloxides considered in previous sections. Silicon is also present in somesteels and alloys but generally in small proportion.
There are non-documented, recent reports of fuel integrity problemsrelating to fouling by hard crud based on mineral elements in European PWRs.Such a problem could be aggravated by newer PWR designs in which localizedboiling is intended to occur. This is believed to be the reason behind recentchanges to PWR coolant chemistry specifications to include new limits forsilica (e.g.< 0.2 ppm), aluminium (< 0.05 ppm), calcium (< 0.05 ppm) andmagnesium (< 0.05 ppm).
A full review of operating experience with those impurities isin IAEA TEC-DOC 356.
4.2.____Modellina of Corrosion Product Behaviour and Crud Build-up in PWR andBWR Circuits
The behaviour of corrosion products in the primary heat transfercircuits of all water reactors is governed by the same general principles.However, there are distinctions between reactors of the PWR and BWR typeswhich result in different emphasis are reflected in the reviews for PWR ad BWRsystems incorporated in this report as Annexes II and III respectively. Thischapter will draw together the two reviews, identifying those features whichare common and those which are specific to each reactor type.
Firstly a brief description will be given of those characteristics ofPWR and BWR systems which underline the differences in corrosion productbehaviour. Then individual corrosion product mechanisms and their modellingwill be considered under headings which correspond broadly to the stages inthe construction of a comprehensive model:
i) Corrosion of alloys;ii) Nature of circulating material;
iii) Transfer to and from surfaces;iv) Interactions with surfaces;v) Performance of purification plant.
Finally, mechanistic models and computer codes intended to providecomplete descriptions of corrosion product behaviour will be surveyed.
66
4.2.1 Characteristics of PWR and BWR Systems
The major differences in corrosion and corrosion product behaviour areattributable, directly or indirectly, to the natures of the primary coolantcircuits and their heat transfer processes. In a PWR, heat is transferredfrom the core to the boiler via a recirculating flow of predominantlysingle-phase water and steam is generated in a secondary circuit. Sub-cooledboiling may occur locally on the surfaces of highly rated fuel in someadvanced reactors. In a BUR, heat is transferred from the fuel to the coolantpredominantly by boiling processes; steam is separated and is passed to theturbine while the water is recirculated to the core. Condensed steam from theturbine is returned to the recirculating primary flow via a feedwater system.The direct consequences of these distinctions are thus:
i) The release of corrosion products into the recirculating coolant inBWRs generally has a major, if not dominant, contribution from therelatively low temperature surfaces in the turbine condenser andfeedwater plant.
ii) Transfer of corrosion products in a BWR occurs in two-phase as wellas single-phase water environments. This distinction isparticularly critical in respect of the influence of boiling heattransfer on deposition on fuel surfaces.
The thermal-hydraulic environments also affect corrosion and corrosionproduct behaviour indirectly since they have a strong influence on the coolantchemistry regimes which may be sustained. The chemistries differ in two keyrespects:
i) Dissolved hydrogen, typically 10 to 30 or 30 to 60 NTP cm /kg(see Section 2.1), is generally added to the coolant of a PWR inorder to promote the recombination of oxygen produced by theradiolysis of water and thus to maintain a reducing, essentiallyoxygen-free environment.
In a BWR, the hydrogen and oxygen produced by radiolysis of water arestripped efficiently during steam separation and are subsequently removed bythe condenser off-gas system. The effectiveness of hydrogen addition istherefore limited. Oxygen is usually present in the recirculating coolant at
67
levels between about 100 and 300 ppb, although somewhat lower values areachieved in those BWR in which hydrogen is injected to the feedstream toprevent stress corrosion cracking
ii) Corrosion of the alloys in the oxygen-free environment of a PWR isgenerally minimized by the adoption of an alkaline regime. Thealkalising agent can be lithium, potassium or ammonium hydroxidessingly or in combination. Boric acid may also be added to thecoolant as a neutron absorber to control reactivity; itsconcentration is varied throughout a cycle in order to compensatefor fuel burn-up. The concentration of the alkalising agent mayalso be varied, for example to maintain a constant pH.
A low-conductivity, neutral regime has generally proved satisfactory inBWRs. Chemicals are not added to the primary coolant or feedwater,paticularly to avoid the possible generation of aggressive chemicalenvironments in the boiling regions on fuel surfaces.
4.2.2 Corrosion of Alloys
The corrosion product input to the recirculating coolant in most modernBWRs is dominated by iron which arises primarily from the uniform corrosion ofstainless and carbon steels in the feedwater systems. Corrosion of thesematerials (considered in Annex III, Section 1.2) is strongly dpendent on theconcentration of dissolved oxygen and on the flow conditions. The corrosionfilms which form on carbon and low alloy ferritic steels between 200 and 300 Cgenerally exhibit a two-layered structure whose formation is most oftendescribed in terms of the model originally proposed by Potter and Mann. Theinner layer is composed of magnetite which is produced at the metal/oxideinterface by the reaction of metal with the oxygen bearing species diffusinginwards. Half of the ferrous ions produced by the metal reaction are assumedto diffuse outward to the oxide/water interface where they may either go intosolution or deposit to form the outer layer.
The corrosion mechanisms for stainless steels and higher nickel alloysunder PWR conditions are not so well understood but there is evidence tosuggest that a two-layer description may be appropriate with an outer layerconsisting of accumulations of crystals (Annex II, Section 2.1).Unfortunately it is not clear whether corrosion ions are transported through
68
the inner layer by solid state diffusion or by aqueous phase diffusion throughpores. There are consequently major uncertainties in the descriptions notonly of corrosion ion release but also of contamination of growing films bysoluble radioactivity.
Wear, erosion or erosion-corrosion of hard facing alloys such asStellites can be significant contributors to cobalt inputs in both PWRs andBWRs. Data on these processes are sparse (Annex II, Section 2.2; Annex III,Section 1.2).
4.2.3 Nature of Circulating Matertial
It is shown in Annex II (Sections 5 and 6) that it is valuable todistinguish between colloidal (< lum) and inertial (> lum) particles since notonly their transport to and from surfaces but also their interactions withsurfaces are governed by distinctly different mechanisms. However, it isconventionally assumed that coolant-borne corrosion products may becharacterised as "particulate" or "soluble" depending on their retention orotherwise on 0.4Sum microporous filters. Thus some of the material whichwould be conventionally described as "soluble" should be more appropriatelydescribed as colloidal particulate. This distinction is crucial for modellingsince truly soluble and colloidal material exhibit distinctly differentbehaviours.
The characterisation of circulating corrosion products from filtersamples is also complicated. Firstly, it is necessary to ensure that thesample line obtains and delivers a representative sample. In practice, thesample will be modified during its passage down the line. Secondly, as notedin Annexe III (Section I.3.(l), the mechanism of particle filtration is notsimply the effect of sieving. There is also an effect of the electrokineticinteractions between colloidal particles and the filter surface; thisinfluence is strongly dependent on the coolant pH and ionic strength,consequently there are circumstances in which particles with diameters muchsmaller than the filter medium's nominal pore size may be collectedefficiently. For example, haematite particles of O.lSum diameter insuspension at neutral pH are readily collected on a Millipore filter withnormal 8 jim pores.
It is apparent, therefore, that adequate modelling of corrosion productbehaviour may require the definition of at least three types of material:
69
i) Truly soluble;ii) Colloidal particulate (< l urn);
iii) Inertial particulate (> 1 urn).
A complete description will require that the detailed size distributions ofthe latter two categories should be known. It is also clear that considerablecare needs to be exercised when attempting to distinguish between these formsusing filtration techniques.
The physico-chemical forms and elemental compositions of circulatingcorrosion products vary from one system to another reflecting differences incoolant chemistry and materials of construction. Thus, the majority ofcoolant-borne crud in the reducing environment of a PWR is present asmagnetite or combined oxides of the ferrite type whereas haematite is thepredominant form in the oxidising environment of a BWR. Evidence on the sizedistribution of this material is generally sparse but there are indicationsthat significant fractions of mass and radioactivity in both systems may beassociated with particles smaller than 1 urn.
Soluble species may arise directly from the corrosion process or fromthe dissolution of solid oxides. They are believed to play a particularlyimportant role in the transport of the major elements, iron and nickel, in PWR(Annex II, Section 3.1.1). For example, the dependence of solubility on pHgenerally has a dominant influence on the choice of optimal strategies forcoolant chemistry control. Solubilities for model systems, such as magnetiteand nickel ferrite, have, therefore, been widely studied both theoreticallyand experimentally under PUR conditions although there are as yet no data forreal corrosion products. Thennodynamic calculations for magnetite solubilityare well-founded and generally account for both ferrous and ferric species.The resulting solubilities are expected to depend on coolant pH andtemperature and, to a lesser extent, on the partial pressure of hydrogen.Agreement with experimental data is generally good. Thermodynamiccalculations for mixed oxides, such as nickel ferrite, are less well-foundeddue to uncertainties in the most appropriate values for the activities of therespective solid phases. It is, therefore, generally assumed that thesolubilities of these oxides may be adequately represented by that ofmagnetite. The available experimental data suport this assumption in respectof iron species, other elements, such as nickel are accommodated by referenceseither to the oxide stoichiometry or to experimentally observed solubilities.
70
Cobalt is chemically a minor component although it is of greatradiological significance as the precursor of cobalt-60. Its presence insoluble form is likely to be controlled by equilibria with the solid oxides ofmajor species. Adsorption of cobalt ions on oxide particles plays a major rolein BWRs (Annex III, Section I.4.(D), although it is not clear whether thetransfer occurs predominantly to the particles in suspension or followingtheir deposition. Sorption processes are considered in the section oninteractions with surfaces. Transfer of soluble species from the coolant bulkto the surface of a particle may be described by a mass transfer coefficientdetermined from an appropriate correlation.
The sources of particulate corrosion products are very uncertain (AnnexII, Section 3.2) but in principle they may originate from three types ofprocess:
i) Erosion of deposits or outer layer crystals by hydrodynamicinteractions and/or transients in coolant chemistry,
ii) Spalling of oxide films as a result of thermal transients,iii) Nucleation in the coolant bulk.
The first two processes may be significant but their contributions aredifficult to quantify. Classical theories of nucleation are currentlyadequate to draw preliminary conclusions about the importance of this processin the bulk coolant; the surface energies of corrosion product oxides andhydroxides are not known with sufficient precision to warrant moresophisticated descriptions. Calculations show that it is possible to dismisshomogeneous nucleation in the coolant bulk as a major source due to therelatively high surface energies of oxides and hydroxides. However,heterogeneous nucleation cannot be dismissed without a careful considerationof the possible sources of particulate impurity which could act as nucleationsites.
The importance of particle interactions in the coolant bulk, includingdissolution and growth (Annex II, Section 3.3) and agglomerate formation andbreak-up (Annex II, Section 3.4), remains unclear.
4.2.4 Transfer to and from Surfaces
Transfer between surfaces and the coolant bulk in single-phase water canbe considered in predominantly mechanistic terms. Mass transfer coefficients
71
for soluble species and colloidal particles may be readily derived fromempirical correlations with very little extrapolation (Annex II, Sections 4.1and 5.1.1 respectively). Thermophoresis may exert a significant influence on0.1 to 1.0 urn diameter colloidal particles to heat transfer surfaces but itsdescription requires validation under appropriate conditions (Annex II,Section 5.1.2). The transport of inertial particles is well understood buttheories have only been validated for aerosol systems (Annex II, Section5.1.3).
Particle deposition on boiling surfaces is a key process in BWRs. Inview of its complexity it is generally dealt with empirically (Annex III,Section 1.3.(3). As a consequence, a clear distinction cannot be drawnbetween transfer in the coolant and the interaction with the surface. Firstorder rate constants are considered adequate to define the deposition andrelease processes. Little is known about the latter. However, there isexperimental evidence showing how the deposition rate constant depends on:
i) The nature of the boiling process, i.e. whether it is sub-cooled ornucleate.
ii) The coolant pH. This influences interfacial interactions via thesurface electrical properties of the particle, deposition substrateand vapour bubble,
iii) Particle size,iv) Hydrodynamics.
Plant data show that deposits on fuel surfaces exhibit inner and outerlayers. It is inferred that the latter is transformed into the former butnothing is known about the rate constant.
4.2.5 Interactions with Surfaces
Precipitation and dissolution of the major soluble species iron andnickel are important mechanisms in PWRs (Annex II, Sections 4.2 and 6.4). Asurface film may be precipitated or dissolved depending on whether or not theconcentration of soluble species in the bulk coolant over the surface exceedsthe saturation solubility at the surface. The kinetics may be adequatelydescribed as a first order rate process providing boundary layer mass transferis limiting; higher order rate processes may be more appropriate if thekinetics of surface reactions are limiting.
72
It has already been noted that uptake of cobalt ions by oxide plays animportant role in BWRs (Annex III, Section I.4.(D). At room temperature itis essentially a surface phenomenon and may therefore be described by anappropriate form of adsorption isotherm e.g. Langmuir or Freundlich. At hightemperatures it is possible that adsorption may be followed by absorption, inwhich the absorbed species penetrates the interface and diffuses into theoxide bulk. Under such circumstances it is also necessary to include adescription of this diffusion process which may become limiting at longtimes. Experimental investigations have concentrated on adsorptionequilibrium and its dependence and pH. However, there have been few studiesof this mechanism at high temperature and there are very limited data on thekinetics of the adsorption and desorption processes.
A minor element such as cobalt may also be incorporated in both theouter precipitated and inner grown-on oxides (Annex II, Section 4.3). Theformer process is effectively co-precipitation and will occur at a rategoverned by the precipitation rate of the major species and the relativeconcentration of the cobalt. The latter process requires the cobalt todiffuse through the inner and outer oxide layers to be incorporated in thegrowing oxide at the metal-oxide interface. It has already been noted thatthe present knowledge of film growth is inadequate to enable a confidentdescription of this mechanism.
The initial interactions of particles with each other and with surfaceswill be governed by gravity, van der Waals, magnetic, electrical double layerand hydrodynamic forces. Well validated theoretical descriptions areavailable in all cases for simple ideal systems (Annex II, Section 5.2).Their application to real systems is limited in two respects. Firstly, it isnecessary to account for the influences of surface asperities and interfacialdeformation. Secondly, there are inadequate data for the bulk and surfaceproperties of corrosion product oxides. The Hamaker constant can be estimatedwith a fair degree of confidence. However, surface electrical properties arevery uncertain, particularly with respect to the influences of operatingtemperature, radiation, and specific ion adsorption. Haematite is notspontaneously magnetised so that in the absence of an externally appliedmagnetic field there is no significant magnetic interaction with an oxidesurface. Magnetite and nickel ferrite are spontaneously magnetised andparticles may exist as single domains up to 0.1 to l.Ojum in size, it is,therefore, the domain structure of a particle which introduces the greatestuncertainty rather than the saturation magnetisation.
73
In the longer term, the adhesion of particles to a substrate may beaffected by processes such as sintering and ion bonding.
The surface reactions during deposition and resuspension of colloidalparticles may both be described by first order rate processes (Annex II,Sections 5.4 and 6.3) using theories which have been validated for modelsystems. The interactions are dominated by van der Waals, magnetic andsurface electrical forces. Resuspension is influenced strongly by changes incoolant chemistry and temperature via the electrical double layer interaction.
The interactions of inertial particles with surfaces during depositionand resuspension are strongly influenced by hydrodynamic forces (Annex II,Sections 5.3 and 6.1). A sticking probability is generally defined for thedeposition process by comparing observed deposits and predicted arrival ratesin model experiments. Erosion, or resuspension, is also described as a firstorder rate process with values for the rate constant derived from limitedexperimental correlations. Mechanistic descriptions of both processes have sofar involved arbitrary normalisation to experimental data. However, apromising approach has been developed recently which enables sticking andentrainment to be described from first principles in terms of the turbulentmotions of the fluid adjacent to the surface and the interaction potentialbetween the particles and the surface.
4.2.6 Performance of Purification System
It was noted above that the release of corrosion products into therecirculating coolant of a BWR has a major contribution from the turbinecondenser and feedwater plant. The majority of this material is crud whichpenetrates through the condensate treatment systems. The performance ofcondensate demineralisers, deep bed ion-exchange resin columns and/or powderedresin filters is therefore crucial to the control of corrosion product levelsin the final feedwater. Theories for filtration of particles in deep beds areconsidered in Annex III (Section I.3.(2)). The removal efficiency is observedto depend on the size of the suspended particles and the pH and ionic strengthof the coolant. The main features of the process can be satisfactorilyexplained in terms of double layer interactions providing allowance is madefor the modification of the surface electrical potential of the resin beads ascoverage by crud particles increases.
74
The primary coolant of PWRs is generally purified through a by-pas.; flowtreating up to 1% of the total flow rate. The by-pass flow rate is usuallymore important than the filtration efficiency so that a full description ofthe filtration process is not essential. However, the principles areidentical to those described for BWRs.
4.2.7 Mechanistic Models and Codes
Comprehensively documented codes descibing the formation, transport anddeposition of radioactive corrosion products in BWRs are surveyed inAnnex III (Section II). Annex II did not include a similar survey for PWRs;fully documented models and codes are, therefore, briefly described in thefollowing paragraphs.
- PACTOLE
i) Corrosion of the base metal (stainless steel and Inconel) leadsboth to the formation of an oxide layer in situ and to the releaseof ions into the fluid. The release rate is governed by thesolubility gradient between the bulk coolant and the pores of theoxide.
ii) If the concentration of a given element in solution in the coolantis less than its thermodynamic solubility in the immediate vicinityof a surface, then deposits are dissolved. The dissolution rate isproportional to the difference between the solubility and theactual concentration of soluble species. The equilibriumsolubility is calculated from thermodynamic equilibria using thePOTHY code module.
iii) If the concentration of soluble species in the coolant exceeds thesolubility in the vicinity of a surface then precipitation mayoccur.
iv) Particles may nucleate in the coolant, agglomerate to a sizelimited by turbulent shear (generally less than 1 or 2um), and aresubsequently deposited on coolant circuit surfaces.
v) The deposits formed in this way are subject to erosion and possiblydissolution.
75
vi) The ions and particles deposited on in-flux surfaces undergoneutron activation.
vii) Account is taken of removal and possible sources in the by-passclean-up plant.
- CORA II. It is based on available theory but is keyed to operating plantdata and observations such that it is basically a semi-empirical model. Themajor features are:
i) Corrosion of materials such as stainless steel and Inconel releasesproducts in soluble forms at a rate proportional to solubility.
ii) The oxide films on circuit surfaces are multi-layered. Corrosionof the base metal produces an oxidised region over which aprotective film grows. The outer layers are formed by corrosionproduct deposition; there is a permanent layer adjacent to theprotective film and an outermost transient layer.
iii) Transport in the coolant is via both soluble and particulateforms. Particles may deposit on all circuit surfaces. Depositionand dissolution of soluble species are controlled by the relativemagnitudes of the concentration in the coolant bulk and the localsaturation solubility.
iv) Transient deposits may be eroded or converted to a permanent form.This erosion process is the dominant source of coolant-borneparticles.
v) In-core structural alloys and permanent and transient depositsundergo neutron activation.
vi) Removal of circulating corrosion products in the purification isaccounted for.
- BURRILL* s mechanistic model was developed to describe the behaviour ofcorrosion products in the CANDU Pressurised Heavy Water-Reactor (PHWR). Thissystem is a pressure tube reactor whose operation is characterised by anessentially constant coolant chemistry: no boron addition, lithium hydroxidedosing, pD about 11 at 25 C. The hypothesis is that:
76
i) Corrosion products from the out-core materials appear both asparticles and as solute in the coolant.
ii) The high coolant pD inhibits precipitation of soluble species onin-core surfaces. However, particles of corrosion products depositon all circuit surfaces including the fuel sheath and pressure tubesurfaces in-core.
iii) Particles deposited in-core are irradiated in the neutron flux,
iv) Dissolution is the only release process from in-core surfaces.
v) Dissolved radionuclides are carried by the coolant to out-coresurfaces and are incorporated into the corrosion layer being formedthere.
- BABCOCK and WILCOX a mechanistic model was developed following success witha phenomenological description of dynamic crud behaviour in Oconee Unit 1.The key features of this model are:
i) Most of the crud transport, both non-radioactive and radioactive,takes place by mechanisms involving particulate removal, transportand redeposition.
ii) The corrosion of stainless steel and Inconel surfaces releasessoluble species which are subsequently converted into particulatematerial in the coolant bulk.
iii) Particles deposit on all primary circuit surfaces. The deposits onin-core surfaces are converted from an initially loosely depositedlayer to a more tightly adherent form by a sintering process.
iv) The loosely adherent deposits on all surfaces are released byerosion.
It is apparent that complete mechanistic descriptions of the formation,transport and deposition of radioactive corrosion products in both PWR and BWRsystems require a diversity of complex phenomena to be modelled.Unfortunately, it is not yet possible to identify with certainty all those
77
mechanisms which are crucial to an adequate description and those which may beomitted. Thus, models are developed from a simplified network of routes, eachof which may be described either mechanistically or phenomenologicallyaccording to the state of understanding. It must be expected, therefore, thatall models will require some refinement as the detailed understanding ofme c h an i sms improve s.
78
5. CONCLUSIONS
This Coordinated Research Programme has provided a survey on the stateof the art of all aspects related to CCI. It is therefore a comprehensivepicture of the relevant phenomena and their impact on fuel behaviour, of thetechniques used to monitor and control the key parameters and of thedevelopment of modelling approaches. No conclusions are drawn at this stageis to what actions and constraints are involved in improving the CCI behaviourof the fuel and operation of the power plant. In fact, it is a survey, not amanual of good practice: it provides an exhaustive documentation on thelessons learned in this field and on basis for each organization to guidedecisions on practices. Amongst the conclusions on each different improvedaspect, the following can be mentioned.
- On water chemistry specifications and their control, the observedtrend to increase simultaneously: the coolant enthalpy (exit temperature onPWRs, void fraction in BWRs), the fuel rod surface heat flux, the operationalflexibility (load following with transient coolant conditions) and the fuelresidence time in the reactor core, leads to reductions in the failure marginsof fuel by CCI. It results in a need for improved water chemistryspecifications and controls.
- On behaviour of fuel cladding materials, optimized thermohydraulicdesign of fuel assemblies and improved cladding varieties are gainingimportance to restore CCI margins in both operating and accidental conditions.
- On corrosion product behaviour and crud build-up in reactor circuits,the collection of an experimental data base and the assessments by modellingapproaches should be pursued to improve the understanding of the parametricinterplay and guide the modifications or adaptions to be implemented.
At this stage, it is useful for the international nuclear community tolaunch a continuation of the work initiated through this CCI program and toattempt, under the IAEA auspices, to produce a guidebook which couldconstitute a "Manual of good practice on reactor water chemistry in nuclearpower plants".
79
Annex IANALYTICAL TECHNIQUES FOR MONITORING WATER CHEMISTRY
A. ELEKCentral Research Institute for Physics,
Budapest, HungaryK.S. VENKATESWARLU
Bhabha Atomic Research Centre,Bombay, India
Table I.I
Code Listing of Nuclear Power Plants
Reactor Type Country Code No.
PWRPWRPWRPWRPWR
PHWRPHWRPHWR
SGHWR
BWRBWRBWR
RBWK - 1000
BelgiumCzechoslovakiaFranceHungaryJapan
ArgentinaCanadaIndia
United Kingdom
FinlandIndiaJapan
Soviet Union
BE PW 01,CZ PW 02, 03.FR PW 04 to 30.HU PW 31.JA PW 32 to 42.
AR PH 43.CA PH 44 to 51.IN PH 52 to 54.
UK SG 55.
FI BW 56, 57.IN BW 58, 59.JA BW 60 to 70.
SU RB 71, 72, 73, 74.
In all the subsequent tables, the Code No. is employed to identify the powerreactor.
This Annex is based on the responses recieved by the end of 1983 on the IAEAquestionnaire sent in the frame of the CCI Coordinated Research Programme.
81
Table 1.2
Overview of NPPs answering the questionnaire
Code No. Title of Country Place ReactorNPP Type
BE PW 01 D -3 Belgium Doel PWRCZ PW 02 Bohunice-1 Czechoslo- Jaslowslce PWR
vakia Bohunice WWER-440CZ PW 03 Bohunice-2FR PW 04 SENA France Ardennes PWR
a ChoozFR PW 05 Blayais-1FR PW 06 Blayais-2 " "FR PW 07 Blayais-3FR PW 03 Blayais-4FR PW 09 Bugey-2 "FR PW 10 Bugey-3FR PW 11 Bugey-4FR PW 12 Bugey-5FR PW 13 Chinon B-lFR PW 14 Cruas-1FR PW 15 Dampierre-1 " "FR PW 16 Dampierre-2 " "FR PW 17 Dampierre-3 " "FR PW 18 Dampierre-4FR PW 19 Fessenheim-1 " "FR PW 20 Fessenheim-2FR PW 21 Gravelines-BlFR PW 22 Gravelines-B2FR PW 23 Gravelines-B3 "FR PW 24 Gravelines-B4FR PW 25 St.Laurent-Bl " "FR PW 26 St.Laurent-B2FR PW 27 Tricastin-1FR PW 28 Tricastin-2FR PW 29 Tricastin-3FR PW 30 Tricastin-4
by end 1983
EffectiveFull powerin MWe
9502x440
2x440320
910910910910920920900900870880890890890890880880910910910910880880915915915915
Startup
19821980
19851965
19811982198319831978197819791979198219831980198019811981197719771980198019801981198119811980198019811981
82
Table 1.2 (continued)
Code No. Title of Country PlaceNPP
Reactortype
Effectivefullpowerin MWe
Start up
HU PW 31 Paks-1
JA PW 52 Mihama-1
JA PW 33 Miharaa-2JA PW 34 Mihama-3JA PW 35 Takahama-1JA PW 36 Takahama-2JA PW 37 Ohi-1JA PW 38 Ohi-2JA PW 59 Ikata-1
JA PW 40 Ikata-2JA PW 41 Genkai-1JA PW 42 Genkai-2AR PH 43 Atueha-1
Hungary Paks
Japan
PWRWWER-440
Fukui PWRPrefecture
Ehime Prefecture
CA PH 44 Pickering-1 CanadaCA PH 45 Pickering-2 "CA PH 46 Pickering-3CA PH 47 Pickering-4CA PH 48 Bruce-1CA PH 49 Bruce-2 "CA PH 50 Bruce-3CA PH 51 Bruce-4IN PH 52 RAPS-1 IndiaIN PH 53 RAPS-2IN PH 54 MAPS-1 "UK SG 55 Winfrith United
Kingdom
" Saga Prefecture>• » it
Argentina Buenos PHWRAiresOntario PHWR.CANDU
Anushakti PHWRfl It
Kalpakkam PHWRDorches- SGHVRter
440
340
1982
1970
500826826826
11751175538
566559559335
500500500500750750750750210210235100
1972197619771975197919791977
198219751981
197119711972197319771977197819791972198019831967
83
Table 1.2 (cont inued)
Code No.
FI BW 56FI BW 57IN BW 58IN BW 59JA BW 60
JA BW 61
JA BW 62
JA BW 63
JA BW 64JA BW 65
JA BW 66JA BW 67JA BW 68JA BW 69JA BW 70SU RB 71
SU RB 72SU RB 73SU RB 74
Title of CountryNPP
TVO - I FinlandTVO - IITAPS - 1 IndiaTAPS - 2Tsuruga-1 Japan
Tokai - 2
Shimane "
Hamaoka-1
Hamaoka-2 "Fukushima-1 "
Fukushima-2 "Fukushima-3 "Fukushima-4 "Fukushima-5 "Fukushima-6 "Leningrad-1 Soviet
UnionLeningrad-2 "Leningrad-3 "Leningrad-4 "
Place Reactortype
Olkiluoto BWR•i fi
Tarapur "•i i*
Tsuruga, "FukuiTokai ,IbarakiKashima,ShimaneShizuoka "Prefecture
» uFukushima "Prefecture
ii iiii «•i n•i iiit ii
Sosnovij RBMK-1000Bor
ii ii•i n» n
Effectivefullpowerin MWe
660660200200357
1100
460
540
840460
78478478478411001000
100010001000
Start up
19681979196919691970
1978
1974
1976
19781971
197719761978197819781974
197619811981
84
Table 1.3GENERAL CONCEPTS OF SAMPLING AND CONTROL
POINTS OF SAMPLING AND PARAMETERS TO BE MEASURED
Unit Code No. C o n c e p t s S a m p l i n g P o i n t s P a r a m e t e r s
BEPW 01
C2PW 02CZPW 03(WWER)
FRPW
FRPW 05to
FRPW 30
Control of boron, lithium anddisolved hydrogen concent-ration. Performance evalua-ion of filter, déminéraliser& fuel cladding integrity.Control of boron, potassiumand ammonia concentrations.Evaluation of fuel perform-ance. There are 163 samplingpoints, 4 sampling boxesand 11 sampling channels.Coolant flow 6500 m^/hr.Purification flow 20 m3/hr.
The primary circuit has4 loops, with isolationfacility, inlet temp.284°C and 138 bars andflow 4700 m3/hr. Samplingconcepts are same as inBEPW 01.Same as abovepH control by Li
ooInlet temp: 286 C andOutlet: 324°C andpressure 155 bars.
From primary loops at thecold legs, Chemical VolumeControl system.
Inlet and outlet of mixedfilters of the continuouspurification plant of theprimary coolant, pressuri-sers, impure condensatetank, outlets of mechanicalfilters, emergency boricacid injection tank andboron concentrate tank,primary coolant makeup andcooling circuits of safetyand control assemblies.Vapour and liquid phasesof pressurisers at coldlegs of loops.
Sampling is from the coldmain line of the primarycircuit and after theChemical Volume ControlSystem.
Boron, Lithium, Calcium,Magnesium, Aluminium, Silica,pH, conductivity, Crud, dis H2,fission products.
pH, conductivity, hardness,turbidity, organics boricacid, chloride, oxygen,hydrogen, ammonia, potassium,sodium, lithium, iron, copper,calcium, silica, phosphate,nitrate, radioactive Na, K,Kr, Sr, I, Xe, Cs, Ba,corrosion products and tritium.
pH , B, H2, NH/., Li, Na, Ca,Ke, Cr, Na.Zp , CU, grossßand ^-activities, tritium,radio-iodine and crud.
Same as above
ooo\ Table 1.3 (continued)
1
HUPW 31(WWER)
JAPW 32to
JAPW 42
ARPH 43
CAPH 44to
CAPH 51 (CANDU)INPH 52to
INPH 54UKSG 55(SGHWR)
Sampling philosophy is toget necessary confirmation toadd chemicals to the prima-ry coolant. Control of boricacid and hydrogen In thecoolant is maintained. Fuelelement integrity, corrosionand radioactivation in theprimary circuit is monitored.Performance of demineral-izers & filters is evaluated.Same as above.
Sampling philosophy is toensure Isotopic purity ofheavy water and chemicalcontrol to minimise corro-sion & radioactivation ofthe primary circuit.Same as above.
Same as above.
Same as aboveIntegrated sample analysisover 2 or 3 days in additionto spot sampling.
Coolant samples are takenfrom the primary loops andreactor auxilliary system:purification systems,reagent vessels andpressuriser.
Either the primary loop orthe reactor auxiliary system,residual heat removal systemand Chemical Volume ControlSystem.Sampling points are inprimary Ü20 coolant loop,in both moderator D2Û loopsand pressuriser.
At least one sampling pointper reactor loop.
Reactor outlet header,after crud filter and afterpurification démineraiizer.Moderator and Coolant loopsalong the fe,ed train. Forcobalt level integratedsampling over several weeksfeed.
Boric acid, fission & corrosionproduct activity, dissolved02, H2, noble gases, chloride,pH, conductivity, hardness,Li, Na, K, NH3, silica,total Fe.
Same as above except K & NH3
Li, D2, 02: chloride, pD,conductivity, silica,dissolved Fe and Crud.
pD, conductivity, dissolvedDÎ and 02-» chloride, fissionproduts and crud.Same as above plus Cu, Ni,tritium.
and
Chloride, silica, metallicimpurities and radioactivation Afission products and specially60Co.
Table 1.3 (continued)
F1BW 56FIBW 57
INBW 58INBW 59
JABW 60to
JABW 70
SURB 71toSURB 74
1.2.3.4.5.
Control of specificationsControl of performance andcleaning system. Control ofcorrosion product concen-trations.To ensure fuel claddingintegrity and to minimizecorrosion and radioactivationof the primary circuit.The philosophy is forimprovement of fuel claddingintegrity and piping andequipment, for reduction ofradiation field build-up andconfirmation of waterquality and evaluation ofmaterial balance of impuritiesInsuring fuel clad integrityand chemical control withintechnical requirements.
In the primary circuit,condensate, heater andreheater drains, feedwater& reactor water. Makeupwater.Before filter in the clean-upsystem and after ion exchangepurification in the same andoff gases.Condensate, condensate demin-eralizer effluent, feedwater,reactor water and reactorwater clean-up demineralizereffluent & fission gasrelease.
Sampling from all mainprocess fluid lines.
Continuous monitoring of boron onlyContinuous monitoring of dis 02 onlyContinuous Multipore/dultipore samplingContinuous sampling and monitoring of conductivity and pH.Same as above plus dis 02
Ionic impurities, corrosionproducts, oxygen, conductivitypH, Na and fission products.
pH, conductivity, chloride,Si02 , crud, Fe, Cr, Li, Ca,fission and corrosion products.
Same as above.
pH, conductivity, hardness, Na,Fe, Ca and chloride.
BEPW 01ARPH 43UKSG 55JABW 60, 61JABW 62 to 70
oo
oooo Table 1.4
CHEMICAL ANALYTICAL TECHNIQUES
Row numberType ofuni t s
1BEPW 1
CZPW 2-3
Item to bemeasured
2Chloride,boroniron etc.Boric acid
Concentration Method and Instrumentsmethod, equip- usedment, factor
3 4ion selective elctrodeICP, titrator
Titration by NaGH, addition
Analytical procedure LowerStandardisât ion
procedure
5Comparison
detectablelevel/accuracyof the method
6ppb range
50/Cg H3BO-,/Kg
Remark
7
Chloride
Ammoniurn
Iron Evaporation
Copper
Si 1 icate
Nitrate
Evaporât ion
of Uranite or glycerine onphenolpchale in,Colorimetry, carminic acidmethod in strongly acidsulphate medium at 6lOnmfor 2 - 22g B/Ug. range.Mercuritnnietric method for morethan 5Xg Cl/fcg. Turbidimetryon silver chloride for lessthan 5/ig Cl/kg. at 450nm.Colorimetry, Nessler's method400 - 410 nmColorimetry, Orthophenanthrolinemethod, 490 - 520 nmColorimetry, Kuprizon/biscyclohexano - Oxaly-dihydrazlnemethod, 590-610 nmColorimetry, molybdate-hydrofluoric acid method, 625-750 nmColorimetry, sodium salicylatemethod, MO nm
g B/Kg
3/i 8/kg
Table I.A (continued)
1 2FRPW It Chloride
FluorideBoronNitrateSilicateMetal / Li, Na,Ca, Fe, Cr,Ni, Zn, CH, Al/Crud
FRPW 5 to 30 B, Cl, Na , Li,corrosion products: Fe, Ni,Cr, Co, Mn.
HUPW 31 Water hardnessNitrate
Silicate
Hydrazine
PHConductivityChlorideAnton la
3 4 5
ion selective electrode- do -
potent iography; colorimetryion-selective electrodecolorimetryatomic absorption
YesFor corrosion UV, Visible spectrophotometryproducts, soluble atomic absorption, ionion exchange paper selective electrode, X-rayinsoluble: filter fluorescence.paper.
tritration or spect rophotometer Standard Solut.spectrophotometer/reduct ion - do -to nitritespectrophotometer/Si Hco- - do -molybdo-blue/titration, PADAB/Spectrophoto- - do -meterpH meter - do - +_conductivity meter KC1 solution +•»pectrophotometer Standard solutionNessler-reagent titration - do -
6 7
5ppblOppm10ppm; 0.1 ppm5ppb5ppb5ppb
0.1 g/kgfor corrosionproducts
0.001 mg equtv./kg20ppb
2-5ppb
0. IppmlOppb0.010.01 /»S/cm0.05 ppm0.02- ppm
00vO
Table I. ft (continued)
I
HUPW 31contd .
JAPW32-38
2 3
Boric AcidDissolved oxygenDissolved hydrogenSodiumPotassiumLithiumI ronOil
pHConduct ivityBoron
4
Titrât ionleukomethy len bluegas chromatographyflame photometryflame photometryflame photometryspec t rophotometerextraction, spectropho-tometerpH meterconductivity metert itrat ion
5
Standard solution- do -- do -- do -- do -- do -- do --' do -
Standard solutionKC1 solutionHydrogen potass-
6 7
0. l-0.2g/dm3
2-5ppbIcm3/kgIppmIppmIppm2-5ppblOppb
(0.01 pH)(O-D/^S/cm2ppm (0.27,)
ChlorideFluorideLithiumDissolved oxygenDissolved hydrogenAbsolute turbidityI ronSi 1 ica
spec trophotometerion-electrodeatomic absorptionIndigo carmine methodgas chromatographyturbidity meterspec t rophotometerspcct rophotometer
i um phthalateStandard solution
- do -- do -
Standard solution- do -
- do -
0.05ppm (57.)
O.OSppm (57.)
0.1 ppm (57.)
0.005ppm(57.)
Icm3 /kg STP (5%)
O. lppm (107.)
0.002ppm(10'/!.)
0.002ppm(107J
Table 1.4 (continued)
1 2 3
JAPW p||39-40 Conductivity
Boron
ChlorideFluorideLithiumDissolved oxygenDissolved hydrogenTurbidityIronNickelCobaltSilica
JAPW pH41-42 Conductivity
Boron
ChlorideFluorideLithiumDissolved oxygenDissolved hydrogenAbsolute turbidityIronSilica
U
pH meterconductivity metertitrât ion
spectrophotometerion electrodeatomic absorptionIndigo carmine methodgas Chromatographturbidity meteratomic absorption
- do -- do -
spectrophotometer
pH meterconductivity metertitration, spectropho-tometerspectrophotometerion electrodeatomic absorptionIndigo carmine methodgas Chromatographturbidity meterspectrophotometer
- do -
5
Standard solutionKC1 solutionPotassium hydrogenphealate
Standard solution- do -- do -- do -
Standard gasStandard solution
- do -- do -- do -- do -
Standard solutionKCl solutionStandard acidPotassium phtalateStandard solution
- do -- do -
Standard solution- do -- do -
6 7
(0.01)(O.Ol)^SXcmIppm (0.27.)
O.OSppm (5V.)O.OSppm (57.)O.I ppm (57.)O.OOSopm (507.)1cm3 /kg STP(57.)O.lppm (107.)O.OOSppm (57.)O.Olppm (57.)0.01 ppm (57.)O.OlSppm (107.)
(O.Ol)pH '(0.01/(S/cm2ppm (0.27. )
O.OSppm (57.)O.OSppm i 57.)0.1 ppm (57.)
1cm3 /Kg STP(57.)O.OSppm (107.)0.002ppm (107.)0.02ppm (107.)
Table I.A (continued)
ARPH 43
CAPH
Nickel
BoronD20TritiumCRUDpDConductivityLithiumSodiumSilica
Chloride
Iron
Chromium
pD, Lithium
Conductivity,Dissolved oxygen,Dissolved deuteriumChloride, Crud,Radio-iodines
filtration
SpectrophotometrydimethylglyoximeCarminic acid or CurcuminSpectrophotometerLiquid scintilatorweighingpH meterconductivity meterflame photometerflame photometerSpectrophotometry andammonium molybdateSpectrophotometry nitratemixtureSpectrophotometrythioglycollie acidSpectrophotometrydiphenylcarbazldcASTM methods are generallyused.
Table l.it (continued)
INPH52-54
UKSG55
Copper
Nickel
pH
Conduct ivity
Chloride
DissolvedoxygenAmmonia
Iron
Rout ine:Chloride
DissolvedoxygenOff gashydrogen
Evaporation, concent- spectrophotometer,ration factors, 10-100 neocuproine method
linear calibration 10ppb(207.)
- do -
Evaporation, concent-ration factors10 to 100Continuous-on-line
Weekly
Daily on-line
spectrophotometer,dimethyl glyoximemethod.
mercuricthiocyanatemethod, UV-VisiblespectrophotometerIndigo carminemethodNessler's reagent,colorimetric methodspectrophotometer,ortho-phenanthrolinemethodby ferric-thiocya-nate method, usingauto-Analy«er
redox potentialmeasurementKacharometer anddiscrete/CambridgeLtd. Instrument
- do -
- do -
color comparator,fixed color charts
- do -
linear calibrationfixed color charts
ICPES, atomic absorp-tion and spectro-photometry is stand-ardised by multipointcalibrations over the
10ppb(207.)
Using diluted com-mercially availableand checked. Standardsolutions, conductivityof diluent water is lessthan 0.1 S/cm.
20ppb(207.)
5ppb(207.)
O.lppm(107.)
30ppb(207.)
Integrated sample ispassed through a singlemicroporous filtermembrane, 0.45 m.porcsire and pack of threecation and three anion
5ppb exchange membranescontained in a l u m i n i u m
17. in air/ filter holder on tso-kinetic lines, fittedwith floating ballsintegrated flow meters.Iron and Copper routinedeterminations is done3 times weekly using100 dm3.
Table 1.4 (continued)
lUKSG55
contd.
Iron
Copper
"ReactiveSilicon"
Chloride
3 times weekly frontintegrated samplecollected by 0.45 umfilter membrane andion exchange membranes,fitted in filter holderson the isokinetic lines,concentration fromprimary water in(3-8). 10, from feedwater (1-3). 104
see iron
me r cap to-aceticacid methodspectrophotometer
Continuous-on-line
Also daily fromgrab sample
0-5ppb
zinc dibenzyldithio-carbonate spectro-photometer
ammonium molybdateand 1 amino-2napthol-4 sulphonicacid, reduced B moly-bdat« silicic acidmeasured continuously,in-line using TechniconMonitor IV
SiC>2 fused withsodium carbonatein platinum meltdissolved in water
O.Ippb
20ppb
NaCl dried at150°C for twohours, dissolvedin water the samefor routinechloride
20ppb
integrated samples prefilteris dissolved in 5cm-3 concen-trated hydrochloric acid onboiling water bath. After10 minutes, O.Scnr concent-rated nitric acid is added,heated for further 20 minutes,cooled and diluted to 25cm3Cation membranes are elutedwith 25 cc IM H Cl. Inspecial cases filtered materialis prepared above, withprolonged digestion, separationof any acid insolubles bymicro filtration and fusingwith 20mg recrysta11izedsodium fluoborate. Fusedextract and acid solublesare combined and dilutedto 25 cm and reducing it tomolybdenum blue by adding1 ammo - 2 naphthol -4 salphonic acid followedby colonmetry.
Silicon - do - by the method ofreactive silicon usingAutoanalyser-1
20ppb
Table 1.4 (continued)
1UKSC55
contd.
FIBW56-57
INPW58-59
2Special
-Iron and-Cobalt,Sol . /Insol .Mult ielementanalysesSulphate
ConductivityChlorideMetallicImpuritiesSilicaChloride
Fe, Cu, Ni, Cr
IronCopperNickelChromiumSilica
3
From integrated samplerat start up, mid-run andshutdownquarterly grab sample
grab sample duringtransients/highconductivity.
evaporationfilteration plusmembranesnone
4 5
ICPES. Cobalt checkedregularly by flame andflameless atomic absorption.ICPES for AI, Cd, Ci, Cr,Cu, Fe, Mg, Mn, Ni, Znion chromatography/Dionex/
s pec t r op hot ome teratomic absorption
spec t r opho t omet erNo prêt reatment .Mercuric Linearthiocyanate methods, UV calibrationVisible spectrometer.No prêt reatment , atomic Linearabsorption UV, Visible calibrationspect rophotometry and standard
addition.bipyridyl methodneocuproine methoddimethyl glyoxime methodS-dipheny Icarbazide methodmolybdcnun blue method
6 7
Ippb0.02ppb
Zn 0.2ppbothers O.lppb0. Ippb
O.Ol^S/cm5ppb0.05-0. Ippb
5ppb20ppb (207.)
lOppb (207.)
5ppb (20%)5ppb (207J5ppb (207.)5ppb (207.)5ppb (207.)
VO
1 2 3JABW 60 pH
ConductivityBoronChlorideDissolved oxygenMetallic filtration, ionimpurities exchange(Sol/Insol.) like: evaporation
- Iron
- Copper
- Cromium
- Nickel
- Zinc
- Cobalt
JABW 61 pH
Conductivity
Boron
Chloride
Dissolved oxygen
4
glass electrodeflow type cell
mercuric thlocyanate methodWinkler Kinetics KitX-ray fluorescence
atomic absorption flamemethod
- do -
- do -
- do -
- do -
- do -
glass electrode
flow type cell
atomic absorption
mercuric thiocyanate method
diaphragm type electrode
5 6
/O.OlpH-
Ippm2ppb (17.)
lOppb1 g/ 0.17.
0.05ppm(57.)
0.03ppm(57.)
0.05ppm(57.)
0.07ppm(57.)
0.08ppm(57.)
0.05ppm(57.)
0.01
(27.) FS
50ppb(57.)
20ppb(17.)
lppb(17.)
7
Concentrât ionequipment 0.45 mmillipore membranefilter and ionexchange paper infilter holder/in-linesampler/; for otherionic species: resincolumn ion Chromato-graph, for colloidalcobalt rotary quartzevaporator, concentration factors are1000-10,000 100,1000and 10,000 respec-tively. Metallicimpurities aredissolved in 1:3HC1solution for deter-mination by AAS
Table 1.4 (continued)
1 2 3JABW 61 Metallic filtration, ioncontd. impurities exchange and
Sol/Insol . like: evaporation- Iron
- Copper
Chromium
- Nickel
- Zinc
- Cobalt
Other ionicspecies like:- Sodium- Chloride- Silica
JABW 62
Conduct i vit y
Boron Solventextract ion
4 5
X-ray fluorescence
atomic absorption flamemethod
- do -- do -
- do -
- do -
atomic absorptionflameless method
Ion Chromatograph- do -
Molybdenum blue methodStandardisationbased on J1S
- do -
absorption photometry - do -1,2 - C2H2Cl2, HF
6 7
1 m(0.17.) Concentration equipment0.41 m Millipore membranefilter and ion exchange
O.OAppm (57.) paper in filter holder/in-line sampler/, for otherionic species: resin column/
O.OAppm (57.) ion chromatography/forcolloidal Cobalt rotary
0.04ppm (57.) quartz evaporation. Conce-ntration factors are 1000 -
O.OSppm (57.) 10,000, 100 - 1000 and10,000 respectively.
0.02ppm (57.) Metallic impurities arcdissolved in 1:3HC1
Ippb (5"/i) solution for determinationby AAS.
(0.01-0.005)ppb(O.Ol-O.OOS)ppb20ppb (17.)
10-4~l(T3mg(10-30%)
methylen blue
ooTable (con t inued)
1JABW 62cont d .
2
Chloride
3 A 5
absorption photometry Standardizationbased on JIS
Metallic lon exchange paper for atomic absorption - do -impurities Sol., Mtlltpore paperSol/Insol like: for Insol Concentra-
tion factors are- Iron 20 - 2.105, Insol.
JABW63-64
- Copper- Chromium- Cobalt- Manganese- Nickel- Si 1 ica
PH
conduct ivity
Boron
ChlorideDissolvedoxygenMetall icimpuri t iesSilica
elutriated from filterand dissolved in HC1and HNO 3
absorption photometry Standardizationbased on JIS
in-line monitor
in-line monitorspect rophotometermethylen blue
Silver nitrate turbiditytneterin-line monitor
300dm3 grab sampling, atomic absorptionion exchange
molybdenum blue method
6 7
0.02-0.5mg(10 - 27.)
1 - 10 ppb«•
If
tt
II
If
M
2.5-15 g(10-207.)(0.02pH)X "Error of
reading;(0.01/tS/cm the error of
sampl ing is 5%SOppblOppb0.05ppm
lOppb
lOppb
Table 1.4 (continued)
JABW65-70
SURE71-74
pHConduct ivityBoron
Chloride
MetallicImpuritiesSol./Insollike:
- Iron
- Copper- Chromium- Nickel- Silica
pHConductivityhardnessChlorideSodiumIronCopper
precipitation ofchloride assilver-chloridefiltering the sam-ple through Milli-pore and ion excha-change papers is themethod for Sol/Insol-uble distinction.Concentration factoris 1000
Spectrophotometerméthylène blue methodtyrbidimeter Chlorimetrie
scale
X-ray fluorescentanalysis of filters
50ppb
20ppb
O.lppb
colorimeter
evaporation & ion-exchange- do -- do -- do -
Boron is converted intoBF^ in sulphuric acid,and with addition of HFcolored with méthylèneblue and extracted with1,2-dichlorocthane. Theconcentration it calcu-lated from optical density.Metallic Impurities arefiltered on by Millipore,cation and anion exchangefilters. Filters are driedout and analysed by X-rayfluorescence. Silicais determined by adjustingpH of the sample to 1.2-1.5,then adding ammonium moly-bdate for molybdenum yellowreaction.
Table 1.5 RADIOCHEMICAL AND PHYSICAL MEASUREMENT
Code number/Type ofUnits
Radiochemical :a) dose rate measurementsb) isotopic contribution to the dose ratec) gamma - spectrometryd) Möbsbauer measurements
Physical:a) crud thickness, crystal formb) crud density, thermoconductivityc) diameters of particles, etc.d) observation methods of fuel surfaces
1
BEPW 1
FRPW 4
FRPW 5 - 3 0
HUPW 31
JAPW 32 - 38
a) Babyline Teletector Automates
a) Canberra Jupiter
b) Ge Hp semiconductor, TNl) TracorNorthern
a) Babyline or total gamma dose rate
b) Isotopic contribution to dose rateis calculated from the isotopecontent measured in the vessel.
c) Ge or Ge/Li detectors
c) Ge/Li semiconductor, camberra series 30
d) No experimenta) lonization monitor
c) Ge/H semiconductor
a-c) The only physical measurement isweighing of crud collected from100-200dm3 coolant on O.A^Xfmmembrane. Particle sizing in withdifferent pore size membranes.
d) Scraping of deposits by a specialFRAGEMA device and collecting onMillipore filter, followed bychemical and radiochemica1analysis.
a) From amount of crud and form it'sspecific amount, mg/dm , X-raydiffraction
c) SEM
d) TV Camera and periscope
Table 1.5 ( c o n t i n u e d )
2
JAPW 39 - do
JAPW 41 - 42
ARPW 43
CAPH
a) Ionisation monitor
c) Ce/Li/Semiconductor
The same as for 1-7
a) As usual
b) External: 15% 3H, 457. 60Co, 407.other gamma emittersInternal: negligible
c) Canberra series 80d) Occasionally, in aELRON Hossbauer
Spectrometer ina constant acceler-ation mode with an Ar/CO2 filledReuter Stokes R.S.G.-61-M2proportional counter, with amultichannel analyser operated inthe time mode. Source used ^'Co inPd matrix, samples are collected onfilters piled up between thinacrylic disks.
a) Gamma survey meters used to measurepredetermined positions during shut-downs
b) shut down surveys
d) TV camera and periscopeThe same as for 1-7
a) By X-ray diffraction and Mössbauerspectroscopy
d) Telescopic
a-c)The isothermal corrosion couponsgive data on corrosion rates, bothgeneral and localized
oK> Table 1.5 (continued)
INPII 52 -54
UKSC 55
a) Boiler cabinet radiation fields aremeasured at predetermined locations24 hours after each reactor shut-down, employing gun monitors
b) The main contributor to the doserate is H which Is monitored byliquid scintilation counter. 131I,13'Cs and dissolved activated cor-rosion products are also measured bya high resolution Ce/Li - gamma -spectometer and quantified. On-linegamma-spectrometry-carried-out atfixed locations during long shut-downs has shown Co and 58Co to bethe major deposited activity on thecoolant system piping surfaces.
c) Measurements made on filterablecrud from the coolant system.
131,c) Routine weekly iodine balance oncircuit samples, determination oand »5I. baily "»I, »"l andExamination of solutions prepared fromintegrated samples for
and 51Cr.60Co, 65Zn,
Examination of grab samples, takenquarterly for short-lived nuclides andnuclides detected in the energy range100 to 18DO keV/about 40 isotopes/,
is separated by ion-exchange.
a) About 1 i/m thick magnetite co.iting wasformed during hot conditioning in thecomissioning phase.
a) Cross-section and profiles of cruddedpipework, measured by SEM; elementaldistribution determined by energy dis-persive X-ray analysis. Resolution lOruj
c) Particle diameter of surface depositedmaterials determined by SEM that ofwater born insolubles by Royco particlesize analyzer.
d) Scanning electron microscope ;uitoradio-graphy employed for the identification ofradioactive particles, generally inrange 1 tolOy^. m diameter, in crystallinennd some amorphous deposits.
Table 1.5 (continued)
FIBW 56 -57
INBW 58 - 59
a) Regular program during shutdownb) Calculated Ci/m2 - mretn/hc) One system in laboratory, one mobilea) By gun monitor at different locations
of priamary circuitb) Mainly Co during shutdownc) Gamma-ray spectrometry for fission
product iodines and cesiums and foractivation products like:58Co, 6°Co, 64Cu, etc.
d) Visual
a) Occasionally measuredd) Boroscopic examination
JABW 60
JABW 61
JABW 60-61
a) Ion chamber type gamma survey metera) Direct proportional gamma survey
meter
b) Calculated from the gamma-spec tro-metric measurement and gamma-emissionconstant of Isotopes
c) Using Ge/Ll - semiconductor detectorand PHA
a) Direct measurement at fuel surface duringpost irradiation examination (PIE).Estimation based on crystal form, chemicalcomposition and sampling area. Crystalform analysis is done by X-ray diffraction
b) Crud density is estimated on above data
c) Screening by various pore size f i l t e r sautomatic particle size counting andSEM is applied
d) TV camera, periscope, PIE
OUi
Table 1.5 (continued)
JABW 62
JABW 63 - .
JABW 65 - 70
a) TLD
b) 60Co, 58Co, 54Mn
c) Ge/Li Semiconductor detector
c) Gross Gamma activity is measured usingNal/Tl - detector, 1311 on silver iodideprecipitate and radioactivity balanceis measured using Ge/Li - semiconductordetector.
a) Gamma Survey mater
b) 58Co, 60Co, 5Sn
c) Extracting radioactive Isotopes ofiodine in presence of inactive carrierfrom ehe coolant and precipitatingthem as silver iodtde, their activityin measured by Oe/Li/ semiconductordetector. Using the same detectorradioactive isooopes retained on Millipore.cation and anioo-exchange filters from 5dmsample are measured in the reactor water.
Annex IIMODELLING THE BEHAVIOUR OF CORROSION PRODUCTS
IN THE PRIMARY HEAT TRANSFER CIRCUITSOF PRESSURIZED WATER REACTORS —
A REVIEW OF PRINCIPLES
R.S. RODLIFFE, M.V. POLLEY, E.W. THORNTONBerkeley Nuclear Laboratories,
Central Electricity Generating Board,Berkeley, Gloucester, United Kingdom
NOMENCLATURE
AaBbF
CC,
Hamaker constant (J).First order surface reaction rate constant (m s"1).
C , ,Csi so
CRYST.
CRYST
, fC. fcmi io ow
Rate constant defining oxide dissolution (kg m~ sFirst order constant for escape of colloidal particles froma surface (s~*).Number concentration of particles in coolant (m~ 3)Concentration of soluble species in bulk coolant (kg m~ 3) .Drag coefficient for particle in still fluid.Number concentration of particles in bulk coolant (m~3).Specific heat at constant pressure (J mole" K~ *) .Solubility (kg nT3).Corrosion product solubility at metal-inner oxide and outeroxide-coolant interfaces respectively (kg m~3).Concentration of soluble species in coolant adjacent tosurface (kg m~ 3) •Resistance to incorporation of trace corrosion Ion atmetal-oxide interface (s m~^).Resistance to incorporation of trace corrosion Ion inprecipitation on outer oxide (s m~*).Concentration of trace soluble species in bulk coolant(kg aT3).Aqueous concentrations of trace soluble species in oxidefilm at metal-inner oxide, inner oxide-outer oxide andouter oxide-water interfaces respectively (kg m~3).Concentration of trace soluble species in coolant adjacentto surface (kg m~3).
105
DD.
DIFF
DIFFc
dd
EeFF
i
m
CgH
Ionic or molecular diffusion coefficient (m2 »~ *) •Particle Brovnian diffusion coefficient (m2 •"*)•Corrosion ion diffusion coefficient in oxide film(m2 .-;Aqueous diffusion coefficient for trace corrosion ion(m2 s"1).Resistance to diffusion of trace corrosion ion in inneroxide layer (s m"1).Resistance to diffusion of trace corrosion ion in outeroxide layer (s m"*).Characteristic dimension of a flow system (m).Particle or agglomerate diameter (m).Mean height of surface roughness elements above meansurface level (m).First order resuspension or erosion rate constant (s"1).Electronic charge (C).Faraday number (C mole"*).Nett force of adhesion (N).Tangential drag force (N).Electrical double layer forces for constant potential andconstant charge interactions respectively (N).Gravitational force (N).Lift force (N).Magnetic force (N).Reaction force on deposited particle (N).van der Waals force (N).Factor modifying diffusion coefficient in oxide film,incorporating porosity and tortuosity.Factor to allow for proximity of surface and velocitydistribution normal to surface.Particle sticking probability.Free energy (J mole"*).Gravitational acceleration (m s"2).
Separation between spherical particle and plane
surface (m).
Surface reaction rate constant describing the incorporationof trace species into a growing oxide film (m s" l).Flux of particles towards surface (m~2 s"*).Corrosion ion flux across inner oxide layer (kg m"2 s"1).Rate of formation of critical nuclei (m~3 s"1).
106
J Flux of trace corrosion ion towards corroding surfacew(kg m~2 s"1)-
K , K Thermal conductivity of particle and water respectivelyP a
(J K'1 m"1 s"1)-
K , Solubility constants for dissolved species, Fe(OH).z, b oK.,K_,K, Hydrolysis constants for dissolved species.K H.,0 lonizatlon (or dissociation) equilibrium constant.w *k Boundary layer mass transfer coefficient (m s"1)-k Boltzmann constant (J K~1).Bk Deposition velocity for inertial particles (m s"1).i kik » — , diraensionless deposition velocity for inertial
tparticles.
k Boundary layer mass transfer coefficient for colloidalparticles (m s"1).
k' k modified to account for the effects of thermophoresis orelectrophoresls (m s-i).
M Deposit mass (kg m~2).MA Magnetisation (A m"1).m ,m Masses of inner and outer oxide layers respectively
(kg n'2).N Number concentration of those ions or molecules in solution
which are capable of participating in the nucleationprocess (m~3).
NA Number concentration of critical embryos (m~3).N(«) Number concentration of anlons or cations remote from a
surface (m~3).n Particle number concentration (a~3).P.,P Partition coefficients for trace corrosion ions at metal-1 o
inner oxide and outer oxide-coolant interfacesrespectively.
Q Number density of particles in deposit (m~2).R Universal gas constant (J mole"1 K~l).
udRe • — , Reynolds number.
u'dRe - ——^ , particle Reynolds number.rA Radius of critical embryo (B).S Entropy (J mole'1 K"1)
vSc • — » Schmidt number.
107
kdSn " D~ ' Sherwo°d number.
kdSh » ' , Sherwood number for mass transfer to a particle.P Ds Supersaturation, defined as the ratio of the concentration
of ions or molecules in solution to the solubility.T Absolute temperature (K) .T Reference absolute temperature (K) .ot Time (s).+t
tu2
U ,U Absolute mobilities of anion and cation respectively,a cdefined as drift velocity established by application ofunit potential gradient (m2 s~1 V"1).
U Mobility of colloidal particle (m2 s~ 1 V~l).P .u Characteristic velocity of a flow system (m s"1).u' Velocity of particle relative to fluid (m s"1).u Fluid velocity averaged over particle projected area
(m s"1).u Electrophoretic velocity (m s~*).u Particle relative velocity due to turbulence (m s~ 1).u Thermophoretic velocity (m s~*).u Friction velocity (m s~*).u Instantaneous dimensionless fluid velocity in neighbourhood
of particle: fluid velocity/u .v Volume of a molecule in solid oxide or hydroxide (m ).v Instantaneous dimensionless particle velocity: particle
veloclty/u .X Thickness of inner oxide film (m).y Distance normal to surface (m).y Separation at minimum of potential (m).y Separation at maximum of potential (m).z , z Valence of anion and cation respectively.& Gz Valence.a. Surface proximity drag correction factor.a a, at maximum of potential.Dla. Coefficient of static friction.ß Kinetic coefficient defining the rate at which ions or
molecules encounter and adhere to a critical embryo(m s"1)-
108
3 PfVelocity gradient in fluid (s"1).
, at maximum of potential (J m~_ ifi" " dH2
Y Activity coefficient of species with charge q.qAC. Corrosion ion concentration difference across inner oxidefilm (kg m~3).
AG^ Free energy of formation of a critical embryo forhomogeneous nucleation (J).
ÛG^ Free energy of formation of a critical embryo forheterogeneous nucleation (J).
A<t Height of potential energy barrier (J).6 Maximum extent of surface interaction potential (m) .e Eddy diffusivity (m2 s-1).E Permittivity of free space (F m~ *) .oEJ Dielectric constant.6 Parameter analogous to the contact angle for a liquid-
solid-gas Interface.6j Angle between normal to surface and line joining particle
centre to pivot.1/2
, inverse diffuse layer thickness
u Fluid viscosity (kg m~ 1 s"1).u Ionic strength.u Permeability of free space (H m"1).u. Relative permeability.v Kinematic viscosity of fluid (m^ B~*).p ,p Fluid and particle densities respectively (kg m~ ').p ,p Densities of inner and outer oxide layers respectively
(kg m'3).a Interfacial energy of the oxide- or hydroxide-water
interface (J m~2).dt Component of dimensionless r.m.s. velocity of particles on
projection into boundary layer normal to the wall.T*" Dimensionless particle relaxation time.T ,T Tortuosities of inner and outer oxide layers respectively.* Particle volume fraction in suspension.4>! Factor relating critical free energy for heterogeneous
nucleation to that for homogeneous nucleation.
109
4> , 4> Porosities of inner and outer oxide layers respectively.$ Surface interaction potential (J).<i> Maximum surface interaction potential (J).0 Electrical double layer interaction potential (J).
tf
è Magnetic interaction potential (J).n* van der Waals interaction potential (J).(Ko) Surface electrical potential (V).
110
1. INTRODUCTIONThe corrosion of the surfaces composing the primary heat transfer and
auxiliary circuits of water reactors is minimised by the selection ofconstruction materials and the rigorous control of coolant quality. Despitethese precautions corrosion products are inevitably released from surfaces,are transferred in the coolant and are subsequently deposited on primarycircuit surfaces unless they are removed by the clean-up plant. Thistransfer can potentially cause several problems in reactor operation (e.g.Berry and Diegle, 1979):
(1) Deposits may reduce flow areas and increase roughness, leadingto increased flow resistance and reduced coolant flow rates.
(2) It is possible for fuel failure to be induced if the claddingenvironment is substantially modified. The thermal conductivity of depositson fuel element surfaces is relatively low so that increased thermalresistance leads to raised cladding and fuel temperatures. If the fuelrating is sufficiently high local boiling may be experienced and,if porous deposits are present, concentration of chemicals «ay occur. Theresulting enhancement of corrosion may then induce cladding failure.
(3) Radioactive corrosion products may contaminate out-of-fluxsurfaces, contributing to shutdown radiation fields and thus to theradiation exposure of workers during inspection, maintenance and repairoperations.
Corrosion is generally sufficiently well controlled to ensure thatthe influences of deposits on system thermal-hydraulics are not limiting.However, the trends to higher fuel rating and higher coolant temperature inadvanced reactor designs result in a significant fraction of the fuelsurface experiencing boiling with the potential for concentration ofchemicals in any porous deposits which may form. The resulting environmentmay also be aggravated if it is considered desirable to Increase theconcentration of alkalising agents in order to operate with higher coolantpHj.
It is well established that radioactive corrosion products aredominant or major contributors to shutdown radiation fields on all waterreactors (e.g. Hlnson and Murphy, 1979; Beslu, Frejaville and Jeanson, 1979;Mishima, 1983; Varovin, Eperin, Konstantinov, Sedov, Senin and Flllppov,1983; Barber and Lister, 1982; Jarnstrom, 1982). In this case the impetusto understand corrosion product behaviour results from the ICRPrecommendation that plants should be designed and operated to ensure thatthe radiation exposure of personnel involved in tasks such as maintenance,inspection and repair should be as low as reasonably achievable (ALARA)(ICRP, 1977).
I l l
This review covers all reactors in which the primary heat transfermedium is predominantly single-phase water and in which steam is generatedin a secondary circuit, i.e. including CANDU pressurised heavy waterreactors (CANDU PHWR), Soviet WERs, etc. These systems will be denoted aspressurised water reactors (PWR) throughout the text. A full appreciationof the effects of boiling and two-phase flow is necessary to extend thiswork to direct cycle reactors.
Reviews of the formation, transport and deposition of radioactivecorrosion products in PWR have been presented by several authors (Berry andDiegle, 1979; Darras, 1980; Vanbrabant and de Regge, 1982; Gautsch, Lanzaand Weisgerber, 1979). More general reviews of relevance to corrosionproduct fouling have also been published: fouling in heat exchangers(Epstein, 1978); corrosion products in power generating systems (Lister,1979a); particulate fouling (Gudtnundsson, 1979); and precipitationfouling (Hasson, 1979). This review focusses attention on the principleswhich must form the basis for any mechanistic model for predicting thecontamination of PWR primary circuit surfaces by radioactive corrosionproducts.
There are two noteworthy exceptions from this review. Firstly,neutron activation. In the simplest terms, the production rate of anisotope by a given reaction may be determined from a knowledge of the parentabundance in the target material, the cross-section for the reaction and therelevant neutron flux. It is implicit that a complete description of thisprocess will include appropriate allowances for the neutron flux spectrum,its variation from one component to another within the reactor vessel andits temporal variation. A detailed discussion of this topic would belengthy and is considered to be beyond the scope of this review. Secondly,the effects of boiling. There have been numerous studies on corrosionproduct behaviour In boiling conditions, principally of relevance to directcycle reactors. This topic is the subject of a separate review(K. Ishigure, private communication) whose implications for the boilingconditions encountered in advanced designs of PWR will need to be consideredat a later date.
Radioactive corrosion products may be generated either directly orindirectly. In the former case, corrosion may result in the release ofneutron radioactivation products from in-flux structural materials. In thelatter case, corrosion products from out-of-flux surfaces may be releasedinto the coolant, may deposit on in-flux surfaces and undergo neutronactivation, and may subsequently be resuspended into the coolant.
The radioactive corrosion products dominating shutdown radiationfields are generally 60Co and 58Co with lesser contributions from nuclides
112
such as 59Fe, 51Cr and 5l*Mn (Berry and Diegle, 1979). 58Co Is generated byfast neutron activation of 58Ni, which is 707. abundant in nickel. 60Co isgenerated by thermal neutron activation of 59Co, which is 100Z abundant incobalt. 59Fe and 5l*Mn are generated from 58Fe and 54Fe respectively; 51Cris generated from 50Cr. Although cobalt is typically present in primarycircuit stainless steel and alloys such as Inconel-600 and Incoloy-800 atlevels from 100 to 1000 ppm it produces a potentially greater hazard thannickel because of its much higher (approximately x200) neutron activationcross-section. Also, since cobalt is present in the majority of primarycircuit materials at such low levels, it is possible for high cobaltmaterials, such as Stellites (approximately 55Z cobalt), to contributesignificantly to cobalt corrosion product input despite being used inrelatively few locations and, therefore, being associated with relativelylow exposed surface areas.
Radioactive and non-radioactive corrosion products may be transportedin the coolant both as true soluble species and as inertlal (> 1 )j,m) orcolloidal (< 1 (jrn) particulate species as defined by Ponting and Rodliffe(1983). Particulate material is conventionally classified by Its retentionon a 0.45 urn microporous filter membrane; material which is not retained,i.e. non-filterable, Is often classed as 'soluble'. The non-filterablefraction, therefore, inevitably contains some colloidal particulatematerial. Indeed, Blesa, Larotonda, Maroto and Regazzoni (1982) havesuggested that most if not all of the non-filterable 60Co circulating In thecoolant of the Atucha I Nuclear Power Station is In the form of particulatematerial. On the other hand some workers (e.g. Lister, 1978; Lister,Kushneriuk and Campbell, 1983) regard true soluble transport as dominantwhen coolant crud levels are low. Practical difficulties are experienced Insampling at high temperatures and in identifying and analysing particlessmaller than 0.1 urn diameter so that reliable data may not be available tosubstantiate models.
It is often assumed, for the purpose of simplifying calculations,that iron, nickel, chromium and cobalt species exhibit identical behaviour,e.g. vith respect to solubility, nucleation and precipitation, so that theelemental composition of deposited and circulating corrosion products wouldbe predicted to be uniform around a circuit. In practice this is notobserved. The Ni:Fe ratio in deposits on Zircaloy fuel may varyconsiderably along a channel, between channels, between the loosely andtightly adherent layers, and between different cycles of the same reactor(e.g. Oldenkamp, Paulson and Solomon, 1961; Rless, 1976; Solomon andRoesraer, 1976; Neeb and Riess, 1977). Such variations are generallypostulated to be due either to varying contributions from particulate
113
deposition and precipitation of soluble species or to differences in thesolubilities of nickel and iron. Materials such as stainless steel andhigher nickel alloys exhibit an inner corrosion layer which is enriched inchromium relative to the precipitated outer layer (Michael and Plog, 1977;Mclntyre, Zetaruk and Owen, 1979; Bergmann, Roesiaer and Péroné, 1983; Pick,1983). Obviously all these variations must be accommodated in any completedescription of corrosion product behaviour.
There is some international debate on the relative importance ofsteady operation and transient conditions for the transfer of activity fromin-flux to out-of-flux surfaces. Direct evidence is available for transferof activity during refuelling shutdown. For example, the radioactivityliberated and finally retained in the clean-up system during the firstrefuelling shutdowns at Fessenheim I and II was comparable to or greaterthan that deposited on out-of-core surfaces (Beslu, Frejaville, Brissaud,Nunse and Ridoux, 1982). In this case in-situ gamma spectrometry showedthat there was little or no change in out-of-core deposited radioactivity.This observation has been confirmed by measurements on US PWRs (Kormuth andBarkich, 1983). The role of less major transients remains to be resolvedbut the indications are that the contamination of out-of-flux surfaces isdominated by transfer during steady operation. However, it is worth notingthat the observation of such transients stimulated investigations of thermaland chemical cycling techniques for decontamination in Douglas Point CANDUPHWR (Montford, 1973) and Atucha I (Blesa, Baumgartner and Maroto, 1983).
A simplified diagram of the processes governing corrosion productbehaviour is shown in Fig. 1. Each of these mechanisms and their analyticaldescriptions will be considered in the following sections.
Release CoolantInteractions
Deposition
'/
/
-.§
X
O1
Corrosion ionrelease .
Oxide dissolution, 1desorption [
Recoil (in- fluxonly )
Nucleationand growth,adsorption
~ Erosion. Ny —— —— iresuspension 1PO
i Spoiling 1
Agglon
ons
r tic
ner<
Precipitation,crystallisation.
1 adsorption
]1 Trace speciesx co-preapitatior
incorporation
Dissolution,desorption
les Brownion,\ méritai and f\
thermophoretictransfer »
KesJ adhesion
/',
tl•o'K
°/
FIG 1. PWR Corrosion Product Formation. Transport and Deposition Mechanisms
114
2. CORROSION AND CORROSION ION RELEASEA detailed description of the mechanisms of corrosion film formation
is fundamental to any model for radioactive corrosion product behaviour.However, it is particularly necessary in order to define (i) the quantitiesof non-radioactive material released from out-of-flux surfaces and thus toestablish the potential for deposition on in-flux surfaces; (ii) thequantities of radioactive material available for release by corrosion of in-flux structural materials; and (iii) the interaction of soluble radioactivespecies with the corrosion films on out-of-flux surfaces.2.1 Uniform Corrosion
The uniform corrosion of stainless steels and higher nickel alloys inautoclaves and loops under conditions appropriate to PWR primary coolantshas been comprehensively reviewed by Cohen (1969), Berry and Diegle (1979)and most recently by Lister, McAlpine and Hocking (1984). Studies ofuniform corrosion of Stellite hard facing alloys have been reported byMclntyre, Zetaruk and Murphy (1979), Taylor and Arrason (1983) and Lister etal (1984). Corrosion rates for all these materials are generally observedto decrease with time and display approximately parabolic kinetics.Corrosion release rates also decrease with time but may exhibit differentkinetics. For example, McAlpine, Lister and Ocken (1984) observed a tdependence for corrosion release rates from Inconel-600 and Inconel-690after about 100 days, where n - 0.6-0.86. Unfortunately, most studies arelimited to relatively short exposure times (generally less than 1 year) andthere have been very few systematic investigations of the effects ofvariables such as surface finish and heat treatment, water chemistry andflow rate. Thus, caution is necessary when extrapolating the results ofsuch studies to typical reactor conditions and operating times. Particularcare should be exercised when Interpreting data from static autoclaves sincethe mass transfer environment is so different from a coolant circuit.
The corrosion mechanisms for stainless steels and higher nickelalloys under PWR conditions are not veil understood but there is evidence tosuggest that a two-layer description may be appropriate with an outer layerconsisting of accumulations of crystals (Francis and Whitlow, 1966; Warzee,Sonnen and Berge, 1967; Michael and Plog, 1977; Ensling, Fleisch, Grimm,Gruber and Gutlich, 1978; Mclntyre, Zetaruk and Owen, 1979; Tachikawa,Hoshi, Sagawa, Yonezawa and Nakashima, 1984; Lister et al, 1984). Suchobservations inevitably invite comparison with the classical two-layeredstructure observed by Potter and Mann (1962; 1964; 1965) during the hightemperature, high pH, aqueous oxidation of mild steel In the absence ofoxygen.
115
One possible set of mechanisms for the formation of such films oncarbon and low alloy ferritic steels between 200 and 300°C has been brieflydescribed by Garnsey, Hearn and Mann (1972) and more fully reviewed by Mann(1976). For these systems it has been proposed that the rate of reaction(total corrosion rate) is controlled by the diffusion of water soluble ironspecies through pores in the oxide layer (Castle and Masterson, 1966).Approximately half of the oxidised iron is incorporated in the magnetiteformed at the metal surface (inner layer) and the remainder is transportedin solution to the outer side of the oxide where it may precipitate (outerlayer) or be released into the bulk solution to precipitate elsewhere(Castle and Mann, 1966). The inner layer is microcrystalline (porosityabout 10%) and contains those alloying elements, such as chromium, in theparent material which form oxides of low solubility; the outer layerexhibits larger magnetite crystals (~ l im) which contain some manganese ornickel. The corrosion rate depends on the dissolved iron flux, which isdefined by the concentration gradient of dissolved Iron, the length of thediffusion path, and an appropriate diffusion coefficient. The corrosionrate decreases with increased film thickness and is closely related to thesolubility of magnetite in the solution generated at the corrosion surface(Potter and Mann, 1962; 1964; Bignold, Garnsey and Mann, 1972). However,the diffusion of ions through the inner layer of magnetite withoutprecipitation in the pores requires some explanation. Bignold et al (1972)proposed that the solubility of magnetite varies through the layer becauseof a change in stoichiometry of the magnetite. A possible explanation insome conditions (of less relevance to PWR) is that a hydrogen concentrationgradient across the oxide layer can be associated, with a diffusion gradientof soluble iron in equilibrium with the pore walls (Berge and Saint Paul,1973; Berge, Ribon and Saint Paul, 1977). The need to resolve blocking ofpores is eliminated if ions transfer through the inner oxide layer by solidstate diffusion as proposed by Effertz (1972): 02~ diffuses inwards alonggrain boundaries and other short circuit paths in the inner oxide and Fe **"diffuses outwards along similar routes. This mechanism has been furtherdeveloped by Tomlinson (1981) in order to account for the fraction ofhydrogen which is observed to be generated at the metal-oxide interface.Garnsey (1979) has suggested that aqueous diffusion through a porous innerlayer governs mild steel corrosion in strongly alkaline conditions whereassolid state diffusion is more appropriate in neutral solutions.
Indirect support for the two-layer structure and aqueous phasediffusion through a porous inner film comes from interpretations of loopstudies of cobalt ion contamination of growing oxide films on type 304stainless steel (Lister, 1975; 1976a; 1976b; Richardson, Castle, Large and
116
Tench, 1983), carbon steel (Lister, 1976a; 1976b) and Inconel 600 (Lister,1979b). However, the fundamental corrosion process for structural materialsunder PWR conditions remains uncertain.
The flux of corrosion ions across the inner layer may bewritten:
ACJi * - f °i Ï7 (1)
The value of the diffusion coefficient, D , is that appropriate either toaqueous phase diffusion through the pores or to solid state diffusiondepending on the nature of the corrosion process being described. In theformer case the parameter f is a constant incorporating such factors as theporosity and tortuosity of the file. In the latter case f is some complexconstant depending on parameters relevant to grain boundary, lattice andprobably dislocation diffusion. The concentration difference, AC , dependson the electrochemical equilibria in the oxide. The total corrosion ratewill depend on the relationship between the corrosion ion flux through theinner layer and the rate of incorporation at the metal-oxide interface.2.2 Other Forms of Corrosion and Wear
Localised corrosion of major circuit materials is not a significantsource of corrosion products compared with uniform corrosion. However,wear, erosion or erosion-corrosion of hard facing alloys such as Stellitesmay be significant contributors to 59Co input (Bergmann, 1982) . Data onthese processes are sparse (e.g. Dufrane and Naughton, 1983; Ocken, 1985).2.3 Solid Corrosion Product Stability
It is convenient to discuss the stability of solid corrosion productsunder the general heading of corrosion but it should be noted that thefollowing comments apply not only to corrosion films but also to circulatingparticulate corrosion products and corrosion product deposits.
Rummery and Macdonald (1975) performed thermodynamlc calculations forthe stability of solid corrosion products In aqueous systems containinghydrogen and oxygen at temperatures up to 300°C. They showed that Fe,0 ,NiFe2(\, NiO, CoO and CoFe2(\ should be stable at 300°C for dissolvedhydrogen concentrations within the range of typical PWR specifications, i.e.10 to 50 STP cm3 H2 kg"1. Thus the description of corrosion productsolubility during operation at power (see Section 3.1.1) should not becomplicated by variations in stability. Rummery and Macdonald (1975) alsoshowed that these oxides should exhibit varying degrees of stability atlower temperatures. ?or example at 10 STP cm3 H2 kg"1 it is predicted that:FejO^ and CoFe204 should be stable over the temperature range 25 to 300°C;CoO and NiO should be unstable bo.low 235 and 290 °C respectively; and
117
u should be unstable with respect to Ni and Fe30H below 200®C. Thesepredictions formed the basis for an interpretation by Bishop, Cunnane,Kennedy, Schraotzer and Stagg (1983) of circulating corrosion product levelsin a PWR during the cooldown and oxygénation stages of shutdown. However,the transfer of activity to or release from out-of-flux surfaces duringrefuelling shutdown is not significant (see Section 1) so that corrosionproduct stability Is unlikely to be a major feature of models for theformation, transport and deposition of radioactive corrosion products.
There have been some studies of the kinetics of relevant solidreactions under oxidising conditions. For example, surface oxidation ofmagnetite to haematite or maghemite has been investigated by Feitknechtand Lehmann (1959), Colombo, Fagherazzi, Gazzarrini, Lanzavecchia and Sironi(1964), Sidhu, Gilkes and Posner (1977), and Blesa, Maroto, Passaggio,Labenski and Saragovi-Badler (1978). Studies of kinetics under reducingconditions have emphasised the formation and subsequent decomposition offerrous hydroxide to form magnetite, the Schikorr reaction (Schikorr, 1929;Evans and Wanklyn, 1948; Shipko and Douglas, 1956; Keston and Lasher, 1959).Hazell and Irving (1966) have considered the thermal decomposition offerrous, nickel II and cobalt II hydroxides, unfortunately, theunderstanding of corrosion film formation under PWR conditions isinsufficient to establish the role of oxide transformations.
3. COOLANT-BORNE CORROSION PRODUCTS3.1 Soluble Species
Soluble species may arise directly from the corrosion process or fromthe dissolution of solid oxides. The behaviours of the major solublespecies, such as iron and nickel, are considered in the context of corrosionproduct solubility in Section 3.1.1. Cobalt is of major radiologicalsignificance although it is generally present at relatively lowconcentrations in the coolant. Its equilibrium with solid oxides and itstendency to precipitate on or to dissolve from solid oxides will be stronglydependent on the behaviour of the major soluble species. However, phenomenasuch as adsorption or ion exchange depend on the cobalt species in solutionand this aspect is considered briefly in Section 3.1.2.3.1.1 Solubilities of Major Species
Thermodynamic calculations (McDonald and Rummery, 1973) show thatFe,0u, CoO and Ni or NiO are the stable species in aqueous solutions\containing dissolved hydrogen (0.1 MN m~2) at temperatures ~ 280°C incontact with pure Fe, Co and Ni, respectively.
118
Sandler (1979) found that fuel pin crud from six US PWRs consistedessentially of the spinel Ni Fe„ 0, containing some chromium. Neitherx 3-x 4nickel metal nor NiO was definitely identified as a separate phase.Ni Fe„ 0, may be regarded as a magnetite structure with some Ni4"*"x 3-x 4substituted for Fe4"*" or as non-stoichiometric nickel ferrite NiFe20^.(Values of x greater than 1 would imply the presence of nickel in a formother than nickel ferrite.) Values of x were in the range from 0.45 to0.75. In Beznau I, one of the six reactors studied (Solomon and Roesmer,1976) data are consistent with x - 1.3 ± 0.4 for high power sub-assemblies(cycle 3). Data are also reported for Point Beach 1 which are consistentwith x - 0.9 ± 0.3 (cycle 1). Vanbrabant and de Regge (1980) found thatparticles in the coolants of Doel I and Doel II were of spinel structurecontaining mainly Fe, Ni and Cr in varying proportions. Superficialparticles on out-of-flux surfaces have been observed with a wide range ofcompositions consistent with 0.3 < x < 2 (Johnson, Griggs and Kustas, 1980;Pick, 1983). Similar ranges of compositions are observed in deposited andcirculating corrosion products in Kraftwerk Union PWRs (Riess, 1976).
Corrosion product elemental compositions, will obviously differbetween circuits constructed of different materials. For example, values ofx in the range 0.1 to 0.5 have been reported for out-of-flux deposits andcirculating crud in the all stainless steel Chooz reactor (Frejaville,Marchai, Beslu and Lalet, 1974). A similar range of values was observed forfuel deposits and circulating crud In the Yankee reactor (Picone and Taylor,1966). Deposits on the fuel cladding in the CANDU PHWRs at Douglas Pointand Pickering 'A1, which have Monel-400 SGUs and carbon steel piping(feeders and headers), exhibit values of x in the range 0.1 to 1.5 (Urbanic,Gray and Lister, 1979). Circulating crud and out-of-flux deposits in thesereactors have values of x towards the upper end of this range (Montford andRummery, 1975; Tomlinson, 1976a). Deposits on fuel cladding at ~ 100 EFPDin Bruce 'A', which has Inconel-600 SGUs, exhibit values of x in the range0.1 to 0.3 (Urbanic et al., 1979).
Studies of the solubility of crud have therefore centred on the'model' systems: magnetite, nickel ferrite and 'non-stochiometric1 nickelferrite.
Thennodynamic calculations of magnetite solubility (Tremaine, vonMassow and Shierraan, 1977; Macdonald, Shierman and Butler, 1972a) and fitsto solubility data (Sweeton and Baes, 1970; Tremaine and LeBlanc, 1980)assume a reaction scheme for soluble ferrous species Fe(OH), , of theform:
1/3 Fe-0, + (2-b).H+ + 1/3 H, - Fe(OH), (2~b)~l" + (4/3-b).H,034 2 a 2 (2)
119
Tremaine and LeBlanc (1980) assume that soluble ferric species Fe(OH),Dmay also be present via:
1/3 Fe304 + (S-b) "*" - Fe(OH)b(3~b)"*" 4- (4/3-b).H20 + 1/6 H2 (3)
The equilibria of equations (2) and (3) for soluble ferrous (z»2) andferric (z«3) species may be combined into:
1/3 Fe304 + (z-b).! * (4/3-z/2)H2 - Fe(OH>b(z"b)+ -I- (4/3-b)H20 (A)
The total iron solubility depends on the solubility constants K . thus,z,b
where y are the activity coefficients of species with charge q and p(H_) is«1 zthe hydrogen partial pressure. For HjO equilibrium:
(6)
[H4*] is obtained from the charge balance equation:
[H+] - -^T + (z-b) ZZb [Fe(OH)b(r-bH] + [Me*] - 0 (7)
where [Me"*"] is the base ion molality, or the negative of the acid molalityin a solution of a strong acid. In a solution of a weak acid, equilibriumconstants are required for the acid dissociation reactions at the relevanttemperatures, further terms must be added to equation (7) and a conservationequation must be introduced for the weak acid. See, for example, Appendix Aof Solomon and Roesmer (1978) for pH calculations in boric acid.
Each solubility constant K , is given by:z,b
- RT Jin K . - AG(T)z,o
- AG(T ) - AS(T ). (T-T } - 1C .T.in(T/T ) + ÛC (T-T ) (8)o o l o' p o p o
where T is usually taken as 298°K.oSolubilities are therefore expected to depend on pH , temperature and thepartial pressure of dissolved hydrogen, p(H2)> Dependence on the latter isfairly weak since for ferrous species C « p(H?)1/3 and for ferric speciessC Œ p(H2)~1/6« The dependence on pH is complex; solubilities areS i
120
generally assumed to be independent of the pH-deterraining additive (boricacid, LiOH, KOH etc).
There have been numerous experimental studies of magnetite solubilityin the temperature range 200 to 300°C with hydrogen typically 10-20 STPcm3kg~*, and with a variety of alkalising reagents, occasionally in thepresence of boric acid, yielding pH3QO in the range 6 to 8 (Sweeton andBaes, 1970; Tremaine and LeBlanc, 1980; Lambert, Montel and Courvoisier,1980; Styrikovich, Martynova, Kobyakov, Men'shikova and Reznikov, 1972;Balakrishnan, 1977; Kanert, Gray and Baldwin, 1976). An example of thevariation of solubility with pH3QO is illustrated in Fig. 2 which isreproduced from the work of Tremaine and LeBlanc (1980).
100
v0
* 0.1
0.01 I '.II6 7PH300
10
FIG. 2. Solubility of Fe70^ at 300° C (fromTremaine ond LeBlanc, 1980)
Thennodynamic calculations of nickel ferrite solubility must takeaccount of the equilibrium between NiFe20|,, Fe^i, and Ni (Macdonald, Rummer yand Tomlinson, 1975; von Massow, Sullivan and Waugh, 1975):
+ 3N1 + (9)
It is also necessary to account for the possible formation of non-stoichiometric nickel ferrite (Lambert, Montel and Courvoisier, 1983):
121
(10)* J—A •*
The solubilisation reactions are described by:
NiFe^j, + H2 + 6H+ •*—- Ni 4- 2Fe + M£> (11)
Ni2* + mOH" -"-7 Ni (OH) (m~2)" (12)m
Fe*+ nOH" Fe(OH)n(n~2)~ (13)
The application of these thermodynamic equilibria to the calculation of thesolubility of magnetite and nickel ferrite at high temperature using thePOTHY code is described by Beslu, Lalet, Joyer, Noe and Frejaville (1983).
Sandier and Kunig (1977; 1981) have measured the solubilities of avariety of synthetic non-stiochiometric nickel ferrites from roomtemperature to 350°C with hydrogen typically 10-20 STP cm^g"1 and pH300 inthe range 5.0 to 8.5.
These solubility studies show that at normal PWR operatingtemperatures (~ 300°C) there is a negative temperature dependence of thesolubility of iron in acidic solutions, a zero temperature dependence insome weakly alkaline solutions and a positive dependence in more stronglyalkaline solutions. Solubilities of species other than iron with respect tomixed ferrites have been less well characterised. Solubilities in solutions/containing boric acid generally increase at low temperatures (~100^C).3.1.2 Cobalt
Co(II) hydrolysis has been studied experimentally by several workersand some of the results are reviewed by Lambert (1975); however, morerecent data (Giasson and Tewari, 1978) have been obtained for cobaltconcentrations in the range 0.01-0.1 mole kg"1, pH (25°C) 4-9.5 and at 25-200°C. The results over this limited pH range were adequately accounted forby a single hydrolysis equilibrium:
Co2*" + R£ -Kl Co(OH)+ + H+ (14)
Values of pK1 for two different standard states are listed in Table 1.More comprehensive values have been extrapolated from thermodynamic
data (Macdonald, Shierman and Butler, 1972b) using two solution equilibriain addition to (14) above:
Co2++2H20 -2 HCo02- + 3H+ (15)and
Co(OH)+ + H 20 Jii- HCo02" + 2H+ (16)
122
TABLE 1
Co(II) Hydrolysis Constants
PK^-D*
pKjCnj-0)
pK
P 5
PK3
Temperature, °C
259.82
10.02
9.82
31.83
22.01
1007.627.798.57
25.71
17.14
1506.59
6.86
7.92
23.23
15.31
2006.026.10
7.31
21.41
14.10
250
6.75
20.11
13.36
300
6.27
18.86
12.59
Reference
Giasson and Te war! (1978). . .
Macdonald, Shierman & Butler(1972b)
«• M ••
- ionic strength
oo
V•8
10-
10-»
i<r4
HCo02-
104 5 6 7 B 9
pH (300°C)
FIG 3. Thermodynamic Calculations of theMole Fractions of Cobalt Solution Speciesat 300° C. Data from Macdonatd .Shierman and Butler ( 1972b).
123
Values calculated for pK,, pK, and pK3 are listed in Table 1 and Fig. 3shows plots of the taole fractions of the three Co(II) solution speciesagainst pH at 300°C. This thermodynamic description is not complete sinceCo(OH)2 has been neglected as a solution species and equilibrium with solidoxides is not considered. However, the available evidence suggests that theCo(II) solution species may be expected to be very strongly dependent on pHand temperature in the conditions encountered in PWR coolant.3.2 Sources of Particulate Material
The sources of coolant-borne corrosion product particles aredifficult to quantify. It is very likely that particulate material isreleased from precipitated outer layers or inner corrosion films byprocesses such as erosion or spalling, considered in Sections 6.1 and 6.2respectively. Formation of particles by homogeneous or heterogeneousnucleation in the bulk coolant to relieve supersaturation is considered inthis section. The review of spontaneous precipitation from electrolyticsolutions by Furedi-Milhofer (1981) provides an interesting background tothis section and Section 3.3.3.2.1 Classical Theory for Homogeneous Nucleation
Several textbooks on precipitation and crystallisation (Nielsen,1964; Walton, 1967; Mullin, 1972) present derivations of the rate offormation of critical nuclei based on classical theories (Volmer and Flood,1934; Frenkel, 1955). The present work will follow the terminology ofNielsen (1964) in which: particles smaller than the critical size arecalled embryos; particles bigger than the critical size are called nuclei;and particles of critical size are critical embryos or critical nuclei.
After Twomey (1977), the rate of formation of spherical criticalnuclei may be written:
J - 4itr.2 ß N N. (17)n » w
The radius, concentration and free energy of formation of thecritical embryos are given by:
The expressions for r^ and UG^ need to be modified for non-sphericalembryos; for example, the factor —=— in equation (20) for ££+ would become
124
32 for a cube. Factors for other geometric shapes are given by Nielsen(1964). The nass transfer coefficient for ions or molecules diffusing to aspherical particle of radius r^ (which is typically 10 to 100 A) establishesan upper limit to the rate at which nucleation is predicted to occur and maybe written (Treybal, 1968):
ß - £- (21)*
The classical theory has been used successfully in many studies ofnucleation in aqueous solutions (e.g Nielsen and Sohnel, 1971; Makrides,Turner and Slaughter, 1980). However, it should be remembered that incrystallisation the interfacial energy is not necessarily independent ofembryo size (Frenkel, 1955) and that extra thermodynamic terms involvingrotation, vibration and translation of the embryos become rather importantfor small embryos (Walton, 1967).
It is immediately apparent from equations (17), (19) and (20) thatthe nucleation rate is strongly dependent on the interfacial energy sincethe cube of this parameter appears in the exponent of the expressiondescribing the concentration of the critical embryos. An indicaton of theappropriate value for interfacial energy may be obtained from the studies ofhydrolysis-precipitation in Iron solutions by Dousma and de Bruyn (1979).They derived values in the range 100 to 200 mJ m~2 for o-FeOOH at pH25»2,temperature 25 °C and high ionic strength. Extrapolation to FVR conditionsby making an appropriate allowance for the contribution of the electricaldouble layer to the interfacial energy (e.g. Kruyt, 1952) yields valuesbetween 250 and 650 mJ m~2. it may be readily concluded that homogeneousnucleation in the bulk coolant is unlikely to be a significant source ofcoolant-borne particles.3.2.2 Heterogeneous Nucleation
Volmer (1939) developed the classical nucleation theory for aspherical cap of crystalline deposit growing from solution on a solidsurface. The situation is illustrated schematically in Fig. A. It wasshown that the critical free energy for heterogeneous nucleation (£G ) couldbe written in terms of the critical free energy for homogeneous nucleation:
AG^ • $, Û3. . (22)
(2+cos9)(l-cose)2where
o - ocos e - -il———Ei
125
ffSL = 'es* "CL cos e
Liquid (L)
FIG
'Crystalline'embryo (C);
///////////////, l
Nucleation on a Solid Surface
and the subscripts si, cs and cl refer to the solid-liquid, crystal-solidand crystal-liquid interfaces respectively. Fig. 5 shows the dependence of4^ on 9. It can be seen that the critical free energy for nucleation can besignificantly reduced for values of 9 less than ~ 90°. In particular, forcomplete affinity 9-0, ^ - 0 and therefore AG^ - 0.
Thus, at least in principle, heterogeneous nucleation may be asignificant source of coolant-borne corrosion product particles if there isa sufficient supply of suitable impurity particles in the charging or make-up flows. It may also contribute to the relief of supersaturation;particulate Impurity initially providing nucleation sites for the solublespecies and subsequently growing by precipitation. The significance of thiscontribution must be determined by comparison with the relief ofsupersaturation by precipitation on circuit surfaces.
I.Or
^ _ (2«cos e)(i- cose)2
OS
0 090e ( 180
FIG. 5. Modification of Critical Free Energy for Heterogeneous orSecondary Nucleation. AG+= «t>1 AG
126
3.3 Dissolution and Growth of ParticlesThe mechanisms which govern the deposition of soluble species on
circuit surfaces (Section 4) and dissolution of deposits or corrosion filmsfrom circuit surfaces (Section 6.4) also govern the growth from solution anddissolution of particles in suspension. Theories for the growth ofprecipitate particles (Ostwald ripening) have been reviewed by Kahlweit(1975). The tendency to precipitation or dissolution may be defined interms of the relative magnitudes of the saturation solubility at theparticle surface and the concentration of soluble species in the bulksolution over the surface; this is described in detail in Sections 4.2 and6.4. It should be noted that the saturation solubility at the surface of aparticle depends on particle size and will be increased relative to that fora plane surface by a factor which is given by equation (18) for a sphericalparticle with a homogeneous surface (Kelvin effect). Adsorption anddesorption are described in Section 4.4. In any case the rate of transferof soluble species to or from the surfaces of particles In suspension isdescribed by a mass transfer coefficient and is considered in this section.
Mass transfer to particles in turbulent dispersion has beeninvestigated by Kuboi, Komasawa, Otake and Iwasa (1974) who correlated theirown and other available data by:
Sh - 2 + 0.49 (Re Sc)1/2 250 < Sc < 1100 .,,.P p (23;0.2 < Re < 1000P
The particle Reynolds number, Re , depends on the velocity of the particlesPrelative to the fluid due to turbulence and gravitational settling. Kuboi,Komasawa and Otake (1974) have presented a method for the derivation of thisvelocity.3.4 Formation and Break-up of Agglomerates3.4.1 Agglomeration
Agglomeration may be considered as a two stage process; transport ofthe particles relative to each other resulting in particle-particlecollisions followed by the interaction of the colliding particles leading tothe formation of agglomerates. In the absence of external body forcesparticle transport may occur as the result of Brownian motion, shear flow orturbulent motion. The treatment of agglomeration due to these variousmechanisms is generally simplified by making the following assumptions:
(a) The agglomerating particles are spherical and monodisperse. Inpractice the particle size distribution will not be monodisperse and Indeedas agglomeration proceeds even an originally monodisperse distribution willbecome polydisperse. Swift and Friedlander (1964) discuss the nature of the
127
particle size distribution in agglomeration by Brownian motion and shearflow.
(b) The particle volume fraction occupied by particles issufficiently small that only binary collisions need be considered. Thisrequires that the volume fraction is less than about 1Z, a condition easilysatisfied in water reactor circuits.
Smoluchowski (1917) showed that when agglomeration Is controlled byBrownian diffusion the kinetics for the decay of total particle concentrationmay be written:
4 k T
It is implicit in this formulation that each particle-particle collisionresults in agglomeration. Fuchs (1934) derived a modification of equation(24) to account for the effect of van der Waals and electrical double layerinteractions between particles. An additional correction for the viscousinteraction between particles has been derived by Spielman (1970) and Honig,Roebersen and Wiersema (1971).
When agglomeration is controlled by shear flow the rate of change ofconcentration for a system of particles of uniform size is given by (Swiftand Friedlander, 1964):
AT*
The agglomeration of particles In turbulent flow may be describedusing simple mean free path concepts (e.g. Delichatsios and Probstein, 1975)The rate of decrease of particle number density is then given by:
(26,
The relative velocity, u , depends on the turbulence scale and may beestimated from the work of Rotta (1972).
Means (1978) showed that the rate of agglomeration in FUR conditionsis likely to be dominated by turbulent motion for particles bigger than aboutl urn and by Brownian motion for smaller particles.3.4.2 Agglomerate Stability
An agglomerate will break-up in a turbulent flow unless the force ofattraction between its constituent particles is sufficient to withstand theturbulent pressure fluctuations across its diameter and the shear forcesassociated with its motion relative to the fluid (Hinre, 1955; Levich, 1962).
128
The former is dominant for particles in water (Delichatsios, 1975;Delichatsios and Probstein, 1975). Thus, in principle, a maximum stableagglomerate size may be defined for given flow conditions and agglomeratestrength; in practice, Means (1978) encountered difficulty in establishingthe agglomerate strength under PWR conditions. Parker, Kaufman and Jenkins(1972) have developed a theory for the kinetics of floe break-up In turbulentflows.
4. DEPOSITION OF SOLUBLE SPECIES4.1 Mass Transfer in the Coolant
Transfer of soluble species from the coolant bulk to the vicinity of asurface, or vice versa, will be limited by Brownian diffusion through aboundary layer. Under these conditions it is appropriate to describetransfer in terms of a coefficient, having the dimensions of a velocity,which may be determined from empirical correlations for the relevant geometryand flow conditions (e.g. Treybal, 1968). Correlations are of the followingform:
Sh - f (Re, Sc) (27)
In a dilute, ideal solution, i.e. one in which the diffusion of thenon-dissociating solvent is not significant, the diffusion coefficient of abinary electrolyte is given by the Nernst equation (e.g. Reid and Sherwood,1966; Erdey-Gruz, 1974).
D -U Uc aU -»-Uc a
zc I_al RTr*rr F (28)
At low temperatures, ~20°C, measured diffusion coefficients are ingood agreement with predictions from equation (28) based on measuredmobilities. At higher temperatures values of mobility are less readilyavailable and the extrapolation from measurements at low temperature mustconsider changes In the size, and possibly ionic form, of the kinetic units.A detailed examination of extrapolation to temperatures up to 300 °C has beenpresented by Lindsay (1981) in the context of SGU secondary water chemistry.An estimate of the diffusion coefficient at temperature may be obtainedfrom (e.g. Reid and Sherwood, 1966):
TDT - D298 198
where subscripts T and 298 refer to the respective absolute temperatures.
129
The Schmidt number for corrosion ions varies from about 100 at 25°C toabout 1 at 300°C.
The flow conditions encountered in reactor primary and auxiliarycoolant circuits are well approximated by fully developed turbulent flow in acircular pipe providing a suitable allowance is made for the effectivediameter of non-circular flow passages. Measurements of mass transfer underthese conditions have been made by many workers (e.g. Harriott and Hamilton,1965; Mizushina, Ogino, Oka and Fukuda, 1971). Berger and Hau (1977) useddata from these studies and their own to derive the following correlation:
Sh - 2 + c Re8 Sc°'33 0.6 < Sc < 10" (30)101* < Re < 106
where c - 0.0165 + 0.011 Sc e"Sc
10and a - 0.86 -(4.7+Sc)3
An indication of the effect of rough surfaces may be obtained from the workof Dipprey and Sabersky (1963). A factor of about 3 enhancement of the masstransfer coefficient may be estimated for soluble species under PWRconditions for 'fully rough1 pipes at Reynolds numbers in the range 10 ** to106.
Mass transfer of ionic species in aqueous systems under other flowconditions and geometries has been comprehensively reviewed by Poulson(1983).4.2 Precipitation
Precipitation may occur on a surface if the concentration of solublecorrosion products in the bulk coolant over a surface exceeds the saturationsolubility at the surface. The precipitation rate has been treated inseveral reviews (Mullin, 1972; Nancollas, 1979; Hasson, 1979; Furedi-Milhofer, 1981). At steady state (see Fig. 6) it must be equated to the rateof mass transfer through the boundary layer and may be written:
The kinetics of surface reaction may be most simply accommodated if firstorder processes are postulated so that:
r - acw - B
130
FIG 6 Precipitation on a Surface with FirstOrder Surface Reaction
At equilibrium (C - C - C ) the saturation solubility is therefore givenw b sby:
C. - T (33>
Thus the precipitation rate may be writ ten:
dt _a+k - c (34)
The above is strictly applicable to a single dissolved species in contactwith the pure solid. For one component of a mixed system the description Isidentical except that the solubility, C , is interpreted as that which IsSappropriate to the concentration of the species of relevance in the solid.
Theories resulting in higher order dependences on (C, - C ) have beenb sreviewed by Nancollas (1979), Nielsen (1981) and Furedi-Milhofer (1981).4.3 Incorporation
Incorporation of soluble radioactive corrosion products, such as 60Co,may occur in either the inner grown-on oxide or the outer precipitated oxide.A description of the processes requires a detailed understanding of themechanisms controlling film growth which were discussed in Section 2. Inparticular, it must be emphasised that it is necessary to decide whethertransport through the inner layer is more appropriately described by aqueousdiffusion through pores or by solid state diffusion. The theoreticalanalysis which follows assumes the former mechanism and Is essentially basedon the model developed by Lister (1975; 1976a; 1976b; 1978; 1979b) tointerpret his measurements of 60Co incorporation into the oxide films oncarbon steel, stainless steel, Inconel-600, Incoloy-800 and Monel-400surfaces under conditions appropriate to CANDU-PHWR.
Lister's fundamental assumption is that the corrosion film develops onthe metal surfaces in two layers as shown in Fig. 7. An inner layer growsdue to corrosion of the base metal and activity is incorporated in this layer
131
Coolant flow
Corrosionfilm
Coarse grainedprecipitatedouter layer
Fne grainedgrown - oninner layer
Pore containingmobile solutionphase ions
FIG 7 Two Layer Oxtde Film Activity is Incorporated n the Outer LayerDuring Precipitation ond m the Inner Layer after Diffusion Throughthe Porous Oxide Layers to the Oxide / Metal Interface
by diffusion of radioactive ions through pores in the material andincorporation into the oxide lattice as it grows at the metal/oxideinterface. The outer layer grows by precipitation from solution andradioactive ions are incorporated in the oxide lattice during the growthprocess. A fraction (assumed by Lister to be one half) of the ions producedby the metal corrosio.i reaction are incorporated into the inner layer oxideat the oxide/metal interface; the remainder pass through the oxide layer andmay be precipitated at the oxide/solution interface to form the outer layeror may be released into the bulk coolant to deposit elsewhere. It is alsoassumed that all soluble metal species behave Identically and that thechemically insignificant quantity of radioactivity simply follows thebehaviour of the major corrosion products, which can be adequately describedby the solubility of magnetite or non-stoichiometric nickel ferrite asappropriate.
It must be remembered that the model as formulated describes only theactivation process. It is implicit that there is no change in coolantchemistry which might cause dissolution of previously formed deposits orprovide a tendency for solid state diffusion into or out of the oxidecrystals.
The incorporation of a trace corrosion ion into the oxide films on acorroding surface is illustrated schematically in Fig. 8. The transfer fromthe coolant bulk may be described by:
klk+I Cb (35)
132
Innerincorf
V
Metal
layer) oration
y
Cmi
Inneroxide
Outerincorp
\
Outeroxide
layeroration
Cow
Fluidbourxoylayer
cb
FIG. 8. Incorporation of Trace Species intoInner and Outer Layers of Growing
Oxide.
The reaction rate constant for the Incorporation mechanisms, I, may bewritten in terms of the resistances of the individual crystallisation anddiffusion processes:
T__J_+ 1___1 CRYST CRYST.+DIFF.+DIFFo i i o(36)
The Individual components may be written:
Incorporation in precipitationon the outer layer surface:
1.5 CCRYST 80
o dm_odt o
(37)
T mo oDiffusion through the outer layer: DIFF • ———T\—r-r° Dipo*o(1~V
(38)
Diffusion through the inner layer: DIFF. » rr——7—r;—r~7i^i i I1.5 C
Incorporation at the metal-oxide CRYST. » -:——interface: i
dt
si
(39)
(40)
The tern DIFF was not considered by Lister but has been included in theopresent work for the sake of completeness. The masses of the inner and outer
dm dmlayer oxides (n. and m ) and their rates of growth -3— and -3— must bel o dt atdetermined from a model for base metal corrosion and duplex layer formation
dm(Section 2). —r - and m will alsodt odepend on the manner in which such a model is coupled with precipitation ofmaterial from the coolant bulk (Section 4.2).
133
4.4 AdsorptionWhile uptake of ions by oxides from aqueous solutions at room
temperature is essentially a surface phenomenon (i.e. ignoring penetrationinto fine pores or lattice cavities as in zeolites), at high temperaturesthe possibility exists that adsorption will be followed by absorption, inwhich the adsorbed species penetrates the interface and diffuses into theoxide bulk. If the oxide crystallite is growing by precipitation this maybe a particularly favourable situation for the uptake of hetero-ions,provided chemical properties are similar (e.g. crystal ionic radius), sincethe surface is continuously being renewed and hetero-ions become incorporatedinto the bulk by the crystal growth process (co-precipitation orco-crystallisation).
Adsorption of Co(II), which may in general be expected to be presentas both positively and negatively charged solution species (Section 3.1.2),has been studied by a number of workers at temperatures up to 300°C on avariety of oxide substrates under idealised conditions (Tewari, Campbell andLee, 1972; Tewari and Lee, 1975; Tewari and Mclntyre, 1975; Rommel, 1978;
BoshoIm, Glasel, Junge and Reinhard, 1978; Rommel, Sachse, Schlenkrich andHittag, 1980; 1982; Blesa, Larotonda, Maroto and Regazzoni, 1982; Tamura,Matijevic and Meites, 1983; Rommel, 1983). The observed behaviour isgenerally describable in terms of Langmuir or Freundlich adsorptionisotherms. The application of these data to reactor conditions is inhibitedby the simultaneous occurrence of other phenomena such as modification of thesubstrate by corrosion, absorption and co-precipitation orco-crystallisation.
Lister, Kushneriuk and Campbell (1983) have studied the contributionof adsorption/desorption to the deposition of 60Co on continuously changingsurfaces of Zlrcaloy oxide at 300°C.
5. PARTICULATE DEPOSITION5.1 Transport to Surfaces
It is convenient to distinguish between colloidal (~ 1 \ja) andinertial (~ 1 \m) coolant-borne particulate material since their transport tocircuit surfaces is controlled by distinctly different mechanisms (Pontingand Rodliffe, 1983). Transfer of colloidal material will be limited byBrownian diffusion through a boundary layer and may be determined fromempirical mass transfer correlations (Section 5.1.1); transfer of inertialmaterial will be limited by inertial projection through a boundary layer andmay be estimated from modification of aerosol theory (Section 5.1.3). Inaddition both colloidal and inertial material will be subjected to the
134
influence of thermophoresis on heat transfer surfaces, tending to inhibittheir deposition on fuel can surfaces whilst promoting it on steam generatortubes. Therraophoresis is only significant for larger colloidal material,about 0.1 to 1.0 Mtn diameter, and is considered in Section 5.1.2.5.1.1 Brownian Diffusion
Transfer of colloidal particulate from the coolant bulk to thevicinity of a surface, or vice versa, will be limited by Brownian diffusionthrough a boundary layer and may be described in terms of a mass transfercoefficient identical in concept to that for ionic species (Section 4.1).Thus the diffusion coefficient may be written (Einstein, 1905):
(41)
For non-spherical particles the diameter, d , must be replaced by anPappropriate effective diameter determined by the shape. If the particlesurface is charged (see Section 5.2.4) the diffusion coefficient will need tobe modified (Bruins, 1931a; 1931b; 1931c; 1932). This effect is potentiallysignificant for particles smaller than 0.1 |jm but detailed calculations arepresently unwarranted in view of the uncertainties in surface electricalproperties.
The Schmidt number for a 1 jm diameter particle In water at 300°C isabout 10**. Thus the correlation of Berger and Hau (1977) for fully developedturbulent pipe flow (Section 4.1, equation (30)) will be directly applicableto all colloidal particulate material In coolant at operating temperature.The Schmidt number is greater at lower temperatures and below about 200 °Csome extrapolation will be required outside the range of experimental data.An indication of the effect of rough surfaces may be obtained from the workof Dipprey and Sabersky (1963). For example, Rodliffe (1980) calculated afactor of about 10 enhancement of the mass transfer coefficient for 55 runparticles in water at about 300°C in a 'fully rough1 pipe at Reynolds numbersin the range 4 x 10H to 2 x 105.5.1.2 Thermophoresis
The phenomenon of thermophoresis arises when particles suspended in afluid are subject to a temperature gradient and as a result experience aforce. This force acts in the opposite direction to the temperature gradientand arises from the unequal velocity distributions of molecules in the hotterand cooler regions of the fluid, resulting in unequal transfer of momentumtangential to the particle surface. When the particle system is inequilibrium the particles exhibit a constant velocity known as thethermophoretic velocity.
135
Much work has been done to derive equations suitable for allconditions in gas systems (Talbot, Cheng, Scheter and Willis, 1980).However, the application of these equations to liquid systems is open tocriticism. The existence of thermophoresis in liquid systems has beendemonstrated experimentally by McNab and Meisen (1973) who determined thefollowing equation for the thermophoretic velocity which should be valid forcorrosion product particles in PWR coolant:
(42)
It can be shown that thermophoresis may be considered to contribute anadditional convection term to the diffusion equation of colloidal particlessuch that:
Jh - -°B ay + "rc <43>Then at equilibrium the boundary layer transfer coefficient, k , can be shownPto be modified by thermophoresis to a value k ' given by:P
UTk ' - ——;—k—r——- (44)p exp(uT/kp) - 1 v '
The thermophoretic velocity may be calculated from a knowledge of thetemperature gradient In the boundary layer.
The effect is likely to be significant only in the size range 0.1 to1.0 pm (Ponting and Rodliffe, 1983). It should be noted that the effect ofthermophoresis will vary along the length of a fuel pin since the temperaturedifference between the fuel can surface and the bulk coolant variessignificantly.5.1.3 Inertial Deposition
The inertial character of a particle in a fluid is generally definedby a parameter known as the relaxation time (Fuchs, 1964), which is a measureof the time taken by the particle to respond to changes in the velocity ofthe surrounding fluid defined in the equation of particle motion(dimensionless form) as follows:
dj/ + (/-u*) , p-f du* (45)
dt t dt
where the Bassett history term has been omitted for simplicity.
136
It may be shown that the dimensionless relaxation time is given by (Ponting,1982):
/du \ 2 / p \ du' < 2 (46)
Inertial behaviour becomes important in deposition from turbulent pipe flowswhen the dimensionless relaxation time is greater than about 0.3, since it isthen possible for some of the particles projected across the laminar layer toreach the wall before their velocities have been reduced to those of Brownianproportions.
The theories of Friedlander and Johnstone (1957), Beal (1970), Liu andIlori (1974) and Reeks and Skyrme (1976) give good agreement with data forInertial deposition of aerosols on perfectly sticky surfaces over relativelywide ranges of dimensionless relaxation time. Comparable data are notavailable for water systems where it appears to be difficult to achieveperfect sticking. The present poor knowledge of the magnitudes of thephysical quantities defining the sticking process (Section 5.3) precludes anunambiguous validation of any theory for water systems.
The flux of particles to a surface may be described in the simplestterms by a Fick's law expression incorporating a coefficient for turbulentdiffusion: dC
Jh « -<VEt> -37 (A7)
This equation may be solved given an expression for the eddy diffusivity, e ,and a suitable boundary condition. In some cases (Friedlander and Johnstone,1957; Beal, 1970) it is assumed that particles need only diffuse to withinone stopping distance from the wall and thereafter travel to the wall byvirtue of their momentum. The stopping distance is defined as the distancetravelled by a particle with a given initial velocity In a stagnant fluidunder the Influence of drag forces alone. The initial velocity is generallyconsidered to be approximately equal to the friction velocity for the flow.Several authors (Browne, 1974; El-Shobokshy and Ismail, 1980; Wood, 1981)have extended this type of treatment to rough surfaces.
Reeks and Skyrme (1976) followed a fundamentally different approach tothe formulation of the boundary condition. They assumed a two-stage processin which:
(i) Particles transfer by eddy diffusion from the fluid bulk to theinterface with the laminar sub-layer.
(11) Particles at the interface exhibit a distribution of velocitiesand thus a distribution of stopping distances so that only a fraction of theparticles may penetrate to the surface.
137
This method provides a better mechanistic description of the process andsince it allows finite velocities for particles reaching the surface it hasthe advantage that phenomena such as imperfect sticking due to bouncing maybe amenable to analysis.
Ponting (1982) has considered the extension of the Reeks and Skyrmetheory to aqueous systems. The dimensionless deposition velocity, ignoringeffects due to the Basset history term in the particle equation of motion,may be written:
k * - 0.7 a erfc"5-(3ßV)l/2
dr~ d
P(12ßV)1/2"
* °iV
The component of the dimensionless r.m.s. velocity of the particles onprojection into the laminar sub-layer normal to the wall, o. , may bedetermined from the structure of turbulence normal to and close to the pipewall (Reeks and Skyrme, 1976). It should be noted that the means ofIncorporating the effect of surface roughness, i.e. the term containing d ,u d ris valid only for hydraullcally smooth surfaces, i.e. ——— < 5, a conditionwhich should be satisfied in PWR coolant circuits under most circumstances.
The theories of Beal (1970) and Ponting (1982) predict depositionvelocities within a factor of 2 for values of T*" greater than 10. However,for 1 < T+ < 10 Ponting predicts that failure to allow for the effects ofparticle size and surface roughness cause Beal to underestimate depositionvelocities by up to an order of magnitude. Values of T4" in the range 1 to 10correspond approximately to particles of diameter 1 to 3 \m under PWRconditions.5.2 Interaction with Surfaces
As in Section 5.1 it is convenient to distinguish between colloidal(~ 1 ifn) and inertial (~ 1 im) particulate material. The generalcharacteristics of colloidal and inertial particulate material are summarisedin Table 2 (Ponting and Rodliffe, 1983).
Most of the forces which will be considered in this review have beenquantified theoretically only for simple ideal systems. Consequently thefollowing considerations will be based on an ideal system defined by aspherical homogeneous particle interacting with the surface of a semi-infinite slab of similar material with water as the intervening medium.Real system effects, such as those due to surface asperities and interfacialdeformation, have been reviewed by Krupp (1967) and Visser (1973) but adetailed consideration in this note is unwarranted in view of the majoruncertainties in the ideal systems.
138
5.2.1 GravityThe gravitational force on a spherical particle resting on top of a
horizontal surface is given by:
Fg-f V* '"p" Pf> * <49>
The particle therefore experiences an effectively attractive force. Aparticle on the underside of a horizontal surface will of course experiencean effectively repulsive force of magnitude defined by equation (49).5.2.2 van der Waals Force
The van der Uaals force of adhesion between a perfect sphere and aflat plate at small separations may be written (Hamaker, 1937):
AdF - (d » H) (50)
The Hamaker constant, A, may be determined theoretically either from amicroscopic approximation involving a simple summation over the individualcontributions from the London interactions between the constituent atoms ormolecules (Hamaker, 1937) or from a macroscopic approximation requiring aknowledge of the optical properties of the materials with respect to thewhole electromagnetic spectrum (Lifshitz, 1956). In the latter case it isshown that the interaction is dominated by the ultra-violet region of theabsorption spectrum and consequently equation (50) is only valid if theseparation, H, is much less than about 1000 A, In which case the force istermed unretarded.
Hamaker constants calculated by both methods have been compared withmeasurements of van der Waals forces for a very wide range of materialsand have exhibited reasonable agreement (Krupp, 1967; Gregory, 1970; Visser,1972; 1973; Coakley and Tabor, 1978). A significant error in the calculationof van der Waals forces in aqueous systems is likely to be introduced by thevalue estimated for the particle-surface separation (H). Krupp (1967)considers that a lower limit to the separation may be estimated from theinteratomic spacing of the materials under consideration and is typicallyabout 4 A. For sub-micron carbon-black particles on cellulose film in waterVisser (1970) determined a separation of 16 A at the zero point of charge(i.e. in the absence of electrical double layer interaction). In a laterstudy (Visser, 1976) of sub-micron polystyrene particles on cellophane inwater a separation of about 20 A was estimated at the zero point of charge.In both studies It was suggested that the separation Is defined by the waterstructure In intimate contact with the surfaces. Some uncertainty Is also
139
likely to be introduced by the particle-surface geometry of real systemsalthough the magnitude of the uncertainty should be readily quantifiableusing simple approximations. Krupp (1967) gives an account of a statisticalapproach to the geometry of the particle-substrate interface which allows fora distribution of the size of asperities. Simple correction factors havebeen proposed (van Bree, Poulis, Verhaar and Schräm, 1974; Czarnecki andDabros, 1980) to estimate the influence of surface roughness on the van derWaals attraction energy. Dahneke (1972) and Derjaguin, Huiler and Toporov(1975) have considered the effects of contact deformations.5.2.3 Magnetic Force
The force of adhesion for a uniformly magnetized sphere in contactwith the surface of a semi-infinite magnetizable medium is (Rodliffe andMeans, 1979):
1 (PT U 2 2F » T u x . 1S M/ d (51)m 5 no ( u. + 1) l p v '
The magnitude of the force is approximately independent of separationproviding H « d .PHaematite is not spontaneously magnetized so that in the absence of anexternally applied magnetic field there is no significant magneticinteraction with an oxide surface. However, both magnetite and nickelferrite are spontaneously magnetised and particles may exist as singledomains up to 0.1 or 1 urn in size (e.g. Kittel, 1956). For particles muchgreater than about 1 \m the spontaneous magnetization will be less than thatat saturation, because many domains will be present, so that the magneticforce will drop off rapidly with increasing size.5.2.4 Surface Electrical Force
There are several possible contributions to the surface electricalpotential of a deposition substrate. Firstly, the oxide film on thedeposition substrate may undergo hydration followed by dissociation or ionadsorption. The interaction under these conditions is considered in thissection. Secondly, a potential gradient may be established by some processsuch as substrate corrosion as considered in the first part of Section 5.4.
The surface charge of solid oxides and hydroxides may be qualitativelyexplained in terms of a two step process: surface hydration followed byeither the adsorption of HjO"1" and OH~ ions or the dissociation of the surface'hydroxide' (Parks, 1965). The ion adsorption or surface dissociation may bedescribed by:
H30+ + MOHg =^ M(H >)+ + H^O (52)
OH~ + MOH ä^ M0~ + H^O (53)where the subscript s refers to surface species.
140
In Che absence of specific adsorption, which will be considered later, theions H30+ and OH~ define the surface charge, and thus the surface potential,and are known as the potential determining ions (pdi) of the system. Thesign and magnitude of the nett charge on the surface will thus depend on thepH of the solution and the affinities of the surface for the potentialdetermining ions.
In the absence of specific adsorption the electrical potential of thesurface relative to the bulk solution may be determined by considering apotential determining ion and equating the chemical potential differencebetween surface and bulk solution to the electrical work required to bringthe ion from the bulk to the surface. The surface potential may then bewritten (Parks and de Bruyn, 1962) in terms of the pH of the bulk solutionand the point of zero charge (pzc), which Is the solution pH at which thesurface carries zero nett charge:
RT<j,(o) - 2.3 =r- (pzc - pH) (54)
It Is recognised that the simple Nernst equation (equation 54) isinaccurate for oxide-aqueous electrolyte interfaces in general (Furlong,Yates and Healy, 1981) although de Bruyn (1978) states that no majorobjection can be raised to Its application to metal hydroxides andoxyhydroxides (e.g. FeOOH). A study on authentic corrosion products attemperatures up to 300°C is obviously desirable to resolve this issue.
The theory for the interaction between two double layers, eachdescribed by Gouy-Chapman theory (Gouy, 1910; Chapman, 1913), was given byDerjaguin and Landau (1941) and Verwey and Overbeek (1948). The case of twodissimilar particles with icd » 1 has been analysed by Hogg, Healy andFuerstenau (1966) and Wiese and Healy (1970) for assumptions of constantpotential and constant charge interaction respectively. For a sphericalparticle Interacting with a plane surface of dissimilar surface chemistry,i.e. different surface electrical potentials 4/(°), and i)>(o)_, the electricaldouble layer force may be written for interaction at constant potential, if H< d :P
tcexp(-icH) - exp (- <H) (55)
For constant charge Interaction, if H < d :P
« «l«odp. . , *2^ <exp(-icH)+ *(o)2) i-exp(.2)CH) exp (- (56)
141
The interaction potentials from which the forces in equations (55) and(56) were derived are strictly applicable to situations in which 12 1 (0) I isless than k T but should be good approximations up to about 200 mV in waterat 300°C.
The assumption of a planar interface, i.e. icd > 10, limits thePapplication of the expressions in equations (55) and (56) to particles whichare defined as inertial (d ~ 1 urn) in this work. The electrical doubleP <layer interaction for colloidal particles (d ~ l im) is considered inSection 5.4.
The two assumptions for the interaction, i.e. constant potential andconstant charge, represent extremes in possible behaviour. When two surfacesare brought together the ion populations at the surface and in theintervening solution will tend to adjust for the effectively differentsolution chemistry. The time constant for the response of the ionpopulations must therefore be compared with the time constant for the processbringing the surfaces together. The latter would have very different valuesfor particle-particle collisions in turbulent flow and for particles settlingonto a horizontal surface. If the time constant for ion response is smallcompared with that for surface interaction, so that equilibrium ismaintained, then the constant potential assumption is relevant. If the timeconstant for ion response is large compared with that for surfaceinteraction, so that equilibrium is not achieved, then the constant chargeinteraction is relevant. The general situation has been considered by Chan,Perram, White and Healy (1975). Lyklema (1980) has considered the rates ofadjustment of ion populations in the diffuse layer and on the surfaces in thecontext of a Brownian encounter.
The point of zero charge may be equated to the iso-electric point inthe absence of specific adsorption. There are some measurements of thepoints of zero charge of oxides similar to those encountered in PWR but thereare very few data on temperature dependences with which to extrapolate to300°C. The theory of the temperature dependence of the point of zero chargeof metal oxides has been discussed by Bé"rubé* and de Bruyn (1968).
The electrophoretic mobilities of Zr02 particles in aqueous dispersionin the temperature range 25 to 80°C have been studied by Maroto, Blesa,Passaggio and Regazzoni (1981). The resulting Iso-electric point at 25°C was6.5 and the temperature dependence followed y pK . Thus a pzc of about 5.2was predicted by extrapolating the dependence on pK to 300°C. However,these values are not consistent with the streaming currents measured byTewari and Campbell (1972) on freshly machined Zlrcaloy capillary at 25 and200°C.
142
Tewari and McLean (1972) and Tewari and Canpbell (1976) reported pzcin the temperature range 25 to 90^C for a number of oxides and hydroxides ofiron, nickel and cobalt. Potentioraetric titration and electrophoretlcmobility measurements on NiO, Ni(OH)2, Co(OH)2 and Co3(\ revealed a similarbehaviour to Zr02, i.e. (pzc - y pK ) vas independent of temperature, andmay be extrapolated to values of pzc in the range 9-10 at 300UC. A thirdmethod based on the measurement of the pH for the primary equilbriumdistribution of H* and OH" ions yielded somewhat lower values of pzc. Thepzc of magnetite showed a change greater than •=• pK and may be extrapolatedto a value of A.I at 300°C If an Arrhenius temperature dependence is assumed.The temperature dependence of magnetite pzc has been confirmed by Blesa,Figliolia, Maroto and Regazzoni (198A) but their interpretation of thecorresponding enthalpy and entropy changes attending ionization suggests thatan Arrhenius extrapolation up to 300°C may not be valid. Room temperaturemeasurements of the points of zero charge or iso-electric points of iron andchromium oxides and hydroxides (Parks, 1965) and nickel ferrite (Tomlinson,1976b) exhibit a wide scatter and are very dependent on the methods ofpreparation but are generally in the range 4 to 7. Kittaka and Morimoto(1980) have studied the iso-electric point of binary metal oxides havingspinel structure and reported values in the range A to 7 for NiO-Fe203 withnickel compositions between 5 and 50%. For oxides dominated by iron andchromium (i.e. Ni - 50%) extrapolation of all these data to 300°C, based oneither -r pK or the Arrhenius dependence for magnetite, yields values whichare less than 6. Hartynova, Gromoglasov, Mikhailov and Nasyrov (1977)measured the electrophoretic mobilities of partlculate corrosion products ofindeterminate composition from the condensate of the Russian MEI heat andpower station at pH_5 " 8*8 and temperatures up to 180°C. The pzc estimatedfrom these data at 25°C is 8.5, which is higher than that observed by Tewariand McLean (1972) for magnetite although it exhibits a similar temperaturedependence. Martynova et al (1977) also report similar measurements, at pH25• A.A and temperatures up to 235°C, on corrosion products obtained byelectrolysis with austenltic steel. The pzc of this material may beestimated as A.8 throughout the observed temperature range« Extrapolation ofboth sets of data to 300°C, based on Arrhenius temperature dependences, givesvalues of about 5.
It should be noted that specific adsorption might be sufficient insome circumstances to reverse the surface charge when the pH is greater thanthe pzc. This effect has been observed for cobalt adsorption on the oxidesof interest (Tewari, Campbell and Lee, 1972; Tewari and Lee, 1975; Tewari andMclntyre, 1975), although the cobalt solution concentrations were much higher
143
than those in a typical operating PWR. Surface flocculation followingadsorption of ions released by substrate corrosion has been postulated(Rodliffe, 1980) as an explanation of auto-retardation in the deposition ofcolloidal magnetite particles on a steel surface (Thomas and Grigull, 1974).It should also be noted that adsorption of boric acid on magnetite andzirconium dioxide in aqueous suspensions has been shown to shift both thepoints of zero charge and the iso-electric points to lower pH values (Blesa,Maroto and Regazzoni, 1984). These major uncertainties in oxide surfaceelectrical properties will only be resolved by measurements on representativesamples, preferably from operating reactors, at temperatures up to 300°C. Itmay also be necessary to consider the possible in-flux modification ofsurface properties by interaction with the products of radiolysis (Sellers,1976).5.2.5 Hydrodynamic Resuspension Forces
Corn (1966) outlines the calculation of tangential drag forces actingon a particle resting on a surface. For a spherical particle the force maybe written:
i _ 2 ^ 2F. - f .C (Re ) — p u —?— (57)d d d p 2 f 4 '
The particle drag coefficient and appropriate methods for averagingthe fluid velocity have also been considered by Corn (1966). The factor fdhas been evaluated by O'Neill (1968) for a sphere in contact with a planewall in a slow linear shear flow (i.e. Re < 2) and has the value 1.7.PThe lift force on a spherical particle in a slow linear shear flow(i.e. Re < 2) may be estimated from the theory of Saffman (1965) and may bePshown to be insignificant In magnitude compared with the tangential drag.The lift force is similarly insignificant in other circumstances (Corn,1966).5.2.6 Other Factors
The adhesion of particles may be affected in the longer term byprocesses such as sintering and Ion bonding (Krupp, 1967; Visser, 1968;1973). For example, Hopkins (1973) and Hopkins and Epstein (1974) havepostulated that the products of substrate corrosion may bind depositedpartlculate material more firmly in order to explain their observations ofthe thermal fouling of stainless steel tubes by flowing ferric oxidesuspensions. These effects have not been systematically investigated forcorrosion products in high temperature aqueous systems.5.3 Inertial Particle Sticking
The interaction of inertial particles with circuit surfaces may besignificantly modified by hydrodynamic forces (Rodliffe and Means, 1979).
144
Sticking requires that a particle should (i) have sufficient energy toovercome any barrier due to electrical double layer repulsion, (ii) shouldlose sufficient energy to be retained within the potential well produced byattractive forces (Dahneke, 1971), and (iii) should be stable against shortterm resuspension, i.e. the adhesion force should exceed the hydrodynamicforce of resuspension. The sticking probability will thus depend on theavailability of suitable sites, the geometries and mechanical properties ofthe particle and the substrate, and the nature of the interaction potential.Theories to describe sticking have not been sufficiently developed to enablereliable predictions or interpretations of experimental data. The depositionrate may be written:
Jh • W (58)
Beal (1978) used data from laboratory experiments to determine a correlationbetween sticking probability, f , and the dimensionless particle relaxationtime, t"*", (a function of particle size and flow conditions). Means, Rodliffeand Harding (1980) measured deposition rate constants as a function ofparticle size in the Winfrith Steam Generating Heavy Water Reactor from whicha sticking probability of ~ 10~3 was inferred for particles bigger than 2 pm.In both cases the evaluation of sticking probability relies on the•prediction of arrival rates using theories which have not been validated forwater systems (see Section 5.1.3).5.4 Surface Reaction Rate for Colloidal Particles
The interaction of a corrosion product particle with an electricalpotential gradient of thermal or galvanic origin at a circuit surface hasbeen considered by Brusakov (1971) and Brusakov, Sedov, Khitrov andRybalchenko (1983). An electrophoretic velocity may be defined:
Ue « '"p ÏÏ7 <59>
The effect of electrophoresis on transfer through a boundary layer In thepresence of a uniform electric field may then be described by analogy withthe expression for thermophoresis in equation (44):
. "e_____kp " exp(u /k )-l (60)
e p
The mobility of colloidal corrosion product particles may be determined fromexperimental measurements (Brusakov et al., 1983) and approximate analyticexpressions (Ohshima, Healy and White, 1972b).
In certain circumstances the electrical double layer interaction mayproduce a potential barrier of the general form shown in Fig. 9 which
145
Potentialenergy,
Barrier to resuspension. û
Barrier to deposition
Separation, y
FIG 9. Interaction Potential for Combination ofAttractive van der Waals and RepulsiveElectrical Double Layer Forces.
must be overcome if deposition is to occur. Various authors (Ruckenstein andPrieve, 1973; Spielman and Friedlander, 1974; Dahneke, 1974; Bowen, Levineand Epstein, 1976; Prieve and Ruckenstein, 1976; 1977; 1978; Adamczyk andDabros, 1978) have shown that deposition in such circumstances may bedescribed by a first order surface reaction rate constant Imposed as aboundary condition on the particle diffusion equation. Such a description isvalid provided the interaction with the surface is confined to a region whichis smaller than the boundary layer, a condition which is satisfied in the PWRcircuit. The deposition rate is given by:
(61)
where the apparent surface reaction rate constant is:
a » (62)- 1) dH
The surface proximity drag factor, CEJ, is generally determined from ananalytical approximation (Dahneke, 1974). For large values of theinteraction potential (i.e. * > 10 kfiT) equation (62) may be accuratelyapproximated by (Bowen and Epstein, 1979):
1/2exp (- (63)
146
Clearly in order to evaluate this surface reaction rate constant it isnecessary to determine explicitly the form and magnitude of the potential forthe interactions identified in Table 2.
TABLE 2
Properties of Colloidal and Inertial Particles(Poncing and Rodliffe, 1983)
Particle Diameter
Boundary layer masstransfer mechanism
Dominant forcesacting on particles
Surface interactionparameter
Colloidal Particles< 1 urn
(i) Brownian diffusion(il) Thenaophoresis
(0.1-1.0 \m) at heat transfersurfaces
(i) van der Waals(li) Electrical double layer(iii) Magnetic (0.1-1.0 ta) if
substrate is magnétisable
Surface reaction rate constant
Inertial Particles> 1 in
(1) Inertial projection(ii) Gravitational settling
.(> 100 un)
(1) van der Waals(ii) Electrical double layer(iii) Gravity (> 100 un)(iv) Hydrodynamic dragSticking probability
In PWR the maximum in the interaction potential occurs at sufficientlysmall particle-surface separations such that only the unretarded van derWaals interaction need be considered. Between a sphere and a flat plate thepotential may be written (Hamaker, 1937):
(64)
The Hamaker constants for the oxides in a PWR may be estimated from data at20°C (Visser, 1972) and are likely to be in the range 2 to 8 x 10~20 J(Rodliffe and Means, 1979).
The magnetic interaction potential between a uniformly magnetisedsphere and a semi-infinite magnetizable medium may be evaluated from theanalysis of Rodliffe and Means (1979):
-96 l (65)
The electrical double layer interaction may be repulsive orattractive, depending on the similarity or otherwise of the signs of thesurface electrical potentials, and will vary significantly with pH and ionicstrength. The inverse diffuse layer thickness (<) Is about 2 x 107m~1 in
147
PWR (in range 107 to 5 x 107 m"1 for pH300 6.2 to 7.6 respectively) so thatthe assumption of a planar Interface (strictly icd > 10) used by Hogg, HealyPand Fuerstenau (1966) in their derivation of the interaction potential Is notgenerally valid for the particles defined as colloidal (i.e. d < 1 |in, icd <P P20). The expression of Verwey and Overbeek (1948), for the interaction oftwo similar particles when icd < 1, can be modified by analogy with theexpressions of Hogg et al to describe the interaction of a spherical particlewith a dissimilar plane surface. For either constant potential or constantcharge assumptions the electrical double layer potential may be approximatedby:
 , ««»i*»^ -KHOg - 0.6 it elEo dp (H+d j—— «P
More rigorous methods for calculating this potential are available (Bell,Levine and McCartney, 1970; Barouch, Matijevic, Ring and Finlan, 1978; Ring,1982; Oshlna, Healy and White, 1982a).
The methods outlined in this section have been used by Ponting andRodliffe (1983) to investigate the ways in which the deposition of colloidalcorrosion products on fuel clad surfaces might be minimised.
6. RELEASE MECHANISMSThe distinction between colloidal and inertial particles (see Table 2)
is also useful when considering resuspension. Hydrodynamic forces tend todominate the resuspension or erosion of inertial particles (Rodliffe andMeans, 1979) whereas Brownian motion dominates the resuspension of colloidalparticles.6.1 Particle Entrainment by Hydrodynamic Forces
Force balance approaches may be used to develop models for particleentralnment which are in good agreement with experimental data andcorrelations (Halow, 1973; Phillips, 1980). Consider the two simpleparticle-surface systems illustrated in Fig. 10. The perfect sphere-flatplate geometry is not considered because the application of a tangentialforce inevitably produces movement irrespective of the nett adhesion force.The criterion for lift-off may be written:
> Fa (67)
The criterion for rolling (Fig. 10 a or b) may be written:
Fd > (Ffl - F p tan &l (68)
148
a) Perfect sphere-steppedsurface Particle aboutto roll
Fd ^ (Fa- F, } tan 6,
Y////A
b) Flattened sphere-flatsurface Particle aboutto roll.
Fd 3»(F0-F,)tan 6,
c) Flattened sphere-flatSurface. Particle aboutto Slide.
FIG.10. Forces Acting on a Particle at Rest on aSurface.
The criterion for sliding (Fig. lOc) may be written:
> a2(Fa - F (69)
As considered in Section 5.2.5 the tangential drag is generally greaterthan the lift so that if the angle 6A is less than 45° it is more likelythat particles in the system under consideration will slide or roll Inpreference to lifting-off. Whether the particle slides or rolls will thendepend on the relative magnitude of tan 9± and the coefficient of friction,o2- A simple description of this type has been used to develop empirical
149
correlations between the force of adhesion and the removal of submicronparticles from various substrates in aqueous solutions (Visser, 1970).Uritier stea.iy flow conditions it is more usual to describe re-entrainment by afirst order rate constant (Epstein, 1978) so that the resuspension rate isgiven by:
f - -EQ (70)
In studies of particulate fouling the resuspension rate constant, E, isgenerally correlated with the fluid shear stress at the wall. However, Beal(1978) derived a correlation with the drag force on a deposited particleusing measurements on sand grains (Watkinson, 1968) and naturally occurringcorrosion products (Pattison and Walton, 1961) in water. The correlation isnot claimed to have any theoretical basis and is subject to considerableuncertainty due to ignorance of the particle size distribution in the latterstudy. A mechanistic derivation of the resuspension rate constant has beenpresented by Cleaver and Yates (1973) who described the interaction betweendeposited particles and the turbulent bursting process in the viscoussublayer. However, absolute predictions with this model requirenormalisation to experimental data.
During transient flow conditions it is possible that resuspension maybe enhanced. For example, Punjrath and Heldrcan (1972) postulated that themaximum probability of re-entrainment could be at the location where theboundary layer changes from laminar to turbulent. Thus a change in flowconditions would change the location of the transition and could result inthe resuspension of previously stably deposited particles.
Finally, it should be noted that particulate deposits may undergosintering or some form of chemical bonding (see Section 5.2.6) so that theybecome too tightly bound to the surface to be resuspended by hydrodynamicforces. Beal (1973) describes this conversion from 'loose' to 'adherent*forms in terms of a first order rate constant.6.2 Spalling
In the present work spalling will be defined as the release ofparticulate material as a result of the inability of an otherwise adherentfilm to withstand the strains Imposed by various operational procedures.Armitt, Holmes, Manning, Meadowcroft and Metcalfe (1978) have comprehensivelyreviewed models for the strain-tolerance of duplex oxide layers specificallyfor austenitic and ferritic steels in steam environments. They presentedexpressions for strains, including those due to cooling, heat flux transientsand oxide transformation, and outlined the options for stress relief whichcould lead to spalling.
150
6.3 Release of Colloidal ParticlesA deposited colloidal particle may be considered to sit in a
potential well as illustrated in Fig. 9. Such a particle may occasionallyovercome the potential barrier because of random Brownian motion and may becarried away by diffusion or hydrodynamic forces. The kinetics ofdetachment of particles have been considered by several authors (Dahneke,1975a; 1975b; Zimner and Dahneke, 1976; Nelllgan, Kallay and Matijevic,1982) and may be described by a first order rate constant:
(71)
A simple analytical expression may be derived for the rate constant where thepotential barrier Is relatively large (û$/k_T ~ 10) so that diffusion overDthe barrier is slow and the process nay be considered as quasi-stationary(Dahneke, 1975b):
Ï1y-y v»y-1
(72)
If the quasi-stationary approximation is not valid, i.e. A$/k T < 10, thenD
numerical or series solutions of the governing equations are necessary(Zimmer and Dahneke, 1976; Nelligan, Kallay and Matijevic, 1982).
The surface interaction potential may have van der Waals, electricaldouble layer and magnetic components as discussed in Section 5.4. Matijevicand his co-workers have published several encouraging comparisons of theoryand experiment for model systems (Kolakowskl and Matijevic, 1979; Kuo andMatijevic, 1979; 1980; Kallay and Matijevic, 1981; 1983).6.4 Dissolution
Dissolution of a deposit may occur If the concentration of solublespecies in the bulk coolant over a surface is less than the saturationsolubility at the surface. For a pure substance and first order kinetics therate of dissolution may be described In an exactly similar way to the rate ofprecipitation (see equation 34) and may be written:
dMdt <Cs * (73)
However, it should be noted that the driving forces for release of thedifferent species from a mixed system will, In general, differ from eachother, (e.g. due to differences in redox potentials of the ions) and willtherefore lead to non-congruent release. It is also possible that agedmaterial may exhibit different solubilities than freshly deposited material.
151
6.5 Recoil58Co and 5**Mn are formed by fast neutron reactions, 58Ni(n,p) 58Co and
51*Fe(n,p) 5l*Mn, with neutron threshold energies of approximately 1.5 MeV.These nuclei recoil with appreciable energy so that direct recoil into thecoolant may be a source of release. Cohen (1969) demonstrated the possiblesignificance of this mechanism using simple approaches to the calculation ofrecoil energies and ranges. However, his calculations may be significantlyin error due to the use of inappropriate range-energy correlations.
7. CONCLUSIONS7.1 A complete mechanistic description of the formation, transport anddeposition of radioactive corrosion products in PWR requires a diversity ofcomplex phenomena to be modelled. Unfortunately, it is not yet possible toidentify with certainty all those mechanisms which are crucial to an adequatedescription and those which may be omitted.7.2 There is evidence that the corrosion films on stainless steel andhigher nickel alloys under PWR conditions have a two-layer structure with theouter layer consisting of accumulations of crystals. However, the detailedmechanisms of film growth are not well known; in particular, it is not clearwhether corrosion ions are transported through the inner layer by solid statediffusion or by aqueous phase diffusion through pores. Consequently, thereare major uncertainties in the descriptions of corrosion ion release andcontamination of growing films by soluble radioactivity.7.3 It is valuable to distinguish between colloidal (< 1 jjn) and inertial(> 1 urn) particles since not only their transport to and from surfaces butalso their Interactions with surfaces are governed by distinctly differentmechanisms. It is also necessary to reconsider the traditional distinctionbetween circulating 'particulate' and 'soluble' material according toretention or otherwise on 0.45 \m microporous filter membranes; somematerial which would presently be described as 'soluble' may be moreappropriately described as colloidal particulate and will, of course, exhibitvery different behaviour from truly soluble material.7.A The sources of circulating particulate corrosion products are veryuncertain. Erosion of deposits or outer layer crystals and spalling may besignificant but their contribution is difficult to quantify. However, it ispossible to dismiss homogeneous nucleation in the coolant bulk as a sourcealthough heterogeneous nucleation cannot be dismissed without a considerationof the supply of suitable particulate impurity in the charging or make-upflows which could act as nucleation sites. This uncertainty is fundamentalsince it influences predictions of the relative rates of transfer byparticulate and soluble routes.
152
7.5 Transport of soluble species and colloidal particles between surfacesand the coolant bulk is well understood and is adequately described byempirical correlations with very little extrapolation. Transfer of inertialparticles frora the coolant bulk to surfaces is also well understood buttheories have only been validated for aerosol systems. Thennophoresis mayexert a significant influence on the transport of 0.1 to 1.0 &m particles toheat transfer surfaces but its description requires validation underappropriate conditions.7.6 Precipitation and dissolution are controlled by the local solubilityof corrosion products. Solubilities for model systems, such as Fe^D,, andNiFejO^, have been widely studied although there are no data for realcorrosion products. These processes may be adequately described by a firstorder rate process providing boundary layer mass transfer is limiting;higher order rate processes may be more appropriate if the kinetics ofsurface reactions are limiting.7.7 Corrosion products within a reactor circuit do not exhibit a uniformelemental composition. Thus, it Is unjustified to assume that cobalt, iron,nickel and chromium species exhibit Identical behaviour, for example withrespect to solubility, nucleation and precipitation. However, it Is notclear that the available data on thermodynamic equilibria, adsorption onoxides, etc. are sufficient to accommodate observed variations in elementalcompositions.7.8 The interactions of colloidal particles with surfaces duringdeposition and resuspension are adequately described by theories which havebeen validated for model systems. The major uncertainty In the applicationof these theories is Introduced by the imperfect knowledge of the surfaceelectrical properties of corrosion product particles and films at reactoroperating temperatures.7.9 The interactions of inertial particles with surfaces during depositionand resuspension are not well described. Determinations of stickingprobability from experimental deposition data are inhibited by the uncertainvalidity of theories for predicting arrival rates at surfaces. Thedescription of resuspension in terms of simple force balances is difficult toreconcile with observations of first order kinetics. In both cases theimperfect knowledge of surface electrical properties introduces a majoruncertainty.7.10 The importance of some processes remains to be demonstrated althoughrelatively satisfactory theories are available to describe them. Forexample, the recoil of nuclei into the coolant as a direct result of theneutron activation reaction is a potentially significant contributor to therelease of 5l*Mn and 58Co from fuel crud. However, the significance of
153
particle interactions in the coolant bulk, including dissolution and growthand agglomerate formation and break-up, is less clear.
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164
Annex niA REVIEW OF MODELS DESCRIBING
THE BEHAVIOUR OF CORROSION PRODUCTSIN PRIMARY HEAT TRANSFER CIRCUITS OF BWRs
K. ISHIGUREFaculty of Engineering,
University of Tokyo,Tokyo, Japan
General IntroductionThe behavior of corrosion products in the primary coolant circuits
is one of the main concerns in relation to the operation and mainte-nance of BWR power stations. It is related to two aspects of reactorsystems : one is an operational problem resulting in an increase in
1) 2 )the core pressure drop or overheating of the fuel pins induced bythe deposited corrosion products on the surfaces of the fuel cladding,and the other is related to occupational radiation exposures arisingfrom the radioactivity transport through the activation of corrosionproducts in the reactor core.
Early operational experiences in some of the first generation BWRplants showed that the release of non-ferrous metals to feed watertook place at rather high rates from the copper based alloys used inthe feed water heater tubes. The high input rates of copper or nickelled to tenacious deposits of metal oxides including these metals on theboiling surfaces of the fuel pins, inducing an increase in the corepressure drop owing to the restriction in coolant flow,and in some cases,the overheating of the fuel cladding as a result of crud cracking andsteam blanketting. The replacement of copper based alloy tubes offeed water heaters with stainless steel, which was carried out in manyBWR plants, significantly reduced the input of the non-ferrous corro-sion products into the core of early reactors, and consequently, broughtabout the reduction of this types of deposit, especially, the coppercontaining deposit, on the surfaces of the fuel cladding.
165
After the reduction of the problem with the copper based alloys,the current concern is mainly concentrated on the radioactivity build-up on the surfaces of out-of-core pipes, which is closely related tothe occupational radiation exposures. It is well established thatshut-down dose rates on the surfaces of the out-of-core pipes mainlycome from corrosion products activated by neutron irradiation onthe surfaces of the fuel cladding. ' A large number of reportshave appeared in the literature on the investigation of corrosionproduct transport in the primary circuits of BWRs in relation toradioactivity build-up. On the basis of these reports, we now havea better understanding than before of the mechanism of radioactivitytransport and build-up in the primary systems of BWR, although thephenomenon is too complicated for full understanding with many problemsstill remaining unsolved.
In Part I of this annex a brief review of the behavior of thecorrosion products under BWR conditions is described. Some of thecorrosion product phenomena are already described in the annex II,and the common phenomena are not repeated.
The description is more or less phenomenological in mostpart of this chapter, and theoretical modeling of the phenomena isgiven only when it is reasonably applicable to present systems orappropriate to explain general phenomena.
The general basic concepts and mathematical formulations employedin the computer codes which are developed to describe the behaviorsof corrosion products and the radioactivity build-up in BWRs are givenin Part II.
References1) R.Gasparini and E.Ioannilli, Proc. Am. Power Conf., 33, 776, 1971.2) A.Garlick, R.Sumerling and G.L.Shires, J. Br. Nucl. Energy Soc.,
1£, 77, 1977.3) G.C.W.Comley, Prog. Nucl. Energy, 2, 153, 1978.4} R.A.Shaw, Nucl Tech., 44^ 97, 1979.5) R.Riess, Nucl. Tech., 23_, 153, 1976.6) J.E.LeSurf, J. Br. Energy Soc., 16, 53, 1977.
166
1. Behaviour of Corrosion Products in BWRs
1. Introduction
The specification of the primary coolant in BWR is somewhat d i f f e r e n tfrom that in PWR, since the direct cycle is adopted in BWR with
boiling of the coolant in core of the reactor. A flow diagram of theprimary coolant system in a typical BWR is given in Fig.l. Thesteam produced in core goes directly to turbines, and is condensed in thecondenser. All of the condensed water is polished with a condensatedemineralizer consisting of ion-exchange resins. Two type of ion-exchange resins can be used in the condensate demineralizers : one isdeep-bed type resins and the other powdered resins. Some newplants have installed dual type, that is, both of the two types arearranged -in series. Originally, the major function of the condensatedemineralizer was the removal of the impurities arising from con-denser leakage. Recently, the removal of the corrosion productsgenerated up-streair. of the condenser has grown in importance withincreasing improvement of the condenser integrity. The corrosion pro-ducts in the primary circuits of BWR are considered to exist in twoforms, soluble species and insoluble oxide particles. The discriminationbetween these two forms is conventionally made by filtration using0.45 yu.m millipore filters. In general, however, "the soluble" fraction
Heater Drain
CBP
LPH
CondDemi
CondFill
Fig.l. Flow diagram of a typical BUR. '
167
contains both trucly soluble species jnd small particles. Corrosion products whichpass through the millipore filters are called filtrates and assumed to be thesoluble ions, while those collected by filters are called crud, which are insolubleoxide particles. This discrimination between crud and ions by millipore filtersresults in some ambiguity to the actual forms of the nominal "ions".
It is widely accepted that powdered resins are more suited to remove crudparticles compared with deep-bed resins, although the ion exchange capacity ofthe former is lower than that of -he latter.
After purification in the condensate demineralizer, feed wateris heated to 180 to 200°C in a series of feed water heaters throughheat exchange with steam bled from the turbine system, and then fedto the reactor core.
It has been recognized from operational experiences in BWRsthat the dissolved oxygen level of the feed water is a very importantfactor in relation to the control of corrosion product releasesfrom the materials consituting the feed water system. It should be notedthat all the corrosion products released into feed water are feddirectly into the core of the reactor.
After heat exchange in the feed water heaters, the steam conden-sate is usually returned to the condenser, and then polished in thecondensate demineralizer. Some BWR plants, however, have the forwardpumped drain systems where the bled steam condensate is added to thefeed water after the condensate demineralizer.
The cooled water is recirculated through the core, and the bypassclean-up blow is taken from the recirculation system.The reactor purification system was intended to control soluble impu-rities and to remove soluble fission products which are released tothe reactor water through small defects in the fuel cladding. Progres-sive improvement in the fuel cladding integrity has uprated theimportance of removing activated corrosion products from BWR coolant.It is now accepted that activated corrosion products are respon-sible for most of the surface dose rates from the piping which in turndominates the radiation exposure to personnel during reactor shut down.
The capacity of the reactor water clean-up system has been 7 percent of the feed water in the early plants, but this has decreased toone or 2 per cent in more recent BWRs.
168
The radiation build-up on the corroding surfaces of the out-of-core piping has become a major concern in relation to the control ofBWR water chemistry. In contrast to PWR,addition of conditioningchemicals to the primary coolant systems is not possible in BWRsbecause of the boiling coolant in the reactor core. Therefore, theonly mean for controlling coolant water chemistry are to keepimpurity levels in the feed water as 2ow as possible by purificationand to minimize the input of corrosion products from the feed watersystem to the reactor core.
The mechanism of the radiation build-up on th primary piping ofBWRs is roughly understood as follows. The corrosion products arefed into the reactor core from the feed water line, a major componentof these products being iron. This is considered to play an importantrole in the radiation build-up process in BWRs, by providing sites forthe deposition and activation of cobalt which is the element of themost concern,making the greatest contribution to the dose rate of thepiping fron Co. There has been some controversy on the sources ofcobalt. It is now generally accepted that the cobalt contributionby release from the core materials is not negligible, which is not thecase with iron.
The corrosion products fed into the core are mostly deposited onthe surfaces of the fuel cladding, and this process is markedly enhancedby boiling OP the fuel surfaces in BWRs. The deposited corrosionproducts are activated by neutron irradiation, and re-released to thereactor water after some period of residence on the fuel surface. Theradioactive nuclides, including Co, in the reactor water are trans-ferred by the coolant to the out-of-core circuits, and taken up on thecorroding surfaces of the out-of-core pipes.
The above description is a very rough summary of the mechanismof radioactivity build-up in BWRs on which general understanding hasbeen obtained on the basis of the model experiments and from the opera-tional experiences in power reactors. The detailed mechanism of theprocess is very complicated with many ambiguous points remaining. In
169
the following sections a detailed description is given to the severalimportant phenomena comprising the overall process of corrosion producttransport in BWRs.
2. Release of Corrosion ProductsThe main component of corrosion products in the reactor water
of BWR is iron which mostly comes from the feed water systems. Theiron levels in the feed water immediately after the high pressureheater arise primarily from the corrosion of stainless steel andcarbon steel in the feed water systems. It is recognized thatsignificant contribution to the iron input to the reactor core ismade by the passage of corrosion products, in particular crudthrough the condensate demineralizers especially if their efficientoperation can not be maintained.
Some experimental results and in-plant tests show that the corro-sion rates and the release levels of corrosion products from carbonsteel and stainless steel are strongly affected by the concentrationof dissolved oxygen. ' As shown in Fig.2, the corrosion rates ofcarbon steel decrease sharply in high purity water with increasingoxygen concentration in the range of 10 to 100 ppb.4' The flow rate
T> 3O
1
Ü? 2O
co
o- 1Oou
Velocity3.5IU/S
/*^ ^*\
\- 1-Ou/s^ \ ^_x
x^ "4 * \S 042mA \ / \:"^' \ s' \\ ^A*^ ^ ^
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-E
S10 <
>v
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1
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1C? 1O' 1C/ iCr4 1O* 1O5
Fig. 2.
Disso lved oxygen (.ppbj
Corrosion rates of carbon steel (SB 46) as3)function of dissolved oxygen.
33 + 2*C. specific conductivity <0.5u /cm,immersion period 500 hours.
170
of water also has a significant influence on the corrosion controlby oxygen concentration, a higher concentration of oxygen being neces-sary to suppress the corrosion at the lower flow rate.
This effect of oxygen has been found to be very marked in thefeed water systems of BWRs ; when 30 to 50 ppb of dissolved oxygenwas injected into the feed water immediately after the condensatedemineralizer, the iron concentration after the high pressure heater
was reduced to the level of a few ppb in comparison with the levelsof approximately SOppb before oxygen injection.
On the basis of these experiments and tests oxygen of 30 to 50 ppblevel is now injected to the feed water in all Japanese BWR plants.In BWRs in other countries, oxygen injection has not been practicedso far, but it is recommended that the oxygen level in the feed waterbe controlled to be 20 to 50 ppb.
The above effect of dissolved oxygen is explained by the assump-tion that some optimum concentration of oxygen is necessary to main-tain the passive oxide films on the metal surface, but a modelhas yet to be developed to explain the mechanism of the oxygen effectin more detail. Several models have been proposed to explain thecorrosion process and the oxide structures of corroding surface ofcarbon steel at high temperature, most being based principally on theoriginal Potter-Mann model. In these models the double oxide layersare assumed, an inner and outer layers being recognized. One suchmodel is schematically shown in Fig.3. The inner layer is composedof magnetite, which is produced at the metal/oxide interface by thedirect reaction of metal with the oxygen bearing species diffusingthrough the inner oxide layer. Ferrous(Fe) ions produced by thecorrosion reaction at the metal/oxide interface diffuse outwards throughthe inner oxide layer, and go into solution at the inner oxide/waterinterface. At the outer oxide/water interface ferrous ions in solutiondeposit as a result of the reactions with water molecules and form theouter layer.
3 (Fe) solution + 4H-O —————> Fe3°4 + 6H+ * 2H (1)
171
Hl
OfTCTOX I DCLATER
METAL
Fig. 3. Model of steel corrosion in high temperaturewater. (pH <_ 11) 7)
In this model it is assumed that a half of the ferrous ions pro-duced by the corrosion reactions diffuse outwards from the metal to theinner oxide/water interface and there enter solution. The ferrousions which escape deposition at the outer oxide/water interface are
(8 9 10)released as soluble corrosion products. One possible explanation ' ' ofthe effect of oxygen present in the solution according to this modelis that ferrous ions react with oxygen in solution and also at theinner ox-de/water interface, precipitating as ferric oxide and, thereby,reducing the release level of ferrous ions.
The details of this process are unknown, but another possibilityfor the reduction of ferrous iron level is the reduction with alreadyexisting ferric oxides.
In the BWR core the intense radiation produces an oxidizingenvironment owing to the radiolysis of water. This situation mayhave some influence on the release or behavior of the corrosion pro-ducts in the coolant circuit. The results of laboratory experimentshave shown that radiation enhances the release of insoluble iron butnot soluble iron from carbon steel and stainless steel in pure waterat high temperature. The chemical form of the released insolubleiron was mainly hematite under the experimental condition. This
172
result was explained qualitatively as follows : the radiolysis ofwater gives several chemical species as shown by equation (2).
HO ——~*—>H OH e- H,O,. H0, H,O+ (2)i aq i f. i 3
Among these chemical species, OH and H_O- are strong oxidizing agents,while e~ and H are reducing. As already mentioned in the previousaqsection, ferrous iron is released from carbon steel or stainless steelby the corrosion process. The ferrous ions released react with oxi-dizing species to yield ferric ions as shown by equations (3)~(5).Some of ferric ions thus produced may have a chance to react with e
(3)(4)(5)
and be reduced to ferrous ions, but the total sum of G values foroxidizing species is larger than G values for e~ . Ferric ionsreadily react with water molecules to yield ferric hydroxide as shownbelow. Ferric hydroxide precipitates and is dehydrated to yieldhematite.
T»J-L "*" i *tn d
£. t e i un ; - — "r re«, U- T Jn-LJ
(6)
The above result and its explanation seem to be very consistentwith the fact that the corrosion products in reactor water of BWRsare mostly crud or insoluble particles mainly composed of hematitewith a very small proportion of soluble iron.
Another species which is also important from the view point ofthe radioactivity transport is cobalt, providing the main source ofCo responsible generally for the highest contribution to circuit
dose rates.
173
C gThere has been some controversy on the predominant sources of Co12)released by the corrosion and wear processes in BWR primary systems.
In one assessment of cobalt sources it was estimated that approximately90% of the cobalt input originates from cobalt based alloys as shown inFig.4, though they are less than 0.1% of the plant's surface area.In another estimation, however, it is reported that approximately 50%of cobalt comes from stainless steel used in the feed water heaters.It is known that the cobalt release rate from stainless steel and cobaltbased alloys depends on the duration of the release experiment andusually decreases with increasing duration of the release test . Thus,available release data is inadequate for a general quantitative esti-mation of cobalt sources.
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':/t '••/Ä 'V
Total Control Structural ValveRod Materials Wear
Mechanism CorrosionWear
Fig. 4. Main cobalt sources in BV7R and P\JR plants. 13)
174
Cobalt is released mainly as soluble ions from stainless steel bythe corrosion process, and the release rate is also dependent on theconcentration of the dissolved oxygen. It is reported that high levelsof dissolved oxygen such as Bppm or 40ppm enhance the release ofcobalt but not of iron. Hence, oxygen injection in the feed waterof DWR is not so effective in suppressing the release of cobalt comparedwith iron.
3. Behavior of Insoluble Species.(1) Properties and size of crud particles.
Insoluble corrosion products or crud particles play an importantrole in the radioactivity build-up in BWRs primary systems.Crud particles dispersed in the reactor water of BWRs are known tobe predominantly hematite, unlike the PWR situation where nickelferrite and magnetite are the main components. This difference arisesboth from the difference in the component materials and the corrosionenvironments in PWRs and BWRs. Reactor water in BWRs, usually, con-tains approximately 200ppb of oxygen arising from the radiolysis ofwater in the reactor core. Therefore, the environment in the coreis more oxidizing than in PWRs where hydrogen is added in the primarysyterns.
In BWR feed water hematite is considered to be the main componentof crud with significant contribution of magnetite. It is believedby some people that there is also some contribution of "amorphous"corrosion products to crud in the feed water system, especially, inthe lower temperature part, though the actual properties of the "amor-phous" crud have not been well defined. This amorphous iron basedcrud may be related to the leakage of corrosion products through thecondensate demineralizers.
When the behavior of crud particles is considered in the modelingof corrosion products, the size distribution of the particles isone of the most important properties of crud. However, data from actu-al plants on the diameter of crud particles is sparse. One of thepossible reasons for the scarcity of data is that experimental techniquesfor the particle measurements are not well developed. For instance,
175
scanning electron microscopy (SEM) is often used for the measurementof the particle sizes,but there is always the possibility of the parti-cle coagulation during the preparation of the samples. One particlecounter, the Coulter counter, can be used to measure the diameter ofcrud particles in the dispersed state, but usually has a limitingvalue for the measurable diameter and can not be applied to the measure-ment of the small particles with a diameter less then O.Sym.
In one of the Japanese BWR plants, particle sizes were measuredwith crud in reactor water and feed water by SEM and values of 5~7pmwere found for average diameters of the particles in both systems. 18)
Relatively large average diameters(2~ 20pm) were also observed 19) , 20)
in the reactor water cf SGHWR at Winfrith, which is a boiling water typereactor with pressure tubes. On the other hand, rather small diameterswere observed with the crud particles in the feed water of another BWR
21)by means of SEM , as shown in Fig.5.
10 ppb 02
JIID
Fig.5. Suspended particle diameter distribution in feed waterline of a BWR, before and during an oxygen dose experiment21)
176
The examination of fuel deposits in BWRs shows that the fuel depos-its consist of two types of particles, small(0.1~ 0.3pm) irregularshaped particles and larger particles(0.5~3um) having sharper egges andfaces. 22)
Nuclepore or millipore filters with nominal pore sizes are some-times used to descriminate particles of different diameter. It isto be noted that the mechanism of colloid particle filtration is notsimply the bulk filtration with the filters. Double layer interactionbetween the colloid particle and the filter surface also plays animportant role, since both of them have their own surface charges depend-ing on pH of the solution. Thus, the filtration efficiency is sharplydependent on pH of the solution, and particles of 0.15pm diameter arereadily collected on a millipore filter with nominal Bpra pores atneutral pH, as shown in Fig.6.
100§90
i 80\
~ 70oÏ (c 50gI 40CD
I 3020
10
0
24)
•.o.
1 1O 11 12 132 3 4 5 6 7 8 !(PH)
Fig.6. Penetration fractions of hematite particleswith millipore filters in model experiment.''
flow rate 0.24cm/sec.filter pore A 8um. SSpm
hematite particle diameter 0.15pn
177
(2) Removal of crud particles in purification units.The majority of the corrosion products in the feed water of BWRs
arises from leakage of crud through the condensate treatment systems,and, thus, performance of the condensate demineralizers is a veryimportant factor in controlling the corrosion product levels in thefinal feed water. Condensate demineralizers,deep bed type ion-exchangeresin columns and/or powdered resin filters are normally in use.Whereas powdered resin filters are very effective in removing crudparticles from the turbine condensate, the performance of deep bedcolumns employing bead resins for crud particles is rather variablefrom plant to plant. Deep bed ion-exchange resins were originallyintended to remove ionic impurities arising from leakage of condenserbut not crud particles. Some of the crud particles, however, areactually retained at the surface of the beep bed ion-exchange resins.The filtration mechanism of colloidal particles with deep beds israther complicated and varies in time with increasing amounts of thedeposited particles on the collectors. From the practical point ofview, it seems that the early stage of the filtration is rather impor-tant, since the deep bed resins are routinely regenerated in shortcycles in normal plant operation.
A conceptual model was proposed to describe the filtration of25)suspended particles with packed deep beds. In this model a single
spherical collector (resin bead) is considered and it is assumed thatthis collector is unaffected by its neighbors and fixed in space inthe flowing suspension as shown in Fig.7. Three transport processesare considered : interception, sedimentation and diffusion. Inter-ception occurs when a suspended particle following a stream-line ofthe flow comes in contact with the collectors as case A in Fig.7.A particle having a density greater than that of water will follow adifferent trajectory affected by the gravitational force field (case B).This is sedimentation. Brownian motion of a suspended particle drivesit to the surface of the collector, and this transport process isdj ffusion.
178
• PARTICLETRAJECTORYSTREAMLINE
COLLECTOR
A INTERCEPTION
B SEDIMENTATION
C DIFFUSION
,1
Fig.7. Basic transport mechanism inwater filtration.
In this model, performance of a packed bed is represented by Eq.(8)
In-CinCeff
(8)
where C. and C ,, are the influent and effluent concentrations for ain errpacked bed, respectively, c the bed porosity, d the diameter of thecollector, L the bed length. n is the contact efficiency of a singlecollector and defined as the rate at which particles strike the collec-tor divided by the rate at which particles flow toward the collector,a is the collision efficiency factor or sticking factor, defined asthe number of the contacts which succeed in producing adhesion dividedby the number of collisions. The contact efficiency n is related tothe transport processes of the suspended particles and evaluated theo-retically as follows :
kT 2,,0.9(-
-f<-ydpdv
18pv
(9)
(10)
(11)
179
(12)
where n , n and n are theoretical single-collector efficiencies,corresponding to diffusion, interception and sedimentation, respectively.H the water viscosity, d the diameter of the suspended particles,v the water velocity, g the gravitational acceleration, p and p aredensities of water and the suspended particles.
The sticking factor a is supposed to reflect the chemistry of thesystem, but is treated as adjusting parameter, since no reliable theoreticalevaluation method is available for this parameter. This model seemsto explain qualitatively the dependence of the removal efficiency onthe diameters of the suspended particles, as shown in Fig.8, but stillsome discrepancy remains between the theory and the experimental results.It was reported chat the double layer interaction plays an important role
9 f*t 97^in the filtration process ' , and closely relates to the sticking
factor. Optimum adhesion corresponded to systems in which the sus-pended particles and the collectors were oppositely charged. It wasalso found that the deposition process of the adhered particles becomesimportant under special conditions, depending strongly on the pH andthe ionic strength of the system.
100
u.O> Lü o-— w.
£ 5u.
OZLUce
80
2O
- d » 0.397mmT » 23°C1 r 0.36L: 5.5 in.
»o1 2 «P™ / »«.II.O - l ( tu«um»d )
|0'2 I0 - l l 10 1C*
SIZE OF SUSPENDED PARTICLES (microns)
Fig.8. Comparison of theoretical model and
experimental data. 25)
180
In another model28^ the desorption term as well as the depositionof suspended particles is incorporated in the rate equation to explainthe rapid reduction of deposition rate with increasing coverage ofthe collector surface.
In a recent model ' the filtration process is classified into4 stages. The first is the early stage during which the resin surfacesare covered with very small amounts of the suspended particles andstill available for the adsorption. During this stage direct double layerkinetic interaction between the surface of the collector and the sus-pended particle affects strongly the adhesion process of the particle.The second is a transient stage where the bead resin surfaces have notbeen covered completely with particles but the interaction between theresin surface and the approaching particle is attenuated or shielded bythe particles already adsorbed. In the third stage when the surfaceof the resin has been covered completely with particles, the interactionbetween the particles already adsorbed and thoseapproaching becomes predominant instead of that between the collectorsurface and the approaching particle. During the final stage decreasein the porosity of the packed bed is no longer negligible, and pressuredrop across the deep bed sharply increase with increasing amounts ofthe adsorbed particles. Finally the bed will become clogged.
The basic conservation equation is expressed as Eq.(13)
o = pcA(l-e)S
where C refers to the concentration of suspended particles in the waterphase, o the concentration of the particles adsorbed on the surfacesof the collectors, 2 the axial distance, DT the modified coefficientLIof axial diffusion and dispersion combined, p the density of the col-lector, A the specific surface area available for the particle adsorp-tion, S the adsorption density of the particles per unit surface area.
Usually the last term is negligible in Eq. (13) compared with theconvection term. Hence Eq. (13) reduces to Eq.(15).
181
*• 3t ' 3C 3z (15)
In the first stage of the filtration, c, Langniuir type rateexpression is introduced in consideration of the saturation effect ofthe adsorption as follows.
-ft" = ka(Ssat ' S)C " kdS (16)
Here S is the limited value of S, and k and k, are the adsorptionand desorption rate coefficients, respectively. Analytical solutionof (15) combined with (13) and (16) is possible and the final form isgiven as follows.
C(g.B) _ X(g,ß)-6 (17)
sat
X(a.ß) = .ß) + T(B.g) + I» (2/6 6) (19)T(ct.ß) + T(ß.a) + I0(2/5ol)
T(a.ß) - e° /° e-TIo(2/7ß^)dT (20)
kd (21)Cinka + kd
g k >_£_ . (22)sat a v
T « t - -2- (24)v
where I (x) is the modified Bessel function of zero-th order.This solution is of rather complicated form to use, but is reduced
to a simplified formula as shown below when the release rate constant
182
(k,) is assumed to be approximately zero as is the case with theearly stage filtration of BWR condensate demineralizer .
ln(Cin/Ceff - 1) = ln(eaL - 1) - (-7 ) )0 (25)
n = -i-t - 1 (27)
p .Eq. (25) means that a plot of In ( in/C ,, - 1) versus n should be linearin this stage of filtration.
On the other hsnd, in the third stage of the filtration the rateof particle adsorption is controlled by the interaction between theparticles already adsorbed and approaching. Therefore, there will be nofurther adsorption limit at this stage until the bed is clogged. Thus,the rate equation should be changed to (28) instead of (16).
= k;C - k^S (28)
If the desorptlon of the particles is assumed to be neglected as inthe first stage, a simple solution is derived from (17) as follows.
S/C = k1 ft-——) e"a' <30>/Lin a ( v ; e
(31)
Eq.(29) shows that the effluent particle concentration C -, is independ-ent of time t as opposed to the situation in the first stage.
In Fig.9 is shown the plots of In( *n/C ff - 1) versus n for themodel experiment using hematite. It is seen that the plots changefrom linear relation in the initial stage to the constant, correspondingto the cha ige from the first to the third stage.
183
HIu
uC
-1
rFig.9. Plots of ln( in/C ff - 1) versus n inmodel experiment using hematite particle
29)(d 0.15 m).
O Co•C0Ac0Ac.
6.73 ppm, L = A cm, v = 0.10 cm/sec.28.9 ppra, L = 9 cm, v = 0.16 cm/sec.31.1 ppm. L =19 cm, v = 0.13 cm/sec,52.0 ppm, L = 9 cm, v = 0.14 cm/sec
(3) Deposition of crud particles on boiling surfacesThe deposition of crud particles on fuel surfaces is one of the
most important step in radioactivity build-up in the primary systems.In BWR, the boiling.of water in core enhances the deposition process,and high heat flux and radiation dose may also influence the morphologyof the deposits on the fuel surfaces. It is known that the depositson BWR fuels again show a double layer structure, inner and outer layers.The latter is assumed to be transformed into the former probablyowing to the high heat flux and radiation dose.
Model experiments show that there are several factors which havelarge influence on the deposition rates of crud particles on boil-ing surfaces. The particle deposition on boiling surfaces is, however,a complicated phenomenon, and the transport process of the particlesaround boiling bubbles is very difficult to deal with theoretically.
184
Most of the relevant reports therefore concentrate on finding the factors
affec t ing the deposition process and treat it rather phenomenologically.
The phenomenological description is given in the following section.
As already mentioned, boiling of water promotes the deposition of
particles on boiling surfaces in comparison with the case without boil-
ing under the same heat f lux. ~ A model has been proposed to
explain the effect of boiling on the heated surfaces. In this model
it is assumed that particle deposition takes place around boiling
points. At the boiling sites bubbles grow and evaporation of a micro-
layer of water formed between the bottom of growing bubble and the
heated surface plays an important role as shown in Fig.10. Only the
particles present in the dried-out microlayer deposit on the heated34)
surface. On the other hand, it is claimed that crud particles
having lyophobic property are trapped on the interfaces betweenwater and b u b b l e , and are collected p re fe ren t i a l ly in the boundary area between
the bubble and the heated surface.
Almost all the reports agree that the deposition rate of the part icles on
boi l ing surfaces is proportional to the concentration of the pa r t i c l e s .
On the other hand, there has been some controversy on the dependence of the deposition
BubbleDry regionMicrolayer
Heated surface
(a)
Microlayer at BMicrolayer at C
fib
fig.10. Schematic model of iron oxidedeposition : (a) bubble growth(b) microlayer evaporation anddryout.33)
185
rate on the heat flux through the heat transfer surfaces. It is reported in one35), 36), 38) . . . . . . .group of papers that the deposition rate is proportional to the
c u L u u 33), 34), 37)square of the heat flux, whereas another group of papers claimsthat the deposition rate is directly proportional to the heat flux.
The pH of the dispersing solution affects the surface charges on dispersedparticles, the heated surface and, consequently, the double layer interactionbetween them. It was pointed out that the electrostatic double layerinteraction is very important in the deposition process of the particles, orespecially when they are small, even under boiling condition. The effect of pHon the deposition process was investigated under boiling condition to examine theinfluence of double layer interaction. It was found ' that the particledeposition rate decreases near the point of zero charge (PZC) of the particles.This cannot be explained by the simple DLTO theory. It is necessary to includethe effects of hydrodynamics and mass transfer in boiling.
It was'inferred on the basis of plant data that smaller particles depositmore effectively on boiling surfaces than larger particles. A modelexperiment, however, showed that the above prediction is partly valid, butpartly wrong. As shown in Fig. 11, there is an optimum particle diameter fordeposition on boiling surfaces. This result is also explained by the assumptionof the optimum interaction barrier described above.
20
o~
91
I '0O
Oa
1.0Particle diameter (pm)
2.0
Fig.11. Relation between the deposition rate andparticle diameter, with hematite concentra-tion = 1 ppm iron and heat flux = 10 I'/cn^.
186
There have been experimental reports on the effect of flow rateof the dispersion solution on the deposition rate of particles. Amodel experiment showed that the deposition rate decreases withincreasing flow rate, which was well explained by the microlayer deposi-
33 )tion model cited previously.At the initial stage of the particle deposition, the release pro-
cess of deposited particles is usually neglected. This does not hold,however,as the deposition proceeds. As shown in Fig.12, the amountsof the deposits level off at long time, and approach an equilibriumvalue.34)
The deposition and release processes are treated by a rather simpleequation as given below.
IF - kic - (32)
where W refers to the deposit weight of particles per unit area, k, therate constant of deposition, c the concentration of the particles andk_ the rate constant of release. As above mentioned, the depositionrate constant (k.) is known to depend on many factors, but little isknown about the release rate constant (k_).
•ii£ ,0> tu'
vu.
&<uO
10 '
NUCLEATE BOILING
SUBCOOLED BOILING -
NO HEATINGJ——————————i—
10s 10«
time (min)
Fig.12. Effect of boiling regimes ondeposition. 34)
187
Plant data show that the deposits on the fuel surfaces takedouble i n y e r oxide s r mr t urr f inner and outer layers, but no model experi-ment has provided the evidence of the double layer formation duringdeposition, probably due to experimental periods being too short forthe transformation of deposits to take place. Therefore, nothing isknown about the rate constants of the outer to inner transformations,which are accepted as taking place in actual EWR plants.
4. Behavior of Soluble Species(1) Interaction of Co ion with crud particles.
The bahavior of cobalt ions is the most important, since cobalt-60makes the greatest contribution to the surface dose rate of recirculationpiping, and the dominant form of Co corrosion products is in solubleionic form. It is well known that metal cations such as cobalt arereadily adsorbed on the surfaces of oxide particles. Therefore,cobalt and cobalt-60 ions in the primary water of BWR are adsorbed onthe surfaces of crud particles, fuel deposits and oxide layers on thepiping. The adsorption of cobalt ions on the oxide surfaces is strong-
(41)ly dependent on the solution pH, as shown in Fig.13 . There hasbeen some controversy on this pH dependence in relation to the mechanismof adsorption. It is generally accepted that metal hydroxidegroups such as ^>FeOH provide the sites for adsorption, and themetal hydroxide groups have amphoteric properties as shown by thefollowing reactions.
>FeOH + H"1" v s FeOH+ (33)>FeOH "* FeO~ + H+ (34)
It has been shown in the previous part (Fig.3-1 in PWR Review)that ions are subject to hydrolysis reactions, depending on pH of the
42) +solution. It was suggested that CoOH is predominant species adsorbedon metal oxide surfaces, and the hydrolysis product, Co(OH) , formedin aqueous phase was claimed to be preferentially adsorbed on the sur-faces of oxides because of its much lower desolvation energy.
188
100
80
60
20
Arls'%)
(CO2*]; l-H5
Cone area 136 m d'r^3
1 00 °C
J---.
6 7 ß 9 pHFig.13. pH dependence of cobalt(II) adsorption
at 30°C.M)
It was proposed, however, that the formation of bidentate surface
complexes are necessary to explain the pH dependence of the adsorp-
tion. , 45)
>FeOH)2 + Co2+>FeO,
>FeO':Co -l- 2H (35)
On the other hand, it was pointed out in the case of other m e c a l ca t ions
such as Cd and Cu that if the e l e c t r i c a l double layers on the sur faces of the
p a r t i c l e s are considered and the complex format ion between hydroxide and meta l
cat ion at s u r f a c e s is due to e lec t ros ta t ic bonding, then the expe r imen ta l r e s u l t s are be t t e r
explained by assuming the presence of two types of the complexes shown
below (36) and (37) than by the introduction of the bidentate complexes
(35) .
189
>FeOH + Co
>FeOH + Co
2 +
2 +
Fc-O . . . . Co
Fe—0 CoOll
(36)
(37 )
In this model the ef fec t of surface potentials on the equil ibrium of
the complex formation is taken into account as follows.
(FeO--Co2 +] - 'FeOH][C°2+1 exp[(e V 2e40/kT] K[ H + ] O B(38)
where [Co ] and [H ] refer to the concentrations of cobalt ions andprotons in bulk solution, respectively. Surface potentials ù and i..
O D
are defined in Fig.14. The cobalt ions are assumed to be placed ino plane, while protons and hydroxide ions are bonded in o plane.
On the contrary, an electron spin resonance (ESR) experiment 'showed that some divalent metal cations are adsorbed on the surfaces ofoxides by the formation of covalent bonds and not only by electrostatic bonds.
Recently, a new model has been proposed^*8' in which the covalencyof the bond between metal cation and oxide surface is taken into account.
SOLID/
Fig.14. Schematic representation of an oxideinterface showing possible locationfor molecules comprising the placesof charge.
190
Here the metal cations as well as protons and hydroxide ions are arrangedin OQ plane in Fig. 14. Therefore, the equilibrium of EC. (36) isrepresented by (39) instead of (38) . The cobalt ion adsorbed on the
[FeOHHCo2*![FeOCo J = ———— -T} ———— exp(-e*o/kT] K (39)
oxide surface is considered to hydrolyze the water molecule to releasea proton, and, therefore, Eg. (37) is rewritten more explicitly asfollows.
>FeOCo+ + H0 ———— ?• FeOCoOH + H+ (40)
This model was reported to account bat isf actor i ly the adsorption isotherms and pHdependence of various metal ions. Any of the proposed models for the adsorption ofhydrolysable cations on oxide surfaces accounts for the adsorption isotherms and pHdependence. The model proposed by Regazzoni, where adsorption is not restricted to àinonolayer, accounts in addition for the observed e lee t rokinet ic behaviour. This idea issupported by the recent adsorpt ion-sur face precipitation model.
Most of the adsorption experiments have dealt with the adsorpt ion-desorpt ionequilibria, and very few data are available on the kinetics of the adsorptionand dcsorption processes, which are very important from the view point of modelling
49)corrosion product transport. Pressure jump methods were applied to measure therate constants of adsorption and desorption, and it was found that the relaxation processesdetected by electric conductivity contain both fast and slow steps. The interpretationof the result , however, not straightforward due to the complexity of the mechanism.
High temperature data on the cobalt adsorption and desorption are also verylimited. It is recognized ' ' that larger amount of cobalt ions are adsorbedon the oxide surfaces at higher temperature as shown in Fig. 15. It was suggestedthat at high temperature the cobalt ions adsorbed on the surfaces of magnetite andhematite are transformed ' ' to a cobalt ferrite phase after diffusinginto the inside of the crystal lattices. This is also supported by evidence collected
41)in p l an t .
191
IÖ4
Ev.oEzo<L1C
S 10"
10 -T
FULL COVERAGE BYDEHYDRATED IONS
FULL COVERAGE BYHYDRATED IONS
30-e
10 10 10 10 10"EQUILIBRIUM SOLUTION CONCENTRATION mol/kg
10"
Fig.15. Log Co(II) adsorbed versus log equilibrium Co(II)solution concentration on NiFcjO, at differenttemperatures, pH 25 6.5 50)
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48) K. Hachiya, M. Sasaki, Y. Saruta, N. Mikami and T. Yasunaga, J. Phys.Chem., 88, 23, 1984.
49) K. Hachiya, M. Sasaki, T. Ikeda, N. Mikami and T Yasunaga, J. Phys.Chem., 88, 27, 1984.
50) P.H. Tewari and W. Lee, J. Colloid Interface Sei., 52, 77, 1975.
51) H. Tamura, E. Matijevic and J. Weites, ibid., 92, 303, 1983.
52) B. Venkataramani, K.S. Venkateswarlu, J. Shankar, L.H. Baetsle, Proc.Indian Acad. Sei., 8_7 A (Chem. Sei. 6) 415 (1978).
53) R. Kuwae, Y. Yamashita, J. Takabayashi, M. Hishida and H. Nagao, J. Nucl.Sei. Tech., 21, 484, 1984.
54) D. Briggs and Y.M. Bosworth, J. Colloid Interf. Sei., 59, 194 (1977.
55) K. J. Parley, D.A. Dzombak, F.M.M. Morel, J. Coll. Inf. Sei., 106, 226 (1985).
194
II. Computer Code Describing the Behavior of Corrosion Productsin BWRs.
1. IntroductionThere are several computer models or codes reported in the
literature to describe the behaviour of products in BWRs and therebycalculate the amount of radioactivity corrosion deposited on the surfaces ofrecirculation piping and corresponding dose rates at the reactorshut-down. Each plant vendor of BWRs has its own code to simulateand forecast present and future situations of its own plants.In these computer codes, scenarios are described for the overallprocess of radioactivity build-up, starting from corrosion productrelease and leading to the deposition of cobalt. Each step com-prising the overall process of corrosion product transport isformuJated mathematically according to the scenario, then representedby several differential .equations, which are solved numerically withplant data provided as input. There are many parameters, such asrate coefficients, included in the models, and these are usually deter-mined by model experiments or fitting to plant data. The mechanismof radioactivity build-up is, however, very complicated as mentionedin the previous section, and is not understood completely. Thus,the computer models are rather semi-empirical and phenorcenological,and sometimes oversimplification is introduced to the formulationwhere the mechanism of the process is not understood well. Therefore,all the models reported are incomplete at present and should be refinedas we understand more of the detailed mechanism included in radioac-tivity transport process.
2. GE modelThe basic transport phenomena assumed in this model are shown
in a diagram in Fig.l. Some important points are hypothesized hereas follows.
195
SPENT FUEL
FEEDWATER
4• MM W
!--1I.1«-•-
1 *-
r -PRIMARYSYSTEMMATERIAL
r-»-111L.
FUEL SURFACE DEPOSIT
-I
1
^
INNER LAYER
M^OUTER LAYER ' '
—— rrpH —————— | ——————— ———l
SOLUBLE JJNSOLUBLE
REACTOR WATER
SOLUBLE . ., ..TOTAL • «O'-«1''»
SOLUBLE 'INSOLUBLELH — i-tT — _J
rt1T
dt
—— «iOUTER LAYER
•• n™INNER LAYER ? I
*/dT - RT~ini(Ul
OUT -OF -CORE DEPOSIT
Ä NEUTRONFLUX
•
••
•
iT
•*— IN-CORE-«-• MATERIAL
~*" HWCUt
6059
Co TransportCo Transport — —
Fig. 1. Block diagram ofmodel 1)
Co transport
1) Some interactions are considered to exist between soluble ions andcrud particles, which include adsorption of ionic species oncrud particles.
2) Both crud and soluble species deposit on the fuel surfaces,crud playing an important role in the deposition of thesoluble species.
3) There are two layers, loosely attached and tenacious ones,deposited on fuel surfaces, and some of the former is trans-formed into the latter.
4) Radioactive species are released from in-core materials andalso from the fuel surface deposits. They are distributedbetween soluble and insoluble species, following Freundlichadsorption isotherms.
196
5) Soluble and insoluble radioactive species deposit on out-of-core surfaces. Double oxide layers, inner and outer layers,are formed on the surfaces of the out-of-core base metal.
Mathematical equations are developed to describe the corrosionproducts transport during the steady state operation of BWRs.
Iron Transporta) Deposition and release of iron on fuel surfaces.
730 + 300 CRÎ, '
Here both 6t and 62 are dependent on properties of iron species.See note after the Nomenclature. 1 in the parentheses is associatedwith i'th fraction of the fuel bundles.
b) Deposition and release of iron in out-of-core deposits.Inner layer
„ S.*to,l\ +o(U- 0.055)1'" (7VW +/)"•« ———
Corrosion rate (R ) is assumed to be a function of water conductivity(ü), and TM is a small time period arbitrarily chosen so that R does notSbecame infinite as t — *0.
Outer layer
.al
c) Iron in reactor water
197
Cobalt (Co and Co) Transporta) Deposition and release of cobalt on fuel surfaces.
Inner layer
*,,(/)- 6 j
where K_ is the rate constant of the transformation of the outerlayer to the inner layer and depends on the characteristic of theiron oxide, decreasing as the total iron deposits on the fuel sur-face increase.
Outer layer
b) Deposition and release of cobalt in out-of-core surfacesInner layer
dt ""'
~2i- (X + *„,
198
The deposition rate constant (ß.) for the inner layer decreasesas the outer layer iron deposits(M ) increase.
Outer layer
dl
dl ~- (X *• koodoo •
c) Co production in in-core materials.
- 1.63X10-"^
d) Balance of cobalt in reactor water.WdCjw ^ —-2T1-"""»I
S - ( II "
dl
xCf° = Cj°w
e) Co concentration on out-of-core surfaces and radiat ion
dose race.
Ri
199
Nomenclature
Reactor ParametersW
F«Ff.
- reactor water mais (10* kg)- RWCU How rate (10* kt/day)- feedwater flow rate (10* kg/day)- core flow rate (10* kg/day)« integer number of fractions of fuel re-
placement per cycle (dimensionless)« total out-of-core surface area (cm1)
- total fuel surface area (cm1)" rated core average heat flux on fuel
surface (W/cm1)
Nuclear Data
X « activity decay constant (day*1)MW - atomic weight of target material (g/
mol)o " activation cross section (b)4>f « thermal neutron flux on fuel surface
(n/cm'-s'1)^, " thermal neutron flux on in-core mate-
rial (n/cm'-s'1)Pf • activity production rate in fuel deposit
(Ci/DP, • activity production rate on in-core
material (Ci/g)
Input Parameter!WT " total iron input from feedwater (g/day)MT " total cobalt input to reactor water
(I/day)DfMf * cobalt release from primary system
material (g/day)M, « cobalt in in-core material exposed to
neutron flux and available for release*(t)
D, « release rate constant forAf,* (day*1)RI • primary system surface total corrosion
rate (g/day)^rer * primary system surface corrosion rate
constant (g/cm'-day"*-')TM « pre-startup time (day)U * reactor water conductivity OtS/cm)a «coefficient relating (he conductivity
with corrosion rate (cm/pS)
Activity InventoriesAf0 » activity in fuel deposit, outer layer (Ci)Af, * activity in fuel deposit, inner layer (Ci)XM « activity in out-of-core deposit, outer
layer (Ci)
AM * activity in out-of-core deposit, innerlayer (Ci)
A0 « activity in out-of-core deposit, total(Ci)
A, • activity concentration on out-of-coresurface (pCi/crn1)
A,, • contact radiation dose rate on recircu-lation piping wall (mR/h)
A, « activity in in-core material avaJbbl: forrelease (Ci)
An - activity removed by RWCU system (Ci)
Concentrations in Water
CFW " iron concentration in fecdwacer {ppb(10'*g/kg)|
Cuir * iron concentration in reactor waterÏPPb)
CRW - cobalt concentration in feedwater [ppt(10-»g/kg)]
CRW ~ total cobalt concentration in reactorwater (ppb)
Cj* * soluble cobalt concentration in reactorwater (ppb)
C|" « insoluble cobalt concentration in reac-tor water (ppb)
Cjfiu - total *°Co concentration in reactorwater (piCi/kg)
C/° = soluble *°Co concentration in reactorwater (jiCi/kg)
CÎS " insoluble *°Co concentration in reactorwater (^Ci/kg)
Material Inventories ,M/0 * weight of cobalt in fuel deposit, outer
layer (g)Mf, = weight of cobalt in fuel deposit, inner
layer (g)MOO ~ weight of cobalt in out-of-core deposit,
outer layer (g)= weight of cobalt in out-of-core deposit,
inner layer (s)« weight of cobalt removed by RWCU
system (g)m weight of iron m fuel deposit (g)= weight of iron in out-of-core deposit,
outer layer (g)= weight of iron in out-of-core deposit,
inner layer (g)= weight of iron removtd by P.WCU s>s-
tem (g)
A/o.
A'o,
A//'
Ml'
Af0(c
A/CFJ
200
Transport Constants
*
ft»
««o
<F
RWCU system removal efficiency forcobalt (dimensionless)RWCU system removal efficiency foriron (dimensionless)
fuel deposition constant (dimensionleu)fuel deposition e f f i c i e n c y for iron(dimcnsionless)
fuel deposition efficiency for solublecobalt, inner layer (dimensionless)fuel deposition efficiency for insolublecobalt, outer layer (dimensionless)
fuel deposition efficiency coefficientfor soluble cobalt (cm/g° *)fuel deposition efficiency for solublecobalt, outer laver (dimensionless)oui-of-core deposition rate constant foriron (ks/cnr day"1)out-of-core deposition rate constant forinsoluble cobalt (kg/cm7 day"1)out-of-core interaction rate constantfor soluble cobalt, outer (kg/g day"')out-of-cor: interaction rite constantfor soluble cobalt, inner (kg'e dav"')
out-of-core deposition rate constant forsoluble cobalt, inner (kj/g)Co,'*°Co isotopic exchange rale con-s tan t (k£/g day"')deposit, on ra te constant coefficient 1
« deposition rate constant coefficient 2(kg)
= "Co soluble/insoluble distribution co-efficient (ppb'1)
m *°Co soluble/insoluble distribution co-efficient (ppb"1)
" fuel deposit release rate constant foriron (day"1)
* fuel deposit release rate constznt forcobalt, inner layer (day"')
= fuel deposit release rate constant forcobalt, outer layer (day"1)
*.«,
Rel
= fuel deposit transformation rate conslant for cobalt (day"')
= fuel deposit transformation rate coefficient 1 (day"1)
= fuel deposit transformation rate coefficient 2 (day"1 g"1)
= out-of-core deposit release rate conslant for iron (day"1)
= out-of-core deposit release rate conslant for cobalt, inner (day"')
* out-of-core deposit release rate conslant for cobalt, outer (day"1)
« out of-core deposit t ransformation rateconstant for cobalt (day"')
«release c o n s t a n t d u r i n g shutdown(day"1)
Empirical Constants
"•i
* empirical constant for th: ra t io of *°Codose ra te to *°Co surface concent ra t ionon recirculat ing piping [(mR/h)/OiCi/cm')|
: empirical constant for ac t iv i t i e s oth;rthan *°Co on piping surface in dose ralecalculation (/jCi/g cm"1)
. a» = empirical constants related 10 the properties on iron species0 (dimensionless)
= empirical constant for Co/*°Co releasefrom fuel deposit, inner layer (d imensionless)
= empirical constant for Co/*°Co releasefrom fuel deposit, outer layer (dimen-sionle«)
= empmcal correction factor for insol-uble *°Co deposition on out-of-coresurface (dimenstonless)--
» empirical correct factor for *°Co releasefrom fuel deposit due to activationrecoil (dimensionless)
= correction for soluble/insoluble distri-bution due to sampling error (dimen-sionless)
= fraction of soluble cobalt deposition infuel outer layer (dimensionless)
It should be noted that in this model difference in crudcharacteristics are throughly emphasized. These are consideredto arise from the different type of the condensate deznineralizers,Powdex or deep bed.and the option of forward pumped heater drainsystems.
This model includes many transport coefficients and empiricalconstants, which are determined on the basis of laboratory informa-
201
tion and plant data from three reactors with different type ofcondensate demineralizers and forward pumping systems. The con-stants determined are assumed to be constant for all BWRs with thesame type of the condensate systems.
Type of CondcnuisTrotment Syilem
Po~dex (without FPriD)
Dc:pb:i (~nhou: FPHD)
Forwici pumped heaterdr.m(FPKD)
*,
1.2
1 0
04
*i
0 7 5
I.D
1.5
*i
1.5
1.0
0 7 5
4.
1.0
1 0
0 5
3. ASEA-ATOM modelIn Sweden a computer model called "CRUD" has been developed to
study radioactivity transport and to determine the origin of themeasured radioactivity on pipe surfaces. A schematic diagram ofthe model is shown in Fig.2. In this model all the species are treat-ed as elements or nuclides, and the core is divided into an optionalnumber of fuel bundle batches and seven axial positions. According toplant experience, fuel deposits tend to turn into a more tenacious layerwith increasing time. The release rates from fuel deposits are there-fore assumed to decrease with time.
0-REFUELING
i
IN-CORE AREA SPACERS
AC[Bq] ASCBq/kg]TCChg] SA[m2JCACm?) CRSCl«9AT>2i"]
— -E>REFUELING
/VKfyuÇ 1 fCW ! PSWl ( AFWC XA7AFCW ^T^AFSW
RWCU— HT F
AH [Bo.]Em EH2WFH[l<g/s]
<&&-
+-1U
REACTOR WATER
AW CBq]TW C^gJWM f>g H2OJ
-_Fwi-C>AFWI ^.
RWCU— IE
AI CBq]EM EI2wFi [kg/sD
?'' •^4?AFWO AFOW!FEEDWATER
FLOwQkg/s]
CONCLU]
OUT-OF-CORE AREAAO C8q]TO CkgDOA [m2]CRO Cfcg/f^.i]
---£> F»x [ kg/i]——— O AFx*[eqA3
- - -D> PI [ Bq/kc.s]- - - • •£> P2 [l/i]
Fig.2. Block diagram of computer code "CRUD„ 3)
202
The mathematical formation is shown below.
Material balance in core
— TC - FUC - FCUd c n ,n n.D n.pxd AC - PI -1C « AFUC - AfCI-37 n.o r>,o B.» n.B n.a
- (P2 * X) 'ACn,« !>•
FCU • . FCU' . RCl • TCn.» "•" n.« n "•"
AFCU - T AFCU - f RC2 • ACn.« "•• n.m n n.
RCln - F • RCl
RC:n • F • RC2
F - In (l »_£)/_£
Here indices n and m refer to differential fuel bundle batches»nd axial positions, respectively.
The fuel spacers in Swedish BWRs are made of Inconel X-750,containing 0.1% of Co. The corrosion rates of the spacer materialsare assumed to follow a parabolic law.
Activation and release from fuel spacers
- T FSU . r „s« *. "•* J-L " IPS— nn."L A>-SU - J" RS - ASn 4 m n,» Q^ n n. m
RS - CRS • SAn n
CRS - CRS • (C » C -t l""2n 2000 0 l cp'
The rates of corrosion, corrosion product deposition and releaseare assumed to be identical vith respect to all out-of-core surfaces ,which take single layer structure. The corrosion races of basemetals are taken to be constant at all time.
203
Material balance in out-of-core areas
3Y TO • FWO - ROI - TOd•^ AO - AFUO - AFOW - A • AO
TOU - R01-TO-CRO-OA-TPOA.FOU - R02-AO
Material balance in reactor water
. FFU-FCU'FSW-'FOU" DC1-DH1-DH-D01
AFCU.AFSU.AFOUDC2»DH2-DI2'D02*A
FUz - Dzl-TV
AFUz -Dz2-AU
(r - C. H. l or 0)
For Swedish reactors the following reactions are studied usingthis model.
Fe-54 (n,p) Mn-54Fe-58 (n,y) Fe-59Ni-58 (n,p) Co-58
Co-59 (n,Y) Co-60Zn-64 (n,Y) Zn-65
Considering the accepted importance of insoluble iron in feedwater and reactor water, an extended version of the code, "CRUD-2" is
4 )now under development. In the new model, corrosion products inthe reactor water are divided into soluble and insoluble species andthe out-of-core surfaces take a double layer structure, the outer looselayer and the inner tenacious layer. The release and deposition ofsoluble corrosion products from and to the in-core area are taken tobe a function of the amounts of iron deposited.
204
ACASAUAOAH
AITCTVTOAFUIC
AFVHAFU1AFUO
AFCW
AFSW
AFOW
FFU
rueFU-HFUIFVO
FCU
F SUFOU
ÇASAWMOATPSTPO
CRS
CRS2000co-ciCRO
- Time (s)- Activity in-£ore area (Bq)- Activity £pacers (Bq/kg)- A_ctivity reactor »<ater (Bq)- Activity out-of-core «rea (Bq)- Activity high temperature filters(HTF)(Bq)
- Activity ^on exchangers (IE)(Bq)- Target vn-core area (kg)- Target reactor water (kg)- Target out-of-core area (kg)- Act i v i t y f_low water to in-core
area (Bq/s)- Activity How water to HTF (Bq/s)- Activity £low water to If. (Bq/s)- Activity flow water to £ut-of-corc
area (Bq/s)- ^ctv-ity f_\ov in-£ore area to water
(Bq/s)- Activity flou spacers to waterTßq/s)
- Activity flou out-of-core area towater (Bq/s)
- Target How fccdtaicr to water(kg/s) ~
- Target £low water to in-£ore area(ks/s)
- Target flow water to HTF (kg/s)- Target How water to l_L (kg/s)- Target flow water to out-of-core
area (Vg/s)- Target flow in-£orc area to ^atcr
(kg/s)- Target ^low £paccrs to water (kg/s)- Target flow out-of-core area to
water (kg/s)- In-£ore £rea (tn )- Jpacer jrea (re )- Reactor _uater «ass £kg)- Out-of-core area (m )- Target £roportion in jpacer oate-
rial- Target proportion in ut-of-core
material t- Corrosion rate spacer material(kg/,. .2)- ~
- CRS after 2000 hours (kg/*.n> )- Constants- £orrosion £ate.out-of-core mate-
rials (kg/s, m 7
DCxDOxDHxDixRCxRSROxx-1x-2FP
CA
- Deposition on in-core area (• )- Imposition on out-of-cor« area (» )- Deposition in KTf (*~ )- Deposition in J.E {»" )- Release from in-core area (s )- Release from spacer «aterial (kg/s)- Release from out-of-core areas (s )- Target material- Activity- Operation time dependent function- Operation time (days)- Constant (days)- Decay constant (s )
P1.P2 - A c t i v a t i o n (Bq/kg , s ) and burn-up ra te (s"1)
205
Removed at CUD>HCRUD
r Corrosion "\•nd )
V^ Efoiion s
F—_Co in Primary Water
l IONMicrolayer >. /'Adsorption on
Evaporation I I Depositing•nd Dryout ^/ \_ Iron Oxide
JLCo on Fuel
( Neutron AIrradiation )
"Co on Fuel
( Release ) ( Dissolution )J \^ _ J
(^Removed it C(JD "Co in Primary Water
CDeposition Diffusion intoOxide Layer
"Co on Pipe Wall
Shutdown Dose Rate
Fig.3. Block diagram of Hitachi model.5)
4. Hitachi modelA mathematical model has been developed to predict radioactivity
transport in some Japanese BWRs. Basic processes involved in themodel are shown in Fig.3. In this model the main concern is focusedon the behavior of iron, cobalt-59 and cobalt-60. The cobalt-60 in thereactor water is divided into two kinds, soluble and insoluble. Theformer is adsorbed by the deposits on the fuel surfaces and the latteris carried by crud particles. The most simple forms of the mathematicalformula are given as follows.5)
VdCFt/<Jt • XFe+
<tHFe/dC - «CCF«V -
VdCCo/dt - Xco
- 6tCCoV -
- Xt+ d*
TVdRc/dt - CeA - Y(
206
dA/dt - k»+
- 4plRi -- «PCRC -(
where6 -
These differential equations can be solved analytically, and usedfor the calculation of dose rates on recirculation piping.
Recently some modification has been given to the model. Forinstance, the Co deposits on the out-of-core surfaces are dividedinto two kinds, soft deposits and hard spinel layer. Crud Codeposits on the pipe surfaces and forms the soft deposits, whilesoluble Co ions are taken up into the spinel layer. It is alsoassumed that a quantity of Co is released from the soft depositsand transferred to the inner spinel layer. The following modificationis added to the mathematical formulation.
dTJdt = 6,AC - (X + f, + 6j)r, -•• f,(r, + IV .and
These new differential equations are solved analytically using somes i mpl i f i cat i ons .
Another modification is also added to take into account the e f f e c t8 )of crud particle size distribution. The particle diameter is assumed
to influence the corrosion product transport in two ways. The deposi-
tion of crud particles on fuel surfaces is affected by the particle
diameter , and it is in fe r red that the larger the par t ic les are, the smal ler the
deposi t ion rate is. The par t ic le diameter also has large in f luence on the
adsorpt ion of Co on the fue l deposits through the specif ic sur face area ofL _, 59)the deposits.
207
Nomenclature
vXF. •*Co *Xi -
«Fe -
«Co -A
CCoR
time. htotal weight of primary cooling water, confeed race of iron, mg/hfeed rate of cobalt, Bg/hfeed race of *°Co (contribution of WVInternal Baterlals), yCl/hamount of iron deposited on the fuelsurface, Bganount of cobalt deposited on the fuel(urface, Bgamount of "Co deposited on the fuel•urface, yClconcentration of Iron in the primary cool-ing water, mg/tonconcentration of cobalt in the primarycooling water, tog/tonconcentration of ionic "Co in water,UCl/Bl
Rc * concencration of crud Co in water,B6
ion and crud removal efficiency in ehecleanup line, h"1deposition rate of ion on fuel aurface,~'
• deposition rac* of crud on fuel surface,h"1
• release rate of ion fro-i fuel surface,h"1
• release rat« of crud fron fuel surface,h'1
• deposition rate of ion on iron oxidesurface. h~ '/r.g
• decay constant of '°Co, h~'• average cross section of cobalt, cm1
• average neutron flux ir. core, n/cm}sNo • nui.ber of ator.s of cobalt in unit oass.
n/mgY ~ specific voluor of coolant, ml/tonF • anount of *°Co deposited on the pipe
surface, uCi/cm*rt - contribution of ionic "Co to the aaouni
deposited on the pipe surface. uCi/cm*Tc - contribution of crud *°Co Co the aaount
deposited on the pipe surface, yCl/cn1
FX - contribution of ionic "Co released fronthe crud to the amount deposited on thepipe surface, uCl/cm 1
6pi • deposition rate of ion on pipe surface,CB/hdeposition rate of crud on pipe surface,CB/hdeposition rate o( Ion release froa the
cCiCe6*Xo
ôpc
«px
»F.
•L-nj -
TJ *
crud on pipe surface, h*"1
• release rate of crud cobalt-60 from pipe•urface. h"1
• release rate of Ionic cobalt-60 fron pipesurface, h"1
- shutdown dose rate, mR/h- geometrical factor for calculation ofdose race. mR/h per pCi/cn2
- removal coefficient of condensatedeminerallzer, -
(J : j th equipment)- release race, •g/(ra*li)• wetted surface, «• ion content In Material, -
cobalt content in material, -number of atoms of cobalt in unit mass.n/otgexposure tine, s
r, - contribution of 60co ion taken up into spinel to theamount of Co deposited on the pip« surface, Ci/n2
Fj - contribution of crud *°Co deposition to the anount ofCo deposited on the pipe surface, Ci/n
T, - contribution Of 60Co cr.nsferr60,
from soft deposits tospinel to the amount of ouCo deposited on the pipe sur-face . Ci/m2
6, - deposition rate of Co ion uptake into spinel, m/s{2 • deposition rate of crud 60Co, m/s6, • transfer tn* oi Co ion from soft deposits to spinel,
-1
dCiCis
XRed*
- release rate of 60Co ion from spinel, s"1
- release rate of crud *°Co, s"1
» transfer rate of spinel to soft deposit by oxidation, s- site number of spinel structure to be prepared for ion
-1
deposition, m-2
g » binding coefficient of ion for a site in the spinel.n3/s
-1« decay constant, a• Reynolds number• diameter of pipe, m• release coefficient of ion from crud surface, m-1
208
5. Toshiba modelAnother model developed in Japan uses a rather complicated
9 )scenario for corrosion product transport as shown in Figs. 4 and 5.The species in the reactor water are divided into four kinds, "crud,"large particles, small particles and ions. On fuel surfacesthere are four kinds of oxide deposited, tightly adhered hematite,loosely adhered large and small particles, and tightly adhered spinels.The out-of-core surfaces have a double layer structure, loose hematitelayer and tenacious spinel layer- According to plant experience insome BWRs a significant contribution to the dose rate is made byradioactive particles sedimented around the bottom of the RPV.
The size of these sedimented particles is rela-tively large (ca. 10v,m) . This situation is taken into account in theToshiba model. Simplified mathematical formulae are given below.These differential equations are solved analytically and used for thecalculation of the quantity of radioactive deposit on out-of-core pipingsurfaces and dose rates. NomenclatureMass Balance AKW- concentration of activated corrosion product
in reactor water, MBq/fcg coolanta. concentration in reactor *aur
AF - activated corrosion products on rue I rod sur-
A, - activated corrosion products on out-of-core- (5 + 7 + ß + f)Cnw V • surface, CBq
b fuel surface *-*" * concentration of corrosion product in reactorwate r. »ig/kg coolant
—r— * &CRW V - f-\lf . Cfu - concentration of corrosion product in feed-water. fig/kg coolant
c. out-of-core surface of primary coolant sys- Ffv = now rate of feedwater. g/hte m
MF " corrosion products on fuel rod surface, g0^7 .
-y— * lCnw V ~ JjA/, . M, • corrosion products on out-of-core surface, gR - rate of activation, GBq/h -g
Activity Balance y = weigh, of reactor W3(eri g
a. concentration in reactor water t> = effective weight of reactor (in-core) water, g
.al ß = rate constant of reactor cleanup system, per' hour
. , . , 7 * deposit rale constant on out-of-core surface.b. fuel surface per nour
—— -* RMr+ & AKW ^ - (X + fMf - * «deposit rate constant on fuel rod (in-core)dt surface, per hour
C. OUt-of-COre surface of primary coolant sys- ? = release rate constant from fuel rod (in-core)tern surface, per hourdAt »7 = release rate constant from out-of-core surface.-JJ7 «7.4/fu- ' ' - (X+nM, . per hour
X = radioactive decay constant, per hourt = carry-over rate to main steam system
209
Cn Fuel Striae«
Fig.4. Block diagram of mass transport. (Toshiba model)9)
On Fuel Surtoce
R*^__ < A n > J^r,
HEMATITE HEMATITE(Urge portOc)
B'
<Ar j> <Ar«>looac*r*Qeoov«< t^lty-oeoosJtctfHEMATITE SPiriELIsmott Doric*)
'< ? ' T" i 1
? i iInReoclor *ote' *''|x)!i *~
Jl i!
U M ?il us& ;| i X
'
— * ~< -
x. **
^^,r
4 T^Ci »'M1 '% i-v r> /?. >
5z
<Aïl> <A ; *> <AiL) <A£CfuO AclivoltO ACIivolea Ion
laoe por itfe v<oi txyteie
II !l
/A : i//V' \ J
* /«.- "j?,\ "Sf•,/ < «n unvV oai ssii^ «on • 'n
°"0S11i°(n(-Cf(Xl ! '1JL ü.
< Ao,>
HEMATITE
Around rh» Bottom of RPV
vK•»>•
*v "^V **
IJ V *IL '..
1 «<» •-s.1> -oJt
> !/i-tioMu.• I s*•* v>*1M m«A «-T~"" r^
O<
7S
<Ao?> <Ao3>looacly OBgokttO 1^^ oecos.lrt
HEMATITE SPINELS
On Out-of-CtXt Ptpings
?
^^«j-sy
fy
Fig.5. Block diagram of activity transport. (Toshiba model)9)
210
Nomenclature for Figs. 4 and 5.
SvmbolList of parameters
Meaning DimensionM massA activity (cone, of activi-
ty in RW)C cone, of metal in RW5 specific activityF flow of waterV volume of reactor waterAR surface area6 deposition rate( release or dissolution ratek equilibrium rateb release rate by self-
diffusion$ release rate by erosion-
corrosionß removal rate by RWCU systemDE D.F. of RWCU systemDF D.F. of fuel deposit
cleaningf fractiona natural fraction of isotope4> flux of neutron
o(ca) activation (annihilation)cross section
w atomic weightX decay constant
Ci(pCi/g)ppbCi/gt/hrtm2t/hr
t/hr/gFehr-ig/hrt/hr
n/cm2/sec-2cm
ghr->
SvmbolList of subscripts
Meaning Example
Under subscriptFWRUCUF0IMOM
feedwaterreactor waterRWCL' systemfuel surfaceout-of-core surfacein-core materialsout-of-core materials
Upper subscriptC crudi*f
ionactivefast neutron
FFWCFWCRW-ARWMCU«ACU
Mp.Ap
6 CO
211
6. Comparison of the BWR ir.odels.All the computer models described in the previous sections
contain some anbiguity on the mechanism of each process comprisingradioactivity build-up on the circuit pipe surfaces. Therefore,they have been refined or improved as more data have beenobtained on radioactivity transport from laboratory experiments andplant operation experience and our knowledge f the mechanisms hasjncreased. This refinement continues. In the course1 of this refinementthe models have become more or less similar to each other.
In Table I a co~iDa-ison of these models are summarized.
212
Table I. Comparison of BWR models
Nuclides
Speciesconsidered
Corrosion andrelease rate
InteractionCo/Fe
In coredeposits
Out-of-coreoxide layer
andCo deposition
Other remarks
CE
Fe, CoCo-60
Mn*-54, Fe*-59
ion, crud
corrosion ratesdepend on electricalconductivity ofcoolant .Co adsorption on
crud
oxide double layers
oxide double layers oncorroding surfacesparabolic law
corrosion proJuctbehaviour dependsstrongly on tvpe ofdéminera 1 izer .
ASr.A-ATOM
Fe, Co, Ni, ZnFe-59, Co-60, Co-58Mn-54, Fe-59, Zn-65
originallynot distinguished,soluble and insolublein the new model
corrosion product re-lease from core spaceris considered. "^ ' s fol-lows a parabolic law.
not considered
oxide single layerrelease from fuelsdecreases with time
oxide double layers oncorroding surfaces inthe newer model .
core divided intosubsections
HITACHI
Fe, Co, Ni**
Co-60, Co-5H**
ion and crud for60„Co
constant releaserate
not considered
oxide s i r g \ i ] a i c rsaturation limits
for deposit
three step*! of Coup-l.ikc
l.crud deposition2. Lister mechanism3. outer —— > inner
effect of particlediameter is taken ininto a i- CO jut 11 a nowversion.
TOSHIBA
Fe, Co, Ni**
Co-60, Co-bo**Fe-59 , Mu-54
ion, crud
largo particle;»small particles
constant releaserate
Co scavenged oncrud
o x l l r j i i < v i > ^
, x i, i1 i L ' i 1 i ^ "corroding surfai_esparabolic law
deposits of largeparticles aroundRPV bottom
* empirical estimation** considered in new version
U)
References
1) C.C. Lin, C.R. Pao, J.S. Wiley and W.R. Dehollander, Water Chemistryof Nuclear Reactor Systems II, BNES, 311, 1980.
2) C.C. Lin, C.R.Pao, J.S. Wiley and W.R. Dehollander, Nucl. Tech.,_54_, 253, 1980.
3) K. Lundgrum, ibid., p.305, 1980.4) K. Lundgrum, EPRI NP-3114, 2, A-82, 1983.5) S. Uchida, M. Kitamura, M. Izumiya, K. Ohsumi and K. Miki, Water
Chemistry of Nuclear Reactor Systens II, BNES, 299, 1980.6) S. Uchida, M. Kikuchi, Y. Asakura and H. Yusa, Nucl. Sei. Eng.,
.fil, 245, 1978.7) S. Uchida, Y. Ozawa, E. Ibe and Y. Meguro, Nucl.Tech. ,_59 , 493, 1982.8) S. Uchida, Y. Asakura, M. Kitanura, K. Ohsuni, K. Yonezawa and
Y. Matsushita, J. Nucl. Sei. Tech.,_2JL 414, 1983.9) Y. Henrai, T. Kamata, Y. Nakayana, A. Tani, Y. Morikawa, Y. Sato,
H. Nagao, S. Sasaki, Y. Sato, M. Kuba and M. Kakefuda, Water chemistryof Nuclear Reactor Systens II, BNES, 319, 1980.
10) K. Yamazaki, S. Fu^ata, T. Kanata, Y. Morikawa, K. Numata, K. Osanuraand Y. Megro, Nucl.Tech. 52. 7, 1981.
214
