BAW-1352 REACTOR TECHNOLOGY (TID-4500) This report was prepared as an account of work 1 sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com =pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use I would not infringe privately owned rights. 1000-MWe LMFBR ACCIDENT ANALYSIS ' AND SAFETY SYSTEM DESIGN STUDY - Topical Report - Candidate Primary Containment Safety Features by J. S. Dossett F. X. Masseth Approved by: M. W. Croft Project Manager November 1970 ANL Contract No. 31-109-38-2339 B&W Contract No. 847-0501 BABCOCK & WILCOX Power Generation Division Nuclear Power Generation Department P. O. Box 1260 Lynchburg, Virginia 24505 SISTRIBUTION OF THIS DOCUM*NT IS UNLIMI™9 1'1
Transcript
BAW-1352REACTOR TECHNOLOGY
(TID-4500)This report was prepared as an account of work 1sponsored by the United States Government. Neitherthe United States nor the United States Atomic EnergyCommission, nor any of their employees, nor any oftheir contractors, subcontractors, or their employees,makes any warranty, express or implied, or assumes anylegal liability or responsibility for the accuracy, com=pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use I
would not infringe privately owned rights.
1000-MWe LMFBR ACCIDENT ANALYSIS' AND SAFETY SYSTEM DESIGN STUDY
This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.
DISCLAIMER
Portions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.
THIS :PAGE iWAS INTENTIONALLY
LEFT BLANK
PREFACE
This report was originally prepared as a technical note to document
the work performed in a specific contract activity as soon as the work
i was completed. The technical editing was limited in order to meet the
objective of timely reporting. The report was issued for USAEC-ANL
use only, and the inte nt was to update and consolidate the information
from all technical notes in a comprehensive phase report before final
publication for public distribution at the end of Phase II.
This plan was changed when the contract was terminated in October
1970 for the convenience of the government. Instead, a final summary
report will be prepared, and the previously issued technical notes will
be published as formal topical reports. In accordance with the modified
plan, this technical note is being published in its original form withoutfurther editing or modification except for minor technical corrections
and change s in the title and date of is sue. Even without updating andtechnical editing, the report provides detailed information that should
be helpful in evaluating and resolving LMFBR safety questions in related
areas.
- iii -
-.
Babcock & WilcoxPower Generation Division
Nuclear Power Generation DepartmentLynchburg, Virginia
This report describes the work performed to complete Attivity243 (Identify and Review Candidate Primary Containment Safety Features)of the 1000-MWe LMFBR Accident Analysis and Safety System DesignStudy. A literature survey was conducted to determine the potentialaccident conditions in the primary containment and to find potentialcandidate solutions to the problems. Underwater TNT-equivalentexplosions were assumed, and the resultant loading conditions werecalculated. Suggested features are presented as possible methods ofsolution.
4.1.5.1. Problem Discussion . . . . . . . . 4-154.1.5.2. Approach .. ........ ...... 4-164.1.5.3. Conclusions and Candidate
Solutions . . . . . . . . . . . . . , 4-20
4.2. Reactor Cover Structure ................. 4-224.2.1. I n t r o d u c t i o n. . . . . . . . . . . . . . . . . . . . . . 4-224.2.2. Shock Waves .................... 4-24
-V-
CONTENTS (Cont'd)
Page4.2.2.1. Problem Discussion........... 4-244.2.2.2. Approach and Conclusions . . . . . . . 4-24
4.2.3. M i s s i l e s. . . . . . . . . . . . . . . . . . . . . . . . . . 4-294.2.3.1. Problem Discussion . . . . . . . . . . . , 4-294.2.3.2. Approach and Conclusions . . . . . . . 4-30
4.2.4. Pressure Problems and Conclusions . . . . 4-344.2.5. Sodium Slug Problem-Discussion and
Babcock & Wilcox (B&W) is conducting the Accident Analysis andSafety System Design Study to develop a better understanding of theinfluence of safety requirements on large LMFBR designs. The refer-ence design of B&W's 1000-MWe Follow-On Studyl, 2 forms the basis forthis study, which is being performed under ANL contract 31-109-38-2339on a cost-sharing basis. Utility companies are also participating unde r
separate agreements with B&W.This activity, "Identify and Review Candidate Primary Containment
Safety Features (Activity 243), ·" 3 parallels the accident analysis studies
(Activities 210 and 220) and similar safety feature studies for the sec-
ondary containment (Activity 244). The major objectives are asfollows:
1. Assess the capability and/or limitations of the reference
design's primary containment safety features to meet refe rence or up-dated functional requirements.
2. Identify revisions that could be made to improve the
capability of the re ference safety features.
3. Determine whe ther alternate or additional safety
features are required.
4. Identify alternate or additional candidate featuresrequired to fulfill or better fulfill requirements. Figure 1 shows theflow of input to this activity and the output to the following activity atwhich stage reference features will be selected (Activity 250).
The primary containment evaluated under this activity is definedas "the system or complex immediately surrounding the primary
system, which constitutea the first external barrier to the release of
energy, missiles, and radioactivity. " For this study, the cover of the
1-1
0
primary tank and the boundary of the primary tank's cavity form
the primary containment. The system is shown in Figure 2, and
complete conceptual descriptions are given in references 1 and 2.
The relationship between the primary and secondary containment
systems is shown in Figure 3, which shows that the secondary contain-
ment is a backup to primary containment.
To accomplish the objectives of this activity, it was necessaryto select reactor core accidents and determine the consequences at the
boundary of the primary containment-and this had to be closely coordi-nated with the accident analysis studies and the secondary containment
studies. Although knowledge of the exact accident conditions is not
critical for this activity, it is important to ascertain that the range ofconditions considered is conservative and to include those postulatedfor the LMFBR. It is also important to include all safety features,
whe ther directly or indirectly associated with primary containment,as intimate parts of the study. Therefore, we had to consider the
design of structures, energy absorbers, and energy diverters that
could be located either inside or outside the primary containment as
This report describes the work performed to complete Activity243, "Identify and Review Candidate Primary Containment SafetyFeatures, " for the 1000-MWe LMFBR reference design. Much of theeffort was devoted to studying conditions created by large core accidents.
However, since the characteristics of the possible accidents and their
energy releases were not available, underwater-explosion calculationalmethods were used. These are the best methods available, and the ygive fairly accurate conditions for positions several radii distancesfrom the explosion's center. In addition, it is felt that they give con-servative results because TNT explosions are much faster acting andrelease a greater amount of the potential energy than the actual postu-lated accidents would.
Five accident conditions have been considered: shock waves,impacting mass, thermal shock, missiles, and pressure. The problemsof maintaining the integrity of components of the primary containmentunder each condition have been studied in some detail for the explosionconditions created from the detonation of from 50 to 1000 pounds ofTNT. Only the rapid absorption of the shock wave energy by the largevolume of sodium surrounding the core has been used to reduce the
energy of the explosion. The effect of core internals is considered
qualitatively only for cases where the conditions severely affect the
primary containment.
For the explosions considered, Figure 4 shows the peak pressuresassociated with the shock wave as a function of the radial distance fromthe explosion's center. The effect of the large volume of sodium can beseen by the slope of the curves. The vertical lines show the positionsof various major components, and it can also be seen that no energy
2-1
Figure 4. Peak Pressure of Shock Wave
36,000
®W = 50 lb TNT = 9 X 104 Btu = 9.5 x 107 J = 95 MW/s34,000 - W = 100 lb TNT = 18 x 104 Btu = 19 x 107 J = 190 MW/s
32,000 -
ab cd W = 300 lb TNT = 54 x 104 Btu = 57 x 107 J = 570 MW/s
absorption is accounted for at the interfaces of these components. Thisomission results in an even more conservative value for the peak pres-sure at the primary tank's boundary.
2.2. Review of Reference Design and Literature Survey
The safety features incorporated into the reference design wereidentified, reviewed, and evaluated. The results are included in the
general discussion of section 3 (see 3.2.2). Concurrently, a literature
survey of other primary containment safety features was initiated. The
survey was limited to information on the safety features of primarycontainments used for operating reactors or for reactors proposed foroperation in the near future. In addition to reviewing the type of acci-dents and forces found in the literature, we evaluated the types of safetyfeatures that have been accepted in the past, or are being considered
for acceptance, and their applicability to the reference design. Themore important documents reviewed in the literature survey are listed
in Appendix C.
2.3. Biological Shield Line r
The biological shield liner must maintain its sealing function
during any accident. Each of the accident conditions previously men-
tioned was investigated. As to shock wave, it has been shown that the
gas gap between the primary tank and the liner controls the peak pres-
sure, and even for the largest explosions considered, the strength of
the shock wave at the liner will be negligible. However, under extreme
conditions the walls of the primary tank might burst instantaneously,
and the steel fragments could acquire the material velocity of the liquidsodium, resulting in a high dynamic impact pressure on the liner dueto the impinging sodium and the steel wall of the tank. To prevent rup-
ture of the liner, one of the candidate Solutions is to install an energy
absorbing blast shield on the liner's surface. The exact design of this
shield depends on the energy of the selected DBA, but it could be simi-
lar to the design used for the EBR-II reactor. For smaller requirements
of energy absorption, a simple steel baffle could be used. Both potential
solutions could be incorporated into the 1-foot space between the primary
tank and the liner while retaining some of the gas gap to dissipate shock
wave energy. The study also revealed that an EBR-II-type absorber
2-3
would present some problems in the reference emergency decay heatremoval system; these must not be overlooked when selecting finalcandidate safetyfeatures.
Thermal shock due to hot sodium striking the much cooler linerwas also considered. In the extreme case examined, the entire interior
surface of the liner was assumed to be wet by 800 F sodium, and theexterior surface was hald at 150 F. .Although the liner would be perma-nently deformed, no rupture is likely to occur.
In considering missile damage to the liner, it was necessary tofind a missile source and accelerate it fast enough tc damage the liner.
Additionally, we had to assume that the primary tank failed in such a
way that it did not resist the missile. Based on this work, a simplemissile shield or the energy absorber required for an impacting massshould preclude missile problems.
The final pressure remaining in the system after a major accidentwas evaluated to determine whether it could result in damage to theliner. Again, this is an accident-dependent situation and requiresfailure of the primary tank before the liner is subjected to any pressure.
Therefore, we considered the biological concrete on which the lineris mounted and the other pressure-relieving mechanisms. The 6-foot-
thick biological concrete and its reinforcing steel should be designedto retain the post-accident high pressure, and the liner should be sup-
ported by the reinforced concrete. The many leakage paths throughthe cover structure will also ease the post-accident pressure situation.
The study of this problem has not been concluded, but the design pressureof 100 psig is not expected to be exceeded.
2.4. Reactor Cover Structure
The cover structure must retain its structural integrity and fulfillits shielding and component supporting functions during any accidentup to and including a DBA. All of the stated accident conditions were Iinvestigated to determine that this requirement can be met or to deter-
mine the type of additional information needed to ascertain the requiredsafety features. As to shock wave and sodium spray pressure, it hasbeen shown that the 18-inch cover gas space between the free surface
of the primary coolant and the bottom surface of the cover structure
2-4
eases the peak shock wave or sodium spray pressure for the largest
explosion investigated. Therefore, no protective features are needed
to protect the cover structure against shock wave or sodium spray
pressure.
The potential problem of missiles being generated during a nuclear
excursion and being accelerated upwa rd against the cover structure has
been investigated. However, as in the case of missiles being acceler-
ated into the liner, much effort is required to find a missile source and
then accelerate the missile fast enough to damage the structure. In
addition, it must be assumed that the core holddown structure does not
reduce the shock wasve energy and that deceleration of the missile due
to the drag of the primary coolant has very little effect on the velocityof the missile. Thus, based on the existing work effort, no missileproblem is anticipated.
The final pressure in the primary containment after an accident
was investigated. In the reference design, the cover structure has
many pressure seals that present many paths for the leakage of pres-sure from the primary containment. In addition, the initiating transient
may already have damaged the seals so that they can no longer fulfill
their sealing function. Therefore, for the reference design it is diffi-cult to assume that a pressure of any magnitude above the design crite-
rion could exist within the primary tank or containment without leakage. . j
The effects of high pressure will be studied further during the Phase IIIS
activities.
The problems involving damage of the cover structure by theimpact of the sodium slug or the effects of internal or external temper-
atures are still being investigated. The ASPRIN computer code is now
being run in an attempt to determine what the impact magnitude is andhow far the plug jumps. After completion of this effort the plug'scondition can be assessed and the post-accident cooling requirementscan be established. This effort is being continued as an integral partof the accident analysis and functional requirements activities.
2-5
2.5. External Protection for Primary Containment
Accidents that could occur during the various handling operations
were evaluated to determine the possibility of damage to the intermediate
and decay heat removal system's IHX or sodium and NaK piping due to
impact by the refueling machine or by one of the large components beingt. , ,
handled. If the damage resulting from any of these accidents were.
severe' enough to cause rupture, then a sodium or an NaK fire wouldoccur in the secondary containment and possibly lead to damage of theprimary containment. Several safety features that could be incorporated
-·r.into the reference design to solve the problem of pipe or IHX rupture
were investigated.In addition, we investigated the possibility of damage to the control
rod drives while removing spent fuel and replacing new fuel in the
storage drums within the primary tank during reactor operation. (This
scheme is used in the refe rence design.) The results indicate that design
features can be incorporated into the reference design to protect the
drives from such damage. These investigations will be continued during
Phase III to ensure that no accidents have been overlooked and that
operation of the drives will not be impaired as a result of damage from
external forces.
2-6
3. GENERAL PROBLEM DISCUSSION
3.1. Background
The principal purpose of the containment system for the 1000-MWe
LMFBR is to contain all radioactive materials, including plutonium,
that are of sufficient quantity to compromise the health and safety of
the public and the operator. In B&W's conceptual design this objective
is met by providing a double containment system in which the primary
containment-the first line of defense-is emphasized, and the secondary
containment functions as the ultimate consequence-limiting barrier.
As designed, therefore, the primary containment must contain the
primary reactor system during all normal, abnormal, and credible
accident conditions without permitting significant leakage to the secon-
dary containment, and, under DBA conditions, limit leakage to levels
that the secondary containment is designed to retain safely.
In studying the primary containment and its safety features we
must have a good understanding of the possible accident conditions
created, the role of primary containment safety features, and the effect
of coupling the two systems of containment. The following discussionis intended to establish this understanding through a general review ofthe reference design. Accidents and their effects are assessed quali-tatively, and in certain cases potential solutions are discussed. Furthe r
details of the accidents and their effect on the primary containment are
given· in section 4, which summarizes the work efforts and the candidate
solutions identified.
3.2. Accident Conditions
The containment requirements of the sodium-cooled fast reactor
will depend on the course and effects of large accidents and, in partic-
ular, on the thermodynamic and nuclear processes occurring insidethe primary containment following the initiation of an accident.
3-1
Figure 5. Sectional Isometric Cutaway View of theLMFBR Reactor Building
\- .- -I--I----
-».1--- --\- /*
0
4 -9
\-
; 6-5# M-- ifijp-*4-1-p1 . -
---* 7 13
- 4 0-4
-« »»«1--14'»8 ,
./cc- 4 .4-1 TUL -- mi
w, 0,#A#4 1 .th /
rs=- T I lilli .6'TI<---C-».
1 111 »11 [ <- ·0220 0./-*--/*..
1
- 03-2
However, because of the sodium and NaK in the piping systems justoutside the primary containment, accidents that could destroy thesesystems and result in sodium or NaK fires must also be considered ina thorough containment study. This problem area is discussed first.
3.2.1. Accidents Outside Primary Containment
The intermediate heat exchangers (IHXs) in B&W's ref- ,erence design are mounted securely on the reactor cover structure,and the intermediate coolant piping (nonradioactive sodium) extends fromthe IHXs, above floor level, to the wall of the secondary containmentwhich they penetrate as shown in Figure 5. Each pipe is protected bya secondary concentric pipe, and the interspace is filled with inert gas.Flexible joint connections at the secondary containment wall compensate
for thermally induced pipe growth, and the external surface is insulated
to limit the heat loss to the building. The environment in the secondarycontainment is air at 1 atmosphere of pressure.
The normal decay heat removal system in the referencedesign consists mainly of two Na-NaK heat exdhangers mounted on thereactor cover structure. The NaK piping for this system extends from
the heat exchangers and penetrates the secondary containment. Except
for size, the piping is identical to that of the intermediate coolant piping.In this study, we are concerned with accidents that could
result in a failure of any portion of the piping (described above) or in
damage to the heat exchangers ' bodies that could lead to large sodiumor NaK fires. This is of major concern because, even though thereactor is subcritical, a large fire on the reactor cover structure coulddamage the primary containment and at the same time the secondarycontainment, resulting in a release of radioactive products to the envi-ronment. If we assume that the detail design of the piping systems andthe inclusion of detection systems can eliminate the possibility of largefires due to corrosion, erosion, and defect failures, then we must stillconsider two other accident causes. First, heavy objects being handledby the crane in the secondary containment may be accidently dropped.Second, the large cask-type fuel transfer machine may be accidently runinto the heat exchangers or the piping. Either of these could result in afailure of the piping or the heat exchanger, a release of large quantities
3-3
of sodium or NaK, and thus.a large fire. The problem is obviously toeliminate the cause of the accident, protect the components to prevent
failure, or limit the size of the fire by adding fire-suppressionsystems.
In the reference design, it doesn't appear practical toeliminate all crane and fuel transfer machine motions from above thereactor, and it seems obvious that fire-supression systems should bea last resort. The solution to the problem is, therefore, to add positiveprotection for the systems. This could be in the form of heavy struc- 1tures, energy absorbing stops, or possibly a combination of stops andelectrical circuit breakers.
Also of concern are secondary containment accidentswhich could damage other primary equipment that could directly orindirectly lead to a hazardous situation. In this category are suchcomponents as the primary coolant pumps, the refueling mechanism,and the numerous service line connections, but the control rod drivemechanisms are of primary importance. Again, the two causal events
of dropped objects and impacting mass must be considered, and the
major concern is to ensure that no accident can cause a control rod
withdrawal from the core or damage to the mechanisms that wouldprevent safe shutdown.
The control rod drives are installed in the central portionof the reactor cover structure and extend 14-1/2 feet above the operatingfloor level (see Figure 5). If heavy objects are dropped vertically onthe drives, then the resultant damage will depend on the kinetic energyof the falling mass, and although the damage could be severe, impair-ment of the shutdown capability would be extremely unlikely. In fact,the impact would most likely cause a scram because of the electromag-netic scram connections on all of the 25 control and safety rods.However, if the dropped mass resulted in a side load on the drive
mechanisms, then a hazardous situation could result; this condition
is analogous to the condition that could ensue if massive objects, suchas the fuel transfer machine, were run horizontally into the drives. Inthis case, the bending action could withdraw rods from the core orcause a binding condition that might inhibit the shutdown capability ofthe rods. This situation exists because all of the electromagnetic
3-4
connections in the reference design are above the operating floor levelduring reactor operating periods and are therefore susceptible to bend-
ing caused.by a side load. Once again, the obvious solution would be
to eliminate all crane and fuel transfer machine movements over the
reactor during operating periods. This solution is possible and was
specified in the reference design although added mechanical and electri-
cal int,erlocks may be required to ensure that such movements cannot·
take place.1
However, such a solution for the fuel transfer machine
is not possible in the refe rence design because of the fuel transfer
scheme proposed. Also this is probably the worst case because the
machine is massive -and operates close to the operating floor level
where the possibility of unsafe damage is highest. The problem requiresserious study; a possible solution might be mechanical or electrical
interlocks to restrict travel, in addition to redesign of the control rodand the cover structure so that the electromagnetic connection is alwaysbelow floor level. The latter redesign is highly desirable for the ref-
erence design because it would drastically reduce the probability of therod withdrawal or the binding consequences.
In discussing accidents in the secondary containment, itshould be noted that the gross failure of Na or NaK piping and the with-drawal of control rods could produce unacceptable consequences. Athird causal event that may also lead to these consequences is the largereactor core accident that could cause high lifting loads on the cover
structure and upward movement. The accidents themselves and the
problems relating to the primary containment are discussed in the next
section, but two undesirable situations are mentioned here. First,upward movement of the cover structure could cause control rod with-
drawal and piping failures, and only a few inches of rod withdrawal
could insert an unsafe amount of reactivity into the core. Second, if
the accident loads acted on the control rod drive housings to cause
failure of their holddown devices, then unsafe rod withdrawal could
result even if the cover structure were held in place.
3-5
3.2.2. Accidents Inside Primary Containment
The primary containment in the refe rence design isdefined as "the system or complex immediately surrounding the primarysystem, which constitutes the first external barrier to the release ofenergy, missiles, and radioactivity. " This design, shown in Figure 2,consists of the steel liner on the biological concrete, the cover struc-
ture, and the sealing components located in and associated with thecover structure. In addition, the overall design has conceptual safetyfeatures and design characteristics that influence (1) the type of acci-dents that could occur, (2) the course that these accidents might takethrough the system, and (3) the resultant releases that must be containedat the boundary of the primary containment. Major features that wereconsidered in this study of primary containment are as f6llows :
1. The system is a pot-type design in which all of the
primary reactor system is located within one large primary tank(essentially no pressure). The primary coolant pumps, intermediateheat exchangers, and other primary equipment are suspended from theprimary tank's cover structure and penetrate into the primary tank.
2. All of the primary sodium coolant is contained withinthe regular-shaped primary tank which contains no nozzles, brackets,or stress risers of any form in the sodium-wetted areas.
3. No drain lines that could lead to accidental siphoning ofthe primary coolant are normally installed in the system, and theannular volume between the primary tank and the biological shieldingis limited to prevent uncovering of the reactor core even in the event
of a major tank failure.
4. The annular volume mentioned above is cooled by aclosed-loop argon subsystem to prevent sodium fires during a tankfailure and to limit the temperature rise of the biological concrete
duri,ng normal periods. The concrete is protected by the gas-tightsteel liner that forms a part of the primary containment system, andcooling coils in the concrete behind the liner provide emergency cooling.
5. The primary tank's cover is a massive structure con-
sisting of a large annular fixed section and three rotatable plug sections.
3-6
There are numerous equipment mounting holes, and each plug section
and each installed component are designed with multiple seals and
retention devices to limit leakage and to prevent the creation of missiles.
The cover structure will be designed so that the large thermal gradient
(approximately 1100 F to 100 F) doesn't reduce its operational or safety
capabilities. It will have an inert gas cooling system.
6. A reactor cover gas subsystem provides a blanket of
argon gas in the space between the primary sodium and the underside
of the cover structure. Argon is supplied through pressure-regulating
valves to the dynamic seals in the primary pumps and the control rod
dri.ves. Contaminated argon is bled from the primary tank through
small-diameter pipes, through pressure-regulating valves, to the
waste gas disposal system.
In this part of the general problem discussion, we are
concerned with accidents that could occur within the primary contain-
ment's boundary to cause failure of the containment. Leakage of radio-active gases to the secondary containment is not given major consider-
ation here for the following reasons:
1. Permissible fission gas leakage during normal, abnormal,
and credible accident conditions is controlled by government regulations
(10 CFR 20), and the design must ensure that these limits are not
exceeded, or, as an alternative, man access must be restricted. (Seediscussion in "Vented vs Non-Vented Fuel Pin Trade-Off Study. "5)
2. The possible consequences of large sodium leakage to
the air environment of the secondary containment and of missiles being
generated as the result of large reactor accidents far exceed the danger
of gas leakage. Inthe reference design, 6 cover gas leakage is permis-
sible following a large reactor accident, but cover structure failures
that would permit the ejection of sodium or missiles are not permitted.
Therefore, potential accidents that could generate
sufficient forces to cause sodium ejection or the generation of missiles
are emphasized. To do this, the conditions created within the reactor
core as a result of an accident must be studied along with considerations
of the paths that the accidents take through the system and the mitigating
3-7
effects of the structural components of core internals and other safetyfeatures. For this purpose, the results of our Phase I activities7 inthis contract are useful.
The B&W reference design was closely surveyed duringPhase I to identify basic fault and malfunction conditions that could
initiate accidents. The paths that these accidents could take throughthe system were traced by constructing fault tree diagrams. Six of
these diagrams, which are especially applicable to this study, a re re -
produced here for reference (Figures 6 through 11). Fi ure 6 is thetop fault tree diagram; the most important parts are identified by the
superimposed capital letter symbols: Part A shows the radioactivity
(R/A) released to the secondary containment resulting from a failure
of the cover structure (B) or failure of the cavity liner (C). Tracing
these trees via the triangular-shaped transfer symbols shows that the
causes of cover structure failure are (1) structural inadequacy, (2) hightemperature,. and (3) abnormal forces. Similarly, tracing the cavity
liner failure diagrams shows that these three causes and one additional
cause-melt-through (see Figure 11)-can lead to failure of the cavity
liner. The seal failure diagram (Figure 9) shows either seals or re-
tention·devices as causes of failure. In all cases a complicated seriesof events must occur, and the only apparent real causal event is the
large reactor accident. Howe ver unlikely, the possibility of large
explosive accidents and the resultant conditions must be considered
because of the consequences.Large accidents have been postulated for the LMFBR
primarily because of these design characteristics:
1. The fuel in the core is not arranged in the mostreactive configuration; therefore, inadvertent movement of the fuel can
lead to an increase in reactivity. For example, bowing of fuel due to
thermal effects may cause a power excursion resulting in overheating
of the coolant, molten fuel in the pins, and cladding failures.
2. The void coefficient of reactivity in sodium is positive,at least in the central region of the core. Loss of effective coolant
flow due to pump failures or mechanical blockage of the coolant flowchannels may therefore lead to overheating and, again, molten fuel or
cladding failures.
3-8
3. Molten fuel may be dispe rsed into the sodium in the
coolant channels during an excursion; depending on how the fuel comesli
into contact with the sodium, a rapid energy release may result.
The destructive component of the large core explosion
consists of three elements: (1) the shock wave, (2) the fluid momentum
due to the passage of the shock wave, and (3) the gas bubble pushing onthe fluid at high temperature and pressure. The magnitude of these
elements and the partition of the total energy is dependent on the size
of the exploding mass, its shape, and the rate of interaction, but each
element has the potential to cause failure of the primary containment.
The shock wave originates in the exploding mass andpasses into the surrounding medium as a steep-fronted compressionwave travelling at supersonic velocity. As the wave expands radiallyoutward its energy is rapidly dissipated. Beyond the close-in effects,
there follows a range of pressures in which the shock wave is sufficient
to vaporize the material behind the shock to a gas. Below the pressure
sufficient to cause this irreversible vaporization, the absorption of
energy in solids or liquids through the "waste heat" process is veryrapid.8 This rapid decay in pressure continues until it falls to the
dynamic crushing strength of the material. At this pressure the waste
heat mechanism becomes small, the wave becomes elastic, and pres-sure decays roughly as 1/R, where R is the distance from the center
of the blast.
The refore, in B&W's reference design, the large volume of
sodium in the pot system has a marked mitigating effect on the final
peak pressure of the shock wave at the wall of the primary tank and at
the interface of the cover gas. In fact, only about 10% of the originalenergy will remain, and coupled with further reduction in the gas spaces,the effect of shock waves on the primary containment may be negligible.However, for larger accidents the peak pressure of the shock wave at
the tank's wall may still be above the design limits, and the tank willfail instantaneously. In this case, the steel particles will acquire the
velocity of the liquid sodium which could impinge on the cavity liner.
In the vertical direction, the larger accident will produce a large shock
wave peak pressure at the sodium-gas interface, and although there
3-9
may be an initial spectacular drop in pressure, positive gains can bemade if the gap thickness is comparable to the positive phase length ofthe shock waves involved.9 For this case, it may be desirable for the 1primary tank to fail, thus producing a rarefaction from the break point,
moving back with the velocity of sound, which would literally tell the
energy in the tank that there was an escape route. Therefore, in bothdirections the shock wave energy must be considered as a potentialcause of containment failure.
The momentum of the sodium coolant due to the passageof the shock wave is the second potentially destructive element of the
core explosion. The major problem envisioned here is the impingingof sodium and steel fragments on the cavity liner following failure of
the primary tank due to the shock wave. Howe ver, we must also con-sider the spray effect created at the interface of the sodium and the
cover gas, and the possibility that the shock wave and the moving cool-ant will create missiles.
The third destructive element is the high-pressure gasbubble. For large accidents this has the potential to accelerate a
column of liquid sodium (sodium-slug) toward the cover structure,
thereby concentrating a large amount of the total explosive energy in
a single and most vulnerable component of the primary containment.
Depending on the magnitude, the impacting sodium slug could result in
excessive upward movement of the cover structure or in seal failures
that could not be tolerated for the following reasons:
1. Seal failure could permit sodium to be ejected intothe secondary containment.
2. Relative motion between the cover structure sections
could cause seal failure or open seal joints.
3. Upward movement of the plug section containing the
control rod drives could extract poison rods from the core and result
in a reactivity insertion (if the core were still intact) that could initiate
a second accident.
3-10
4. Relative motion between cover structure sections couldrupture the piping in the cover structure cooling system. This couldjeopardize the post-accident integrity of the cover structure.'
5. Upward movement of the outer, fixed cover structuresection could rupture the intermediate sodium coolant piping to the IHXsor the NaK piping to the normal decay heat removal system. Eithe rfailure could result in a large fire in the secondary containment.
The problem, therefore, is to (1) eliminate the possibilityof the accident that would produce the sodium slug, (2) minimize thefinal load on the cover structure, (3) provide a holddown device thatwould limit unacceptable movement, (4) design the damage-susceptiblecomponents so that the total movement doesn't affect their integrity,
and (5) provide backup systems and safety features for the ultimate
consequences. Again the refe rence pot system may offer advantagesthat influence the solution to these problems. In this system, no pres-sure vessel wall directly surrounds and directs the rising sodium slugtoward the cover structure. Without this restriction, the sodium willspill over the upper shield tank wall (Figure 2) into the rather largegas space beyond, and may greatly reduce the final slug energy impartedto the cover.
Another effect of expansion of the high-pressure bubble
is the creation of a higher than normal pressure in the system. Againthis final pressure will depend on the magnitude of the explosion, but
it may also depend on prior damage caused by the shock wave. Forexample, if the primary tank has failed a larger gas volume is availableand the pressure will be partially relieved. If the seals in the cover
structure are damaged to the extent that gas can leak to the secondary
containment, then the peak final pressure will depend on the rate of
leakage. Once again, primary tank failure may be desirable. On
the other hand, permissible gas leakage through the seals is a functionof the amount of radioactivity and the permissible leakage from the
secondary containment. Purposely designing the seals to fail at some
,=:t:EEMELTDOWNINADEOUATE HBAT INADEQUATE HEATSINK FOR THE SINK FOR THEINTERMEDIATE INTERMEDIATEHEAT EXCHANGER .IEAT EXCHANGERDUE TO LOSS OF IUE TO HIGHFLOW TEMPERATURE
Accidents that could jeopardize the integrity of the primarycontainment were discussed briefly in section 3. These accidents andthe effects on components of the containment are discussed further here
along with a discussion of candidate solution methods. The first three
sections deal with large accidents originating within the primary con-tainment's boundary, and the last section deals with accidents outside
the containnnent.
p 4.1. Biological Shield Line r
4.1.1. Introduction
The biological shield liner is a 3 /4-inch-thick carbon
steel, gas-tight liner assembled on the biological concrete shield.
Together with the reactor cover structure and interfacing seals, the
liner forms the primary containment boundary, which is the first bar-
rier against the release of radioactive materials to the secondary con-tainment. The conceptual design of the line r and its functions are
completely described in the Follow-On Study report, 1 and an abstracted
summary description is presented in Appendix A. Figure 2 shows the
location, the shape, and the relationship of the liner to the primarytank (indicated by the superimposed symbol 20).
As designed, the primary tank is suspended inboard
from the shield liner, leaving a gap of approximately 1 foot. This gappermits the use of the reference circulating argon gas system, whichreduces the heat load in the biological concrete and allows sufficient
space for periodic inspection of the exterior surface of the primarytank. Of greatest importance here, the gas gap also has a markedmitigating effect on shock waves originated by core accidents, and it
provides space for the installation of energy barriers if they should
be required.
4-1
The primary objective of this study was to identifycandidate safety features that are required and that can be incorporatedinto the refe rence design to protect the integrity of the shield liner and
therefore the integrity of the primary containment. Five accident-causedconditions have been considered along with certain combinations of them:
and (5) pressure. Each of these conditions and the problems of main-taining the integrity of the shield liner are discussed.
4.1.2. Shock Waves
4.1.2.1. Problem Discussion
If explosive reactor core accidents are consid-ered credible in the reference LMFBR, or if such accidents are defined
as DBAs, then the first resultant disturbance to be considered in assess-ing the reactions of the primary containment is the shock wave effect.As a result of the explosion process, the initial mass of material will
be converted into a gas at a very high temperature and pressure. Thepressure wave in the reacting explosive material arrives at the boundaryof the surrounding medium, and the pressure immediately begins to berelieved by an intense pressure wave and outward motion of the medium.Once initiated, the disturbance is propagated radially outward as awave of compression; the steep-fronted wave is described as the "shockwa ve. 1/
In studying the effects on the primary contain-
ment, the reflected pressure of the shock wave from the primary tank'swall is the important characteristic. If this pressure is small and noplastic deformation results, then there is obviously no damage and no
danger to the shield liner. A shock wave is transmitted through theannular gas space, but its strength is negligible. If the reflected pres-sure is sufficient to produce plastic deformation of the tank's wall but
no failure, then the subsequent conditions inside the primary tank be -
come important. In this case, pressure failure or pressure-induced
creep failure become possibilities because of the weakened condition
of the tank, and protection for the shield liner may be required.The third pressure condition that could result.
is that in which the reflected wave pressures on the tank's Wall are
4-2
found to exceed any strength that can reasonably be expected to be de-
signed into the tank. The tank would fail instantaneously, thus acquiringthe velocity of the liquid sodium which could impinge on the shield liner.For this case, protection may have to be added to prevent the liner'sfailure. This is discussed further in section 4.1.3.
As pointed out above, the important character-istic of the shock wave is the reflected pressure on the primary tank'swall. This pressure is not only a function of the distance of the tankfrom the center of the explosion, but also a func tion of the total energyrelease, the size and shape of the exploding material, and the time ofburning. Since this information was not available for the study, themajor problem was how to determine a range of pressure conditions onthe tank's wall that would include actual conditions.
4.1.2.2. Approach
Because the characteristics of reactor accidentsand resultant energy releases are not available, underwater explosioncalculational methods were used to estimate the effects on the primarytank and the liner. These methods use TNT charges exploding under-
water, and the time duration of the energy release assumed is thatwhich produces the highest pressures. The methods are considered
applicable to the sodium system since liquid sodium exhibits physicalcharacteristics approximating those of water, at least in the range ofpressures to be expected. Within this range, liquids behave essentiallyalike in overall aspects and are essentially insensitive to the type ofexplosion (TNT-like or point source).
Ove rall, the methods used should produceconservative estimates of the loads on the primary tank. This assump-tion is supported by the work of several persons, including that of
10J. R. Bohannon, Jr., and W. E. Baker. In determining blast effectson the AF-NETR containment structure, they concluded that the structureis safer than required, and that it "would probably contain a nuclearexcursion several times the total energy release of 1000-MW secondsbecause of the drastically reduced damaging capabilities of the relativelyslow nuclear excursions when compared with explosives. " It should be
pointed out that damage estimates are only accurate at distances
4-3
Figure 12. Peak Pressure of Shock Wave
36,000
® W = 50 lb TNT = 9 X 104 Btu = 9.5 X 107 J = 95 MW/s34.000- W = 100 tb TNT = 18 x 104 Btu = 19 X 107 J = 190 MW/s
32,000 -, , c W= 300 1b TNT=54x 104 Btu = 57 x 107 J=570 MW/s
eew = 1000 lb TNT = 180 x 104 Btu = 19 x 108 J = 1900 MW/s
1.13
26,000- /Vvl/3\P = 2 16 x 104 -1 -FAE4 24,000
peak' R)
3 -O. wherelb, x 24 W = weight of TNT,
22,000 - - S./ R = radial distance from 4, Z4 20,000 - center of explosion. o g
i Q 6 0A 18,000- C
E 16,000 - r „ El-4 - 3\ 0/3 14,000 - 3 0.* f 12,000 3 5 7 \ i\ -0- : . e1.--
Ov E \3 - 8 4 K10,000 - 1 4 k 41 m- 0 1 - a e:8. 0
(1)
8,000- ink - 1 2-44 4 +1 0- .§ .---3 -- Ak / U at -0
6,000-0 - al- i IM U..----8 --.9u' & 0 C· · 5 --...- ABO O- 4 E.-I....A
4,000- B:8, u, Al 83 e 0 . ...-- 40--0z 8, 8'3 > 4 k-- &-82,000-AX|
A 866 8 A0 1 1 1 1 I l i l l i
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Radial Distance (R) From Center of Explosion, ft
approximately 10 charge radii from the explosion's cente r and are not
accurate close to the center. At the larger distances, the strength andthe duration of the primary shock wave are primarily dependent on thetotal energy available.
The equations and related theories used are
those given by Monson and Sluyters and by Cole. 11
The analytical approach was to calculate the
peak pressure of the shock wave resulting from the explosion of 50, 100,
300, 500, and 1000 pound charges of TNT at the center of the reactor
core. The following equation from references 8 and 11 was used:
prn = 2,16 x 104 *12 1.13 (1)
whe re m = Peak pressure of the shock wave at distance R from
the center of the explosion, psi,W = charge of TNT, pounds,R = radial distance from the center of the explosion, feet.
The results of the various calculations are
plotted in Figure 12. The peak pressures associated with the shock
wave are shown as a function of the distance from the center of the
explosion. For example, a peak pressure of 240,0 psi occurs at the
primary tank's wall, at a radial distance of 26 feet from the center of
the explosion, as a result of 50 pounds of TNT exploding. Similarly,·
the peak pressure is 7300 psi for a 1000-pound TNT charge exploding.Due to the waste heat process the equation accounts for the rapid ab-
sorption of the shock wave energy as the wave propagates radially out-
ward from the explosion's center through the large volume of sodium.Howe ver, the equation doesn't account for energy lost to intervening
structures. This makes the peak pressures at the primary tank's wall
more conservative because in the actual case, whenever the shock wave
meets an interface between different materials, part of its energy would
Be lost, either to a reflected wave or in crushing of materials.To assess the effects of the shock wave on the
biological shield liner, the gas space between the primary tank and the
shield liner becomes important. Two conditions were assumed for the
4-5
primary tank: ( 1) non-failure of the tank wall due to the pressure ofthe shock wave, and (2) instantaneous failure of the wall due to thepressure. If the tank does not fail, then most of the energy is reflected
back toward the. center of the core or dissipated in deforming the tank.The remaining energy is transmitted through the wall into the annular
gas space. If the tank does fail, then some of the energy is again re-flected back but most of it remains in the shock wave as it enters thegas annulus. In either case, the acoustic mismatchB between the argonand the sodium will result in an enormous drop in pres sure as the shock
wave enters the rarified medium (several hundred-fold reduction in
pressure). (The acoustic velocity in argon is approximately 1400 ft/sand in sodium is approximately 9000 ft/s.) The final strength of theshock wave is negligible when it reaches the liner.
4.1.2.3. Conclusions and Candidate Solutions
The major objective of this effort has beento determine whether the shock waves generated by large core explosionsjeopardize the integrity of the biological shield liner. Based on the
results of our scoping calculations and on an evaluation of the literatureon the shock wave phenomenon (see Appendix C), we have concludedthat the argon annulus between the primary tank and the liner providessufficient protection against direct shock wave damage. Failure of the
liner is therefore not anticipated unless the accidents are larger thanthose assumed, or unless more rigorous calculations show that the
energy is not dissipated to the degree shown here.
If larger accidents or shock wave energiesare subsequently calculated, then some additional safety features may
be required to protect the biological shield liner. Two possible candi-
date methods were considered: (1) energy-absorbing material close tothe core, and (2) energy-absorbing material on the face of the liner.In the first method, closely packed crushable tubes could be placed atthe core's radial boundary (see Appendix B), say, by redesigning theradial neutron shield assemblies. Another way would be to place thetubes just exte rnal to the shield tank, but it would be ve ry difficult todetermine the size, shape, and location of the explosion accuratelyenough to permit a good design. In addition an energy absorber placed
4-6
2$
close to the explosion's center would encounter a very high shock wavepeak pressure and might be difficult to design. Also, to reduce the
energy level sufficiently to protect the liner, the energy absorber mightcontain the core debris in an undesirable manner. In other words, itmight be better to allow the= core to come apart rather than to compressthe debris against a boundary structure. Still another consideration isthat placing the energy absorber near the core might prevent the rupture
of the primary tank due to the shock wave. Although this rupture may
not always be desirable, some accidents, due to the expansion of the
gas bubble, might result in final pressure conditions in the nonfailed
tank that could otherwise be relieved if the tank had failed first. All
of these problems need serious study, but it may be necessary to placesome amount of absorber in this area; hence, the crushable tube method
is considered as a candidate.
The second candidate method to protect the
biological shield liner involves placing energy-absorbing material in
the annular gas space and possibly directly on the face of the liner.This would take full advantage of the large energy absorption that occurs
during propagation of the shock wave through the pool of sodium, but
some caution must be taken so that the dissipation of energy in the gas
space is not lost. The method is also a candidate solution to protectthe liner against an impacting mass as discussed in section 4.1.3.
4.1.3. Impacting Mass
4.1.3.1. Problem Discussion
The peak pressures at the primary tank's wall
due to the shock waves generated by large accidents are given in the
previous section and shown in Figure 12. Because of the conservatism
in the calculational methods used, the pressures shown will probablybe lower than those given, but as reported by Porzel, 9 "upon meetingthe steel walls the pressures are increased several-fold by the reflec-tion process. " Very generally, then, the pressure on the tank's wall
could be so high as to offer little hope of containing the explosion within
the tank. In other words, in large explosions the tank will probablyfail because of the shock wave unless much of the original energy is
dissipated in inte rnal structures designed for that purpose.
4-7
If instantaneous failure of the primary tank
occurs, then the radial biological shield concrete and its liner will besubjected to the kinetic energy of an impacting mass. This impactingmass could be a composite mixture of primary coolant and steel from
the tank's wall, which is accelerated across the annulus between the
primary tank and the liner with an initial velocity equal to the particle
velocity of the primary coolant. This composite mass will produce adynamic pressure upon impact against the liner and, unless the kinetic
energy is absorbed by intervening structures or crushable materials,could damage the liner or the biological shield severely enough to pre-vent their functioning. For example, if the liner failed, then the sealingand concrete protective functions would be lost, and the biological con-crete may subsequently lose its shielding function because of the damageit undergoes while in contact with sodium. Or, if the kinetic energy ofthe impacting mass is large, then structural damage to the concrete
could directly result in loss of the shielding and supporting functions.
Therefore, if the primary tank failed instantaneously due to the shock
wave effect, means may have to be provided to protect the biologicalshield liner and the concrete against the impatting mass.
4.1.3.2. Approach
If instantaneous failure of the primary tankdue to the shock wave effect is a real problem, then it is important toknow what conditions the biological shield concrete and its liner will
be subjected to. Therefore, our approach has been to calculate the
velocity of the impacting mass and the resultant dynamic impact pres-sure on the liner for the range of accidents considered in the shockwave study.
Based on the work of Monson and Sluyte rB and
Cole, the sodium and the steel particles of the tank's wall at the fail-11
ure point will acquire the particle velocity of the liquid sodium whenthe instantaneous wall failure occurs. Using the following equation, a
conservative estimate of the mass ve16cities was calculated for the 50-
and 1000-pound TNT charge explosions:
4-8
11 - 2 mgc '2)P C
0 0
whe re 11 - particle velocity of the primary coolant = velocity ofimpacting mas s,
pm = peak pressure in shock wave at the primary tank's wallbounda ry (value shown in Figure 11),
Po = density of primary coolant = 51 lb/ft3,C = acoustic velocity of shock wave in primary coolant =0 9000 ft/s.
The approximate mass velocity is 48 ft/s for the 50-pound charge and
147 ft/s for the 1000-pound charge.Based on these results, the dynamic pressure
on the biological shield liner was calculated for each case. It wasassumed that the final velocity of the mass was equal to its original
velocity, that the mass was one-third steel and two-thirds sodium, andthat gravity had no effect. For this calculation, the following equation
from refe rence 8 was used:
2
pd = 1/2 p E- (3)
0 gc
whe rePd = dynamic impact pressure,
po = densityof primary coblant and structural material
mixture = 200 lb/ft3,P = mass velocity calculated using equation 2.
For the 50- and 1000-pound TNT accidents, the dynamic pressures on
the shield liner were calculated as 50 and 466 psi, respectively.
4.1.3.3. Conclusions and Candidate Solutions
Using the foregoing values of dynamic pressure,
the possible damage to the liner and other plant components was assessed.Owing to the conceptual nature of the design, good numerical answers
were not obtained, but it was concluded that some sort of failure could _
occur. Three hypothetical failure modes were considered:
4-9
1. If the liner was supported on structural beams with an
air gap or insulating mate rial behind it, then the allowable stress limitsin the plate could be exceeded.
2. If the emergency decay heat Annoval piping was directlybehind the liner wall, then the stress limits could again be exceeded
and the possibility of pipe failure could exist.
3. If vertical support beams were positioned just behind
the liner, then the dynamic loading could be sufficient to cause failure.Although hypothetical and dependent on a more. detailed design, these fail-
ures would not be acceptable, and some sort of protection should be
added, at least for the larger accidents.
A literature survey was conducted.to findcandidate energy absorber mate rials and designs that could fulfill the
requirements of the reference design.. For the largest accidents, crush-able energy absorbers, such as those used in EBR-II8 or in the machinery
12dome on. Fermi, appear superior for several reasons:
1. They can be laminated in layers, each layer absorbinga portion of the impact energy before crushing of the next layer; thus
large amounts of kinetic ene rgy are absorbed.
2. They·can be constructed of materials compatible withsodium and its heat transfer characteristics, so that the post-accident
cooling function will not be seriously impaired.
3. They have design crushing strengths of approximately350 to 800 psi. 12
4. They will crush up to 75% of their original thickness
without exceeding their design crushing strength.
5. They can be assembled in compact arrangements to fitin areas such as the annulus between the primary tank and the liner,thus presenting minimum obstruction to inspection equipment requiredfor the pe riodic inspection of the primary tank.
For smaller accidents, such as the 50-poundTNT explosion, crushable material may not be the most economicaldesign solution, and a simpler steel baffle may be sufficient.
4-10
This could be a c6rrugated design applied to the liner so that it candeform to absorb energy but can also redirect the impacting mass toavert damage to the liner. The concept has three other advantages:
1. Essentially no interference with the normal gas systemin the annular space would result.
2. Less of the annular space would be used; the refore,there would be minimum interference with inspection or surveillanceprocedures.
3. It would not act as an insulator to prevent proper
operation of the emergency decay heat removal piping system located
behind the liner.
In the event that these candidate solutions are
not sufficient for the impacting mass condition, or if sufficient materialcannot be located in the annular space between the primary tank andthe liner, an alternate method may be required. This would most likelyinvolve the placement of structures or energy-absorbing material insidethe primary tank. The objective, of course, would be to reduce the
impact pressure by reducing the shock wave energy. This potentialsolution was discussed in detail in considering the shock wave (section
4.1.2.3).
4.1.4. Thermal Shock and High Pressure
4.1.4.1. Problem Discussion
In section 4.1.1, the rmal shock and highpressure were given as two of the conditions that could result in failureof the biological shield liner. Separately and combined these conditions
influence the design of the liner; possible problems are discussed here.If the primary tank fails due to the shock wave
pressure as.discussed in section 4.1.3, then the biological shield liner
will be subjected to an impacting mass as well as.to a thermal shock.
This shock will be caused by the 800 F sodium from the primary pool
striking the liner, which is normally at 200 F. Although deformation
of the liner will not be detrimental by itself, we must be assured that
the total stresses and the elongation of the material do not result in
4-11
rupture of the liner. Special attention must be given to liner attachmentpoints and to piping joints serving the annulus cooling system.
A second thermal shock case is set up if failureof the primary tank is catastrophic and the complete annulus betweenthe tank and the liner is suddenly filled with hot sodium. A similarsituation would result when the intermediate coolant system sodium
was dumped into the annulus to effect post-accident decay heat removal
(a reference safety feature for cases where the accident does not causea tank failure that flooded the annulus). Again we must be assured thatthe liner does not rupture.
The accidents that cause failure of the primarytank may also result in an abnormally high primary system pressuredue to the expansion of the explosion gases. The final equilibrium pres-
sure may be retained within the primary containment's boundary for a
long period of time. This factor must be considered along with thethermal shock and gradient conditions set up by the hot sodium on theline r.
4.1.4.2. Approach
To assess the consequences of thermal shock
on the biological shield liner, preliminary estimates of the thermalstresses were made to determine the liner's response when suddenlyexposed to 800 F sodium. For the carbon steel liner, the numericalanalysis was based on a hollow cylinder heated internally and insulated
externally with the biological shield concrete. The boundary tempera-ture increases suddenly at t=0 from T= 150 F( reference liner tem-
perature is 200 F) to T = 800 F assuming an infinitely large heat trans-
fer coefficient. The maximum stresses were found to be 59, 726 psi at
the insulated surface and -186,643 psi (compression) at the heated
surface.
The thermal stresses were calculated assum-
ing that the material was not stressed above the yield point. Since the
calculated values do exceed the yield strength of the nnaterial, plasticdeformation will occur. The percentage of elongation was calulatedas 0.43%, which is well below the percentage elongation to rupture for
carbon steel (39%).
4- 12
The final pressure in the system after a majoraccident has also been considered to dete rmine whether it could result
in damage to the liner. Again this is an accident-dependent problem,and the primary tank must fail before the liner is subjected to thepressure. It was not possible in this study to calculate the pressures
because detail knowledge of the explosion is required. Computer codes,
such as BANG or ASP RIN, will have to be run to follow the expansionprocess accurately. The capabilities of the liner were assessed, how-ever, to determine a maximum pressure limit for the refe rence design
and to study solution methods for higher pressures.
Assuming no support by the concrete, it was
determined that the refe rence liner could be designed to contain final
pressures up to about 50 psi without requiring excessively thick material.
Obviously, the 6-foot-thick concrete on which the liner is mounted and
its reinforcing steel will increase this allowable.
4.1.4.3. Conclusions and Candidate Solutions
The major objective of this effort was to
determine whether thermal shocks or high pressure resulting from
major core accidents could lead to failure of the biological shield liner.
The results are somewhat lirriited at this time because sufficient knowl-
edge of the accident conditions is not available, but the followingconclusions and candidate solutions are given.
An extreme case of thermal shock was consid-ered wherein the interior surface of the liner was assumed to be wet
by 800 F sodium while the temperature of the exterior surface was held
at 150 F. The results show that rupture is very unlikely although the.. ,
liner will deform under this condition. This case«did not consider the
pressure in the containment, but the combination shouldn't present a
serious problem unless the final pressure is very high.
During the study of hot sodium striking the
liner, two potential problem areas were uncove red: (1) liner attachment
points, and (2) piping attachments that serve the normal annular space
cooling system. The primary tank could fail so that the hot sodium
striking these attachments resulted in stress concentrations and failures.
4-13
This possibility could not be considered adequately because of the con- 'ceptual nature of the design, but two conclusions have been made:
1. All attachments and openings for the liner must beadequately reinforced to prevent rupture due to thermal shock.
2. Adequate means must be provided to compensate forcircumferential and axial thermal growth due to flooding the annular
space.
In studying the possibility of hot sodium cominginto contact with the liner, the use of insulating material was consideredas a possible solution. However, when applying insulation to the linerto protect it from the rmal shock, provisions will have to be included toprotect the insulation from impact and from the effects of sodium. Thelatter is required because the types of insulation available for applicationto the reference design are soluble in sodium. In addition, the use ofinsulating materials to protect the liner seriously impairs the post-acci-dent decay cooling of the reactor core materials. This is due to thereduced thermal communication between the post-accident decay heatremoval system and the flooded primary containment.
High pressure in the primary containment
following an accident may be one of the most serious problems to besolved. No satisfactory solution evolved from this study, but prelimi-
nary conclusions can be made:
1. The concrete and reinforcing steel for the biologicalshield should be designed to support the liner, and together they mustretain the final containment pressure.
2. The as annulus between the primary tank and the lineris an important pressure relief volume. The final pressure in theprimary tank following an accident will be reduced when and if the tankfails. Based onthe ratio of gas volumes (25,000 ft3 cover gas and7500 ft3 annulus), this would reduce the pressure by 30%.
3. Pressure may also be relieved by gas leakage throughthe seals in the cover structure or by escape through the pressure reg-ulating valves in the reactor blanket gas subsystem. Although leakage
4-14
through the seals is not planned, the seals could be damaged by earlieraccident effects, or the final pressure may exceed the design limits.
4.1.5. Missiles
4.1.5.1. Problem Discussion
If explosive reactor accidents occur, or ifsuch accidents are defined as DBAs, then the problem of missiles and
their effects on the primary containment's radial biological shield andits liner must be considered. This is necessary because during the
explosion process a high-energy shock wave is produced in the primarycoolant and travels radially outward at an acoustic velocity of approxi-mately 9,000 ft/s. Thus, owing toits high peak pressure, the shock
wave is capable of accelerating components or fragments of structural
materials submerged in the primary coolant radially outward toward
the biological shield and its liner. Therefore, the reference design,in which all of the core components, reactor internal components andtheir respective supporting structures are submerged in the primarycoolant, will have to be evaluated to ascertain the potential for missiles
capable of damaging the radial biological shield or its liner. Thus, itbecomes necessary to answer these questions:
1. Is the energy associated with the shock wave suffi-
cient to accelerate missiles so that they achieve a high enough velocityto damage the radial biological shield and its liner upon impact?
2. Is the deceleration due to the primary coolant resist-
ing the outward trajectory of the missile sufficient to reduce its finalvelocity to the extent that no damage occurs upon impact?
3. If missiles can impact against the radial biological
shield and its liner, what type of energy absorbers or missile shields
are required for their protection?
4. Can the energy absorbers already proposed to miti-
gate the effects of an impacting mass also provide for adequate missileprotection, or is no protection against missiles required?
As mentioned above, the important character-
istics of the core accident to be considered under this task are the
4-15
problems of missiles and the resultant damage to the radial biologicalshield and its liner upon impact. However, since the input informationfrom the accident analysis effort, which describes the core accidentand its energy release, was not available, only scoping calculationswere Inade to assess the problem of missiles in B&W's reference design.Thus, one of the major problems was how to determine a range of explo-sions that will produce shock waves capable of generating missilessimilar to those that may be produced under actual conditions.
4.1.5 .2. Approach
Because the characteristics of the nuclearexcursion and its energy release were not available (as noted previously),underwater explosion calculational methods were used again. These
equations, as stated before, should give conservative results when usedto calculate the various energy releases occurring from the assumedcore accidents. Thus, assumed energy releases and the resultantshock waves were used to approximate missile velocities, their impact,and the resultant structural damage to the radial biological shield andits liner.
The analytical approach utilized the previouslycalculated peak pressures of the various shock waves for 50-, 100-,
300-, 500-, and 1000-pound charges of TNT at the center of the reactor
core. Figure 12, also used as part of this calculation, plots the peak
pressures of the various shock waves as a function of their radial dis-
tance from the center of the explosion. Therefore, the peak pressureof the shock wave acting on a core component or structural fragmentlocated anywhere between the cente r of the explosion and the primarytank wall can be taken directly from the graph for input to the followingequations (given in reference 8):
PAtB = 33- (4)
wherep = initial velocity of missile or component at the end of
its accelerating interval,
A = cross-sectional area of the missile or component,
4-16
t = time required for shock wave to travel the height orthickness of the component or missile,
P = peak pressure of the shock wave at a designated distancefrom the cente r of the explosion and for a given TNTcharge (shown in Figure 11),
M = mass of missile or component.
Using this equation, the initial velocities of
several missiles and components were calculated. Howe ver, it wasfound that input values for the peak pressure of the 1000-pound TNT
shock wave were required before significant missile velocities could
be calculated. This was due to the deceleration of the missile caused
by the large volume of sodium surrounding the core combined with the
rapid dissipation of the shock wave energy in this same volume of
sodium. The deceleration of the missile during its deceleration period
was calculated from
2
= -CDPAp.(5)dt 2M
where du- = deceleration of the missile or component,dt
CD = sodium drag coefficient, which is assumed to equalunity for this problem,
A = cross-sectional area of missile or component,
p = initial velocity of missile or component, calculatedusing equation 4,
M = mass of missile or component,
p = density of sodium surrounding core.
Using equations 4 and 5 and assuming tha tthe primary tank failed before the missile breached the primary tank's
boundary, three types of missiles were investigated. For each investi-
gation only the 1000-pound TNT shock wave peak pressure values fromFigure 12 were used. Next, equation 4 was used to calculate the initial
velocities for two cases at various distances from the explosion's center.
(Case 1, 3-inch-diameter steel sphere; Case 2, 6-inch-diameter steel
sphere. ) The results are tabulated as follows:
4-17
Distance of missile Initial velocityfrorn Peak pressure of of 3- or 6-lb
center of explosion9 ft shock wave, psi rnissile, ft/s
4 60,804 95.5
8 27,700 43.5
12 18,000 28.2
16 12,800 20.1
20 10,000 15.7
24 8,200 12.8
26 7,300 11.5
These initial velocities and equation 5 werethen used to calculate the various initial decelerations for both the3- and 6-pound spherical missiles. The results are as follows:
Distance of missile Deceleration of Deceleration offrom 3-lb nnissile, 6-lb nnissile,
center of explosion, ft ft/s 2 ft/s 2
4 -2838.1 -1419.2
8 -588.8 -294.5
12 -247.5 -123.7
16 -122 -61
20 -76.7 -38.35
24 -51 -25.5
26 0 0
Inspection of these initial decelerations
indicates that missiles of the type represented by cases 1 and 2 will
not excessively damage the liner. However, to assess the problem of
missiles and their impact, it was assumed that the 6-inch-diameter
spherical missile located 4 feet from the center of the explosion strikes
the liner with a final velocity equal to 75% of its initial velocity.(
4-18
The resultant impact force against the liner was calculated as = 2600
foot-pounds. Next it was assumed that the missile impacts against theliner with a knife-edge equal to 2 inches in length and causes a shearingaction similar to that occurring when metal is pierced in a blanking die.The shearing strength of the liner was assumed to be 40,000 psi, anda shearing area of 1.500 in. 2 was calculated. Then, multiplying the
shearing strength by the shearing area produces a shearing force of
60,000 pounds. This is the force required for a shearing edge of thelength noted to completely penetrate the liner of the reference design'sradial biological shield. Next, dividing the impact force (2600 ft-pounds)by the shearing force (60,000 pounds) shows that the assumed missilewould penetrate the liner to approximately 0.52 inch before its kinetic
energy was dissipated.For the third case, it was assumed that a
1,000-pound lower radial neutron shield located 10.5 feet from the
center of the explosion becomes a missile. Equation 4 was used to
calculate the initial velocity when the neutron shield is in the vertical
position. The result was 111 ft/s. Next, it was assumed that the
neutron shield rotated to the horizontal position and impacted againstthe liner of the radial biological shield at a final velocity equal to one-
half the initial velocity. This was necessary because, unless the neutron
shield rotates to the horizontal position, the deceleration calculated
with equation 5 limits the radial movement of the shield to approximately1.8 feet when it stays in the vertical position. Using a final velocity of
55.5 ft/s, the impact force against the liner was calculated to be= 47,836 foot-pounds. It was then assumed that when the neutron shield
impacts against the liner, the outer edge of its hexagonal shape causesa shearing action similar to that of a die punch. The shearing strengthof the liner was again assumed to be 40,000 psi, and a shearing areaof approximately 29.9 in. 2 was calculated. Thus, a shearing force ofapproximately 1.2 X 106 pounds was calculated as the force requiredfor a hexagonally-shaped missile, such as the neutron shield, to pene-trate the liner completely. Dividing the missile's impact force by the
shearing force shows that the assumed missile in this case would onlypenetrate the radial biological shield liner to approximately 0.47 inch
before its kinetic energy was dissipated.
4-19
4.1.5.3. Conclusions and Candidate Solutions
The possibility of missiles occurring in thereference design reactor with impact velocities sufficient to damagethe biological shield liner has been shown to be very small. In consid-ering the likelihood of missile damage, much effort was required to
find a missile source and to accelerate it fast enough to bring it intocontact with the liner. It had to be assumed that the missile was oriented
ideally and that it would not strike any intervening structures, includingthe primary tank. Even with these very conservative assumptions, themissiles were not sufficient to penetrate the liner completely.
Although none of the missiles investigatedresulted in failure of the liner, it is possible that the impact and shear-
ing loads may be combined with other accident loads to imperil theliner's integrity. Also, the liner 's support system (see section 4.1.3.3)must be considered if complete support is not provided by the concrete,as assumed in our study.
Based on the results of this study and informa-
tion from the survey of literature on missile protection in similar designs,it has been determined that no missile protection is required for thereference design. However, since there is a small probability thatmissiles and combined loads could endanger the liner, candidate methods
of solution were investigated.Positive protection of the liner against missile
damage can be ensured by providing a missile shield to absorb impact
energy and deflect the missile away from the liner. Based on the
calculations presented in the design approach, the shield could be
fabricated from carbon steel of about 3/4 inch and mounted in the pri-mary tank's cavity between the primary tank and the liner. A radialgap of approximately 3 inches should be provided between the missile
shield and the liner to ensure that the post-accident cooling function
is not impaired and that no impact loads are transmitted from the
shield to the liner. Mounting provisions for the missile shield could
present a design problem because in all probability the shield will be
attached to the liner. Therefore, mounting attachments will have to
be designed to preclude damage to the liner when a missile impacts
4-20
against the shield or when both the shield and the liner are subjected tothermal shock and the impacting mass. Section 4.1.3.3 presented asimilar candidate solution for protection against an impacting mass; a
single design could serve both purposes if required.An alternate design approach would be to use
an energy absorber material to eliminate the possibility of damagingthe liner. Again, a candidate method of solution for impacting mass
protection has been given in section 4.1.3.3, and a single design could
be considered for both purposes. The energy absorber material for12the Fermi design is especially applicable for missile protection. For
example, an energy absorber with a crushing strength of 90Q psi anda thickness of 8 inches would absorb a missile impact force of 47,836foot-pounds after crushing approximately 5.6 inches. It should be pointedout that a requirement for this energy absorber can affect the perfor-mance of the post-accident decay heat removal system and the operation
of the normal gas cooling system in the annular space. In addition,the crushable energy absorber could require an increase in the diameter
of the primary tank's cavity. This increase in turn could require anincrease in the capacity of the closed-loop argon cooling subsystem,
and this change in volume will have to be considered to ensure that the
reactor core never becomes uncovered. Although these problems seemserious, they are primarily design problems and the crushable materialis considered as a candidate method of solution.
In addition these two methods of protecting theliner could be used in combination. This solution would combine the
high-kinetic-energy absorbing capabilities of carbon steel with the
crushing ability of the crushable energy absorber material. For exam-
ple, a 1/2-inch-thick carbon steel plate laminated to a 3-inch-thick
crushable energy absorber is capable of protecting the liner against
impact from any mis sile investigated. The laminated structure could
be mounted in contact with the liner to eliminate the mounting problemsassociated with an independent missile shield. Also, the laminated
structure could be mounted in the annular space between the primarytank and the liner. This feature would eliminate the problem of increas -ing the diameter of the primary tank's cavity if the crushable energy
4-21
absorber was used. Although this design would downgrade the post-accident cooling system's performance, it should not present a serious Iproblem.
4.2. Reactor Cover Structure
4.2.1. ·Introduction IThe reactor cover structure is a gas-tight, stainless steel,
cylindrical structure with a depth of 10 feet and a maximum diameter of 59 feet. It is filled with 6 feet of serpentine aggregate with an 8-inch
covering layer of steel shot for shielding. The assembly mates with
the primary tank and is supported by and attached to the concrete of the
radial biological shield. Together with the radial biological shield's
liner and interfacing seals, the cover structure forms the primarycontainment boundary, which is the first barrier against the release
of radioactive materials €o the secondary containment. The conceptualcover structure design and its function are completely described in the
Follow-On Study report, and a summary description is presented in
Appendix A. Figure 2 shows the relationships between the cover
structure, the primary tank, the 18-inch cover gas space, and the core
holddown assembly. Figure 13 is a conceptual arrangement of the cover
structure assembly.In the reference design, the reactor cover structure sets
on and seals to the top flange of the primary tank and extends radiallyoutward beyond it. The cover structure 's holddown studs are anchored
to the concrete of the radial biological shield outboard of the sealingsurfaces. The main body of the cover structure extends down into the
primary tank opening for approximately 4 feet and forms an interface
with the cover gas over the primary sodium. This 18-inch cover gas
space between the high-temperature primary coolant and the bottom
surface of the cover structure is an important design feature because
it reduces the severity of shock waves originated by core accidents.
The primary objective of this study was to' identifycandidate safety features that could be incorporated into the reference
design to protect the integrity of the reactor cover structure and thus
the integrity of the primary containment. Five accident conditions
causing accidents were considered along with certain combinations:
4-22
Figure 13. Conceptual Arrangement of CoverStructure Assembly
:C'.Itt.
..=lmtr.
7.„1 1
'46..-346.
tzu,704 COOLANT p"P Se'.24""POR=7 4:=/Aa/0-,i= till".<.... 4.3.VITU.. - COAC . r cON,40. AOD OR,Ve i INriAMED,Ard Hur
(1) shock waves, (2) missiles, (3) pressure, (4) sodium slug and (5)internal .and external temperature effects. Each of these conditionsand the problems of maintaining the integrity of the cover structureunder each condition are discussed in the following sections.
4.2.2. Shock Waves
4.2.2.1. Problem Discussion
If explosive reactor core accidents are consid-ered credible in the reference LMFBR, or if such accidents are defined
as DBAs, then the shock wave effect is the first disturbance to be con-sidered in assessing the reactions of the reactor cover structure. Theshock wave phenomenon discussed in sections 3.1.2 and 4.1.2 should bereviewed.
In studying the effects of shock waves on thecover structure, the pressure reflected at the interface of the covergas and the primary coolant is an important characteristic. Althoughthe shock wave. will be transmitted across the gas space, an enormous
drop in pressure will again occur as the wave enters the rarified medium.The final strength of the shock wave at the interface of the cover struc-ture will be negligible unless th1 wave's original strength is extremely
high. If the pressure reflected at the cover structure is small, thenthere is obviously no danger of failure.
If the pressure at the sodium-gas interface issufficiently high, then a mass of sodium, mostly in the form of a spray,will be propelled' upward. Depending on its original velocity, this masscould impinge on the bottom surface of the cover structure and thus
cause damage. This danger and the danger of shock wave damagemust be seriously considered because of the possibility of a sodium
slug impacting on the cover soon afterward. This spray event is dis-cussed further in the following section, and the sodium slug event isdiscussed in section 4.2.5.
4.2.2.2. Approach and Conclusions
Because the characteristics of the reactor
accidents and their energy releases were not available, the underwater
explosion calculational methods were used to estimate the effects of
4-24
the shock wave on the cover structure. The equations and the shockwave theory of Monson and Sluyters and Colell were used (section 4.1.2.2).The resultant peak shock wave pressures at the cover gas inte rface are
shown in Figure 12. The distance from the bottom surface of the cover
structure is shown, but the large pressure drop in the gas space is not
reflected in the pressure,distance curves. The values of the peak
pressures at the gas interface and equation 6 were used to calculate
the particle velocity of the sodium behind the incident wave:
B. 21 '61
whe rep = particle of the sodium behind the incident wave within
the sodium,pm = peak pressure at the interface of the primary coolant
and the cover gas,
Po = material density in the undisturbed primary coolant,
Co = acoustic velocity in the undisturbed primary coolant.
The particle velocities are tabulated as follows : \
Using only the 1000-pound TNT case from the tabulation, in order to
be conservative, and the information quoted below from refe rence 8,
the maximum spray particle velocity within the argon cover gas was
calculated as 177.8 ft/s.
"Upon arrival of the sodium shock wave at the sodium-argon interface, reflection as a rarefaction wave occurs.This reflection of the incident wave throws upward a mass
4-25
of sodium, mostly in the form of spray. . . . The particle ivelocity of this spray is a function of the particle velocitybehind the incident wave within the sodium. "
( Since the peak pressure of the primary coolantat the interface is known, the latter velocity as determined fromequation"6 is 88.9 ft/s. )
"Because the reflected wave is of strength about equalto that of the incident shock, superposition of the twowave s. causes the sodium particle velocity, B, to doubleupon entry into the argon. "
Next, the peak pressure exerted against thecover structure by the shock wave action in the argon cover gas wascalculated, again using equations frorn reference 8.
Rankine-Hugoniot Equation for Ideal Gases I
Pi (y + 1/9 - 1 1. + P o/P i (7)po= 1 + (y·+ 1/y- 1)(Po/Pl)
where y = 1.666 = adiabatic exponent of argon,
ytl_ A9-1- -i'
Po = density of argon in region ahead of shock wave,Pi .= density of argon in region behind shock wave,Po = pressure ahead of shock wave,
Pi = pressure behind shock wave.
Also used was the relation
1 -(e),=(t- 1)(1-2.) 18)Y
whe reB = maximum spray particle velocity within the argon
cover gas = 177.8 ft/s,
Co - acoustic velocity in argon = 1400 ft/s,
and the relation frorn equation .7,
4-26
Pl 4 + P o/P lPo 1 + 4P, / Pi
Thus, equation 8 becomes
-(t),(R- 1)(1-
1 + 4Po/Pi).4 + Po/Pi )
(9)
Substituting = and 1 = x, and solving for thevalue 2
Y, equation
7 can be arranged in the following quadratic form:
/1 \
- 0.0271.4 -- 1,1(1 4 + x )\X -1 + 4x '
/1 - x 1 + 4 x- 4 - x\ ,- 0.0271 = x 1 + 4x j|
x2-2.084 x + 1.037 = 0. (10)
Solving equation 10, x = 1.263 or 0.821.
Since the pressure of the cover gas is increas-ing because of the influence of the shock wave, the value of x= 0.821
is used to satisfy the relationship Pi/Po = 1/x. Thus Pi/Po is found to be1.22, which is the pressure of the transmitted shock wave. The argonshock wave is reflected when it strikes the cover structure. As this
occurs, the pressure increases again owing to superposition of the
two waves. This pressure is calculated using the following equation:
P2-PO 1 + Y -1/y + 1 .=1+ (11)Pl - Po Po/Pi +Y- 1/y +1
Although equation 11 applies rigorously only to a plane wave, the devia-
tion from the plane condition that might be expected in the subject case
should not appreciably affect the pres sure value calculated.
4-27
wherePo = design pressure of the cover gas in the reference
design = 15 psi,
Pi = pressure calculated based on equation 8 - 18.3 psi,P2 = blaximum pressure exe rted against the bottom of the
cove r structure due to the action of the shock wave.
Solving equation 11 for P2 yields a value of 22.2psi, which is the maximum pressure exerted by the cover gas againstthe cover structure. Since the pressure exerted against the coverstructure is small and acts only for an elapsed time of a few milliseconds,the large pressure reduction due to the use of cover gas in the referencedesign adequately protects the cover structure from shock waves.
Following the pressure increase in the cover
gas due to the shock wave, there is another pressure loading of shortduration against the bottom surface of the cover structure. This loadingis caused by the sodium spray thrown upward from the primary coolant-cover gas interface. (One effect of the shock wave's striking the free
surface of the primary coolant is the production of a reflected expansion(tension) wave. ) Therefore, the upward velocity of the sodium particleis the sum of the upward velocities of the sodium behind the incidentand reflected waves as calculated using equation 2. It should be noted
that the velocity of the sodium particles at the free surface varies as
a function of their radial distance from a vertical centerline throughthe explosion's center. This causes a vari-ation in the time that thesodium spray impacts against the bottom surface of the cover structure.
Howe ver, to be conservative it was assumed that the velocity of thespray particle at the explosion's centerline acts uniformly over the
impact area and that the initial velocity of the spray is equal to thefinal velocity. Using these assumptions and the following equation fromreference 8,
d= / Pfz ' (12)
wherePd = dynamic impact pressure,
B = initial velocity = 177.8 ft/s,p = spray density = 25 lb/ft3.
4-28
i 1The dynamic impact pressure against the
cover was calculated to be 85 psi. Owing to the short duration for whichthis calculated pressure acts against the cover structure, combined with
the conservative assumptions made, no special protective features are
required in the 'refe rence design other than the cover gas space already
provided.
These conclusions for both the shock wave and
dynamic spray effects on the cover structure of the reference design
are in agreement with the conclusions made for the EBR II for the same
type of accident. Thus, a gas s]>ace between the primary coolant and
the bottom surface of the cover structure will adequately protect the
cover from damage by shock waves and dynamic sodium spray impact
during a core accident with an energy release in the range of those
investigated.
4.2.3. Missiles
4.2.3.1. Problem Discussion
If explosive reactor core accidents can occur,
or if they are defined as DBAs, theh the problem of missiles and their
effects on the primary containment's reactor cover structure must be
considered. This is necessary because, during the explosion process,
a shock wave of high energy is produced in the primary coolant and
travels radially upward at an acoustic velocity of approximately 9,000
ft/s. Thus, because of the shock wave's high peak pressure, the wave
is capable of accelerating components or fragments of structural mate -
rials submerged in the primary coolant upward toward the bottom sur-
face of the cover structure. Therefore, the reference design, in which
all of the core components, reactor internal components and their
respective supporting structures are submerged in the primary coolant,
will have to be evaluated to determine the potential for missiles capable
of damaging the cover structure. Thus, several questions must be
answered:
1. Is there enough energy associated with the shock
wave to accelerate missiles so that they achieve a velocity sufficient
to damage the cover structure upon impact?
4-29
2. Is the deceleration due to the primary coolant's
resisting the upward trajectory of the missile sufficient to reduce itsfinal velocity to the extent that no damage occurs upon impact?
3. If missiles can impact against the bottom surface of
the cover structure, what type of energy absorbers or missile shieldsare required for their protection?
4. Can the energy absorbers proposed to ease the effects
of a sodium slug also provide adequate missile protection, or is no
protection against missiles required?
As mentioned above, the important character-
istics of the core accident to be considered under this task are the
problems of missiles and the resultant damage to the cover structure
upon impact. However, since the input information from the accident
analysis effort, which describes the core accident and its energy release,was not available, only scoping calculations were made to assess the
problem of missiles for B&W's reference design. Thus, one of themajor problems is how to determine a range o,f explosions that willproduce shock waves capable of generating missiles similar to those
that may be produced under actual conditions.
4.2.3.2. Approach and Conclusions
Because the characteristics of the nuclear
excursion and its energy release are not available (as noted previously),underwater explosion calculational methods were used again. These 4
equations, as stated before, should give conservative results when used
to calculate the various energy releases occurring from the core acci-dents assumed. Therefore, the assumed energy releases and resultant
shock waves were used to approximate missile velocities, their impact,
and the resultant structural damage to the cover structure.
The analytical approach utilized the previouslycalculated peak pressures of the various shock waves for 50-, 100-,
300-, 500- and 1000-pound charges of TNT at the center of the reactor
core. Also used as a part of this calculation was Figure 12, which
plots the peak pressures of the various shock waves as a function oftheir radial distance from the center of the explosion. Therefore, the
4-30
peak pressure of the shock wave acting on the core holddown assemblyor on a structural fragment located anywhere between the center of the
explosion and the interface of the primary coolant and the cove r gascan be taken directly from the graph for input to equation 13 (given inrefe rence 8 and used previously in 4.1.5.2 as equation 4).
PAt9 - M (13)
where11 = initial velocity of missile or component at the end of
its accelerating interval,
A = cross-sectional area of the missile or component,t = time required for shock wave to travel the height or
thickness of the component or missile,P = peak pressure of the shock wave at a designated distance
from the center of the explosion and for a given TNTcharge (shown in Figure 12),
M = mass of missile or component.
Equation 13 was used to calculate the initial
velocity of several missiles and the core holddown assembly. However,
again it was found that input values for the peak pressure of the 1000-
pound TNT shock wave were required before significant missile veloci-ties could be calculated. This was due to the deceleration of the missilecaused by the large volume of sodium surrounding the core combinedwith the rapid dissipation of the shock wave's energy in this same vol-ume of sodium. The deceleration of the missile during its deceleration
period was calculated from the relation
du CDPAB2-=- -g (14)dt 2M
whe reduIE = deceleration of the missile or component,
C D = sodium drag coefficient, which is assumed to equalunity for this problem,
A = cross-sectional area of missile or component,
4-31
B = initial velocity of missile or component, calculatedusing equation 4,
M = mass of missile or component,
p = density of sodium surrounding the core,
g = deceleration due to gravity.
Three types of missiles were investigated
using equations 13 and 14 and only the 1000-pound TNT shock wave
peak pressure values from Figure 12. . Equation 13 was used to calculatethe initial velocities of the first two cases at various distances fromthe center of the explosion. (Case 1, 3-inch-diameter steel sphere;Case 2, 6-inch-diameter steel sphere. ) The results are tabulatedas follows:
Distance of missile Initial velocityfrom Peak pressure of of 3- or 6-lb
center of explosion, ft shock wave, psi nnissile, ft/s
indicates that missiles of the type represented by cases 1 and 2 will
not excessively damage the bottom surface of the cover structure. How-
ever, to assess the problem of missiles and their impact, it was assumed
that the 6-inch-diameter spherical missile located 6 feet from the center
of the explosion strikes the bottom surface of the cover structure with
a final velocity equal to 75% of its initial velocity. Using this assump-tion, an impact force of about 1023 ft-lb against the cover was calcu-lated. Next it was assumed that the missile impacted against an0.75-inch-thick stainless steel plate covering the bottom surface of
the cover structure with a knife edge equal to 2 inches in length. Thisimpact then causes a shearing action similar to that occurring whenmetal is pierced in a blanking die. The shearing strength of the bottom
plate was assumed to be 40,000 psi, anda shearing area of 1.500 in. 2was calculated. Therefore, by multiplying the shearing strength by the
shearing area, a shearing force of 60,000 pounds was calculated. This
is the force required for a shearing edge of the length noted to com-
pletely penetrate the assumed stainless steel plate. Next, dividingthe impact force (1,023 foot-pounds) by the shearing force (60,000 pounds)shows that the assumed missile would penetrate the plate to approxi-
mately 0.21 inch before its kinetic energy is dissipated. These calcu-
lated results indicate that missiles of the type used for cases 1 and 2
will not seriously damage the cover structure; therefore, no protectionagainst them is required.
For the third case, it was assumed that the
core holddown structure, located approximately 4 feet from the center
of the explosion, is accelerated upward by the shock wave and that thestructure does not rotate during its accelerating period. Equation 14was then used to calculate the deceleration of the core holddown assembly.The result was -3326 ft/sz. Using an average value of -1663 ft/sz forthe deceleration, and considering that the assembly must travel upward
through approximately 18 feet of sodium and 18 inches of cover gasbefore striking the cover structure, a final velocity of the assemblyafter travelling 12.5 feet upward was calculated as approximately zero.
Because no credit was taken for the resistance due to the six jackingcolumns and the 25 control rod guide tubes, it was concluded that this
4-33
is a conservative approach to the problem. Therefore, it was assumed
that the cover structure would not be seriously damaged upon impact ofthe core holddown assembly.
4.2.4. Pressure Problems and Conclusions
If large reactor core accidents are considered credible. in the reference LMFBR, or if such accidents are defined as DBAs,
then a high-temperature, high-pressure gas bubble may form in thereactor core by vaporization of sodium, fuel, and structural materials.The expansion of this bubble will increase the normal pressure in theprimary containment even if the reaction is not violent enough to generate
a shock wave.
The design pressure of the reference primary tank is
30 psig, but the normal operating pressure is only about 5 inches of
water. The reactor cover structure is-designed for 100 psig. Onereason for these design choices was to ensure that the primary tankwould fail before the cover structure did, allowing the post-accidentpressure to expand into the rather large gas volume in the annulus(a 30% reduction in pressure). For an explosive accident it is also
likely that the primary tank would fail because of the shock wave effect.
Therefore, it is assumed that the primary tank has already failed before
any dangerous pressure can be created within the system.In section 3.2.2, the Phase I efforts were discussed.
The fault tree diagrams show three modes of failure that could result
from high pressure in the primary containment: (1) biological shield
liner failure, (2) cover structure failure, and (3) seal failure. Theshield liner is discussed in some detail in other parts of this report,and in section 4.1.4.3 it was concluded that the liner and its supportingconcrete must be designed for the final accident pressure. As to themodes of failure for the cover structure and seals, the latter is the
more probable mode and is thus discussed here.
Although each of the cover structure 's joints is protected
by multiple seal designs and redundant seals, it is possible that earlier
accident conditions, such as the shock wave or sodium slug, could dam-
age these seals and lower or·eliminate their ability to contain pressure.
In a way, this is a good aspect, but it yvill have to be assumed that
4-34
adequate protection will be provided to prevent this occurrence; other-
wise, the 100 psig design requirement (which is post-accident) will notbe met. If the design requirement is met, then no leakage through theseals will occur, and the primary containment will retain pressures
up to 100 psig. It should be pointed out that the reactor blanket gas
subsystem is equipped with pressure regulating valves which are set
to limit the normal gas pressure (5 inches of water). These vent line sbleed to the radioactive gas waste disposal system which consists of
three, 3000-ft3 storage tanks t a resultant 36% reduction in pressure ispossible.
Overall, it is difficult to determine whe ther any pressureabove design pressure can finally exist in the primary containment
because of the many leak paths and pressure relief volumes. Rigoroustreatment of the accident process will produce the final pressures
anticipated for a leak-tight containment. The pressure problems mayhave to be reconsidered at that time. (It should be recalled that leakageof gas, after a major accident, was specified in the reference designas an acceptable condition. )
4.2.5. Sodium Slug Problem - Discussionand Candidate Solutions
If explosive core accidents are considered credible in
the reference LMFBR, or if such accidents are defined as DBAs, thenimmediately following the accident's initiating shock wave effects are
the slower-acting effects of the mass of gaseous products formed duringthe nuclear excursion. This mass of gaseous products is formed in ahigh-temperature and -pressure gas bubble occupying an initial volume
approximately equal to the volume of core materials destroyed duringthe accident's initiating transient period. Therefore, due to the high
temperature and pressure within the bubble, the bubble expands very
rapidly. As this expansion occurs, large amounts of primary coolant
are being accelerated from the regions adjacent to the bubble's boundary.
In s tudying the effects on the primary containment, the
primary coolant being accelerated from the regions adjacent to the
bubble.and upward toward the bottom surface of the reactor cover
structure is of utmost importance. The reason is that if the amount of
kinetic energy associated with the primary coolant moving upward in
4-35
the form of a sodium slug is large, then extensive structural damagecould occur to the cover structure upon impact of the slug unless
adequate safety features are provided. Thus, it becomes necessary to
determine the type of protection required in the form of engineeredsafety features to ensure that the cover structure is still capable ofmaintaining sufficient structural integrity in the event it is impacted
by a rapidly accelerating sodium slug. This will ensure that the cover
assembly or any of the mounted components do not collapse into the
damaged core, that loss of shielding capability is not seriously impaired,and that specific po·rtions of the cover's cooling system are operableto accomplish adequate post-accident cooling of the cover.
The effects of the upward movement of the cover struc-
ture upon impact of the slug must be considered, too. In this case the
consequences of vertical movement on the components mounted on and
attached to the cover structure must be investigated. For example, ifthe entire cover assembly jumbs, the secondary containment could besubjected to damage from missiles due to fragments from the failed
holddown device, and the intermediate or decay heat removal system
piping could suffer damage. If damage to the piping is in the form ofa rupture, then intermediate sodium or NaK will be ejected into the
secondary containment. Also, even if the piping did not rupture, the
secondary containment 's piping penetrations could be damaged to theextent that the integrity of the containment was violated. (This problemis discussed further in section 3.2.1.)
Although the impact energy of the sodium slug is not
known, the following potential candidate safety features have been
identified. Each of these solutions has been preliminarily eva]uated
as having the potential to reduce the slug's impact or to mitigate its
effects. The candidate solutions are as follows:
1. A series of structural steel baffles attached to the
bottom surface of the cover structure and submerged in the primarycoolant to prevent acceleration of the slug.
2. Design the core holddown assembly to absorb the
slug's energy by controlling its upward movement through the crushingof high-impact energy absorbers designed as an integral part of the
4-36
support column's mounting attachments, or through the use of hydraulic
snubbers mounted on the cover structure's body to provide mountingattachments for the support columns and control the upward movement
of the core holddown assembly. Or a combination of crushable energyabsorbers and hydraulic snubbers could be used.
3. Control the direction of the slug by designing one ormore of the core holddown assemblies' support columns to collapse a
predetermined degree and direct the slug sideways so that it spills overinto the large gas space in the primary tank. Candidates 2 and 3 wouldrequire redesign of the reference core holddown assembly.
4. Attach high-impact crushable energy absorbers to
the bottom surface of the cover structure to absorb the slug's energy
by crushing a predete'rmined degree upon impact of the slug.
Accidents of a lesser degree have been investigated inan attempt to determine the effects of vertical movement of the rotating
plugs (due to a sodium slug) on portions of the core that are undamaged
during the accident's initiating transient period. This was necessary
because all the control rod drives are mounted on the rotating compo-
nents of the cover structure, and if an accident resulting in a vertical
upward movement of these components occurred, then the control rods
might be withdrawn. To investigate 'the consequences of this vertical
movement the TARTO computer code has been run. The results indi-
cated that any vertical movement exceeding 5 inches and occurringwithin a period of 0.027 second would result in a prompt critical condi-
tion if the major portion of the core was still intact. Therefore, theholddown of the individual rotating plugs will have to be investigated
thoroughly once the accident conditions have been established.
The problems associated with damage to the cover
structure by the reference impacting sodium slug and the effects of
internal or external temperatures are still being investigated. The
ASPRIN computer code is being run to determine the magnitude of theimpact and the distance that the plug jumps. Afte r completing this
effort the plug's condition can be assessed and the post-accident cooling
4-37
requirements can then be established. This effort is being continuedas an integral part of the accident analysis and functional requirement .
activities.
4.3. External Protection for Primary Containment
The reference design's primary equipment is mounted on the
reactor cover structure and is susceptible to damage from dropped
objects or impacting masses. The accidents and the possible conse-
quences are discussed in section 3.2.1, where it was shown that the
following accident consequences are unacceptable: (1) rupture of theheat exchanger bodies or piping resulting in an Na or NaK fire on thecover structure, (2) damage to the control rod drives resulting in rodwithdrawa1, and (3) damage to the control rod drives impairing their
safety scram function. To identify candidate safety features that could
be added to the reference design to prevent these consequences, the
reference design and applicable literature have been thoroughly reviewed.The major efforts and conclusions are summarized here.
4.3.1. Heat Exchanger and Piping Protection
As described previously, six IHXs are mounted on the
reactor cover structure, and their piping extends above floor level to
the point where it penetrates the secondary containment's wall (seeFigure 5). Each pipe is protected by a secondary, concentric pipe,and the interspace is filled with inert gas. The two exchangers forthe normal decay heat removal system are similarly mounted, andexcept for size their piping is identical and routed in a similar manner.
The only major diffe rence is that the IHX piping contains sodium and
the decay heat removal piping contains NaK.
Review of the conceptual design .disclosed that the heat
exchanger was not adequately protected from damage caused by handlingaccidents. Although three safety features that influence the extent of
damage are specified, they do not appear sufficient. These features
are: (1) the secondary pipe and the inert gas interspace onthe Naand
NaK piping, (2) the limits placed on crane travel, and (3) the mechanicalstops on the gantry rails and the bumper bar that encircles the lowerend of the fuel transfer machine. This leaves the system susceptible
4-38
to damage from dropped objects or from inadve rtent movement of the
fuel transfer machine unless additional safety features are provided.If these accidents are of a large magnitude or properly directed, both
could result in an Na or NaK fire that might jeopardize the integrity
of the primary containment. This is a matter of great importance
because failure of the cover structure combined with failure of the
secondary containment during a single accident could release radio-
activity to the environment in excess of allowable limits. It shouldbe noted that the accident is independent of reactor operation, and the
only recourse would be to suppress the fire before serious damage
were done.Four possible methods of solution are apparent: (1) elim-
inate all crane and machine movements near the equipment, (2) struc-
turally protect the equipment, (3) make the secondary containment so
that no fire will result, or (4) add a fire suppression system that wouldact quickly enough to prevent failure of the primary containment.
4.3.1.1. Crane and Fuel Transfer MachineMovements
In s tudying the refe rence design, it quickly
becomes evident that all potentially dangerous movements of the crane
and transfe r machine cannot be eliminated. For example, maintenance
might require crane operation near the heat exchanger systems. Since
an accident could occur when the reactor was not operating, positive
Stops and interlocks would have to be removed. The fuel transfer
. machine must also travel across the face of the cover structure to gain
access to the two fuel storage drums in the primary tank. .Although
some limitations can be placed on this travel path, the path is somewhat
torturous, and it does not appear possible to impose positive limitations
that would eliminate all chance of damaging motion.
Because the potentially dangerous crane and
machine movements cannot be completely eliminated, this method is
not a candidate solution for the problem at hand. However, the study
of the method did point out the importance of additional stops and inte r-
locks to prevent less serious accident consequences.
4-39
4.3.1.2. Structural Protection
The second possible method of solution is to
protect the heat exchangers' systems structurally. Of several approachesstudied, the first is to add energy absorber material around each of
the heat exchanger and pipe units. The structural members would beinside the energy absorbing material, and switches, electrically inter-
locked with the crane and the machine gantry, would be mounted on the
external face to interrupt dangerous movements before structural dam-
age was done. Although this appears to offer a fairly economical
solution, it is difficult to place sufficient structural and energy absorbermaterial in the limited space available (see Figure 14). As an example,
during maintenance the 40-ton primary pump may have to be raised
above the heat exchanger equipment in route to a repair area.
Also, the 200-ton fuel transfer machine must
be moved across the face of the cover structure to accomplish fue 1
handling. The reference design does not specify the highest rate of
travel, but for a machine of this size, standard bridge travel would
be up to 100 feet per minute. The material required to absorb the
energy associated with the machine's moving at this velocity may exceed
the space limitations. It should be noted, however, that in the referencedesign the operations of the fuel transfer machine are independent ofreactor operation. Therefore, a much lower maximum rate of travel,
say 5 feet/minute, could be specified without penalizing reactor edo-
nomics. The solution of these problems basically involves design.The overall method appears feasible and is considered as a primecandidate for protection against the accident in question.
An extension to the proposed method would
be to add a continuous steel membrane wall inside the energy absorberto act as a gas-tight cell around each heat exchanger-piping unit. Theinte rnal space could be inert, and as a result the need for the secondary,concentric pipe arrangement could be eliminated. Although this does
not actually serve as a safety feature to guard against the subject acci-
dent, it could overcome some of the economic penalty of adding thestructure and the energy absorber. A decision on the means of
4-40
structural protection will depend primarily on the results of a trade-off
study on the two methods of utilizing inert gas.Another method of protecting the heat exchang-
ers and piping structurally is to construct a complete false floor ove rall units. This essentially involves joining the individual sections
proposed in the latter method; the floor would appear as extensions to
the existing side floors in the refe rence containment building, leaving
a keyhole-shaped opening over the reactor center (see the superimposedoutline on Figure 14). This method offers no readily apparent advantageunless its cost is less or unless the concept can be developed into a
complete inert box over the systems, so that the concentric pipes can
be eliminated. A problem is encountered at the interface of the floor
and the six primary pumps. Since these pumps are much taller than
the heat exchangers, the body of each must have a seal, or the entire
floor (or a shell membrane) will have to be raised above the pumps.
In both cases, inspection and cooling of the pump motors must be con-
sidered, and raising the floor's height would require a design changeto eliminate inte rfe rence with the operation of the fuel transfer machine's
gantry. Overall the false-floor method appears to be less economicalthan the prior method discussed, and it presents the same, plus addi-
tional, design problenns.
A third method of structurally protecting the
piping involves burying the pipes in channels beneath the ope ratingfloor level. As can be seen in Figure 14, no apparent protective
advantage is gained because most of the piping is already beneath the
side floors (the inboard edge of these floors is indicated by the gantry
rails). Inaddition, a protective cover over the heatexchangers'bodies
would still be required. Overall, this method cannot be seriously con-
sidered as a candidate unless other reasons exist for placing the pipe s
in the channels.
4.3.1.3. Ine rt Secondary Containment
Although an inert secondary containment will
prevent an Na or NaK fire, it cannot prevent the accident in questionfrom occurring. Therefore, we have concluded that it is not a candidate
safety feature of the primary containment if the structural candidates
are designed to adequately protect the heat exchanger systems fromthe accident. It will become a candidate, however, to prevent theultimate hazardous fire if structural protection is not provided or can-not provide adequate assurance that major rupture of the heat exchanger
systems will not occur.
4.3.1.4. Fire Suppression
Several suppression systems for sodium fires
have been proposed for use in sodium reactor systems; e. g., treated
sodium chloride that would be dropped into the fire. Again, as withthe inert secondary containment, these systems would not prevent the
accident from rupturing the heat exchanger systems. In this case, it
would simply extinguish the fire once it strrted. Once again, the system,
may be a candidate safety feature to protect the primary containment
if the structural protection methods are not adequate. However, thisis considered a less likely candidate than is the inert containment
because the time required for the fire-suppression system to .function
may exceed the time required for the fire to damage the reactor cover
structure.
4.3.2. Control Rod Drive Protection
Twenty-five control rod drives are mounted on the plugsection of the center cover structure. These drives extend 14-1/2 feet
above the operating floor level and are connected to the poison rods in·the reactor core by an electromagnetic scram connection and extensionrods. The possibility of accidents above the cover structure resultingin ·rod withdrawal or damage that could impair the scram capability ofthe rods has been discussed in section 3.2.1. The conclusion was that
protection against accidents that could create a side load on the drives
must be provided, or that the drive-cover structure design must bealtered to eliminate the hazardous consequences.
4.3.2.1. Crane Movements
The pos sibility of eliminating potentially
dange rous crane movements as a method to prevent heat exchanger or
piping ruptures is discussed in 4.3.1.1, which concluded that this
couldn't be done because of the maintenance procedures required.
4-43
This holds true for the control rod drives because the crane must bcused in replacing a drive assembly and in certain repair procedures.There is a difference, however, for these procedures will be attempted
only when the reactor is shutdown, during which time accidents would
not jeopardize the primary containment or the safety of the reactor.
Regardless, positive stops should be placed on the crane rails to pre-vent all movement over the drives during reactor operating periods.
A real danger to safety would occur if other
equipment being handled by the crane were swung into the drives. This
could produce a bending moment that might actually withdraw poisoncolumns or impair the scram function of the rod. This is analogousto the condition that would be created if the fuel transfer machine wereinadvertently run into the drives (discussed in the next section).
4.3.2.2. Fuel Transfer Machine Movements
The fuel transfer machine must be movedover the face of the cover structure to gain access to the two fuel stor-
age drums in the primary tank. If this procedure is carried out duringreactor ope ration, as proposed, the machine must be maneuvered
around the IHXs and the control rod drives. Because of the potential
danger of control rod withdrawal or damage that could impair the scram
function, all possibility of the machine's running into the drives must
be eliminated.
Several methods of preventing damage to thecontrol rod drives have been considered. The first would eliminate
all travel of the machine near the drives. However, since the machineis also used in changing poison rods, absolute elimination cannot be
guaranteed. Positive mechanical stops could be placed on the gantryrails during reactor operating periods, but the possibility of the stopsnot being replaced after removal would always have to be considered.
In the second method, bumpers are placed inthe path of the machine to absorb the impact energy and to electricallyinterrupt the gantry's motion and scram the reactor. This is a likelycandidate solution to the problem, and the reference design already
specifies a. spring-loaded bumper bar encircling the machine that
could serve as a part of this system. Additionally, heavy duty hydraulic
4-44
or other energy absorber mechanisms should be placed on the surface
of the reactor· cover structure. Although this is a design problem, itappears that the solution would eliminate both the machine and crane
movement problems. It should be pointed out again that the maximumvelocity of the gantry should be greatly reduced to minimize the design
problem and the damage if all safety features were to fail (see section
4.3.1.2).
4-45
REFERENCES
1 1000-MWe LMFBR Follow-On Study-Task II and III Final Report- -Conceptual System Design'Descriptions, Babcock & Wilcox, BAW-1328, Vol 2, March 1969.
2 1000-MWe LMFBR Follow-On Study-Task II and III Final Report-Conceptual System Design Descriptions, Babcock & Wilcox, BAW-1328, Vol 3, February 1969.
3 1000-MWe LMFBR Accident Analysis and Safety System DesignStudy-W6rk Plan, Babcock & Wilcox, BAW-1339, August 1969.
4 BAW-1339, Appendix.
5 1000-MWe LMFBR Follow-On Study-Task II and III Final Report-Trade-Off Studies, Babcock & Wilcox, BAW-1328, Vol 4, Section 4,Appendix A, November 1968.
7 1000-MWe LMFBR Accident Analysis and Safety System DesignStudy--Phase I-Fault Trees and Malfunction Catalogs, Babcock &Wilcox, BAW-1344, December 1969.
8 Monson, H. 0., and Sluyter, M. M. , "Containment of EBR-II, "Reactor Safety and Control, Vol II, p 1892, Proceedings of the SecondUnited Nations International Conference on the Peaceful Uses ofAtomic Energy, Geneva, September 1958.
9 Porzel, F. B., "Some Hydrodynamic Problems in Reactor
Containment, " Reactor Safety and Control, Vol II, p 434,Proceedings of the Second United Nations International Conferenceon the Peaceful Uses of Atomic Energy, Geneva, September 1958.
10 Bohannon, J. R., Jr., and Baker, W. E., "Simulating NuclearBlast Effects," Nucleonics, 1€, 3, March 1958.
11 Cole, R. H. , Underwater Explosions, Princeton University Press,Princeton, New Jersey (1948).
12 Atomic Power Development Associates, APDA-303, Evolution and
Design of the Machinery Dome for the Enrico Fermi Atomic Power
Plant.
D
APPENDIX A
Primary Containment ComponentsSummary Description
A-1
Radial Biological Shield and Line r
The radial biological shield and its liner surrounding the primarytank serve'.many purposes in the LMFBR integral pot concept. The
biological shield supports the reactor and all of the integral primary
equipment associated with the heat removal systems. In addition, itsupports the primary tank's cover assembly and, together with its liner,becomes the primary sodium and energy barrier during abnormal con-
ditions, including the DBA. (See Figure 2.)The radial biological shield also serves as biological shielding
for the plant's operating personnel. This shielding system is constructed
of ordinary feinforced concrete and has an inside diameter of approxi-mately 54 feet-4.5 inches and a minimum thickness of 6.5 feet. Thearrangement.provides sufficient attenuation under normal operatingconditions to limit radiation exposure to a level commensurate with
gove rnment requirements for personnel working in the limited access
areas adjacent to the primary containment.
The radi·al biological shield liner·is 3/4-inch-thick carbon steel
with an inside di.amete.r of approximately 54 feet-3 inches. The linerseals and pre.vents loss of the-argon atrrlosphere in the primary tank's
cavity .during normal reactor operation; in the evdnt of a primary tank
failure, the liner seals and protects.the biological shield concrete from
the primary coolant. The interspace between the primary tank and the
biological·shield liner is small enough to maintain coolant coverage ofthe coolant circulators, the core, and the heat·removal equipment if
the primary tank should fail. If all the primary coolant heat removalequipment failed as a result of a DBA, and the primary tank did not
fail, then this annular space would be filled with intermediate system
sodium, and the circumferential biological shield cooling coils in the
biological concrete directly behind the liner would be activated to
accomplish post-DBA cooling of the damaged core. During normal
reactor operation the annulus between the primary tank and the lineris cooled by the closed-loop argon cooling subsystem, which limits the
temperature of the radial biological shield concrete to approximately180 F.
A-2
Sections 2 and 3 of reference 1 completely describe the refe rence
design.
Primary Tank Cover
The cover of the primary tank seals to the tank's flange and isbolted securely to the radial biological shield concrete. The primarytank's cover assembly is a flat, cylindrical configuration consisting ofa fixed.outer section and triple inner rotating plugs. A fill of serpentine
aggregate and steel shot provide shielding, and a variety of access ports
are provided for installation of primary rhactor and refueling equipment.
(See Figure 13.)
The complete cover assembly is supported 'in place by a heavysectioned outer flange, which is an integrai part of the fixed section of
the cover. The all-welded, structural.steel cover is an annular con-figuration consisting of inner and outer ring beams connected by· radially
positioned deep-section beams and interconnecting stiffener plates. The
major radial beams are positioned between access ports, and the inter-
mediate beams are welded into the access ports' liners. All of these
liners are stepped to mate with the equipment to be installed and to
prevent radiation streaming up through the annular spaces. From the
bottom face upward, the fixed cover section consists of a bottom plate,reflective-thermal insulation, a heavy bottom plate, beams, shielding
mate rials, an equipment void space, and a heavy top plate flush with
the reactor ope rating floor. The bottom plates are welded, gas-tight
membranes that prevent reactor cover gas from entering the assembly.The top plate is also welded gas tight and contains equipment compart-
ments that are covered but not sealed from the reactor building'senvironrnent.
The inner cover structure sections are rotatable plugs. Three
plug sections are used. A small rotatable plug is positioned eccentri-
cally within an intermediate rotatable plug, which is positioned eccentri-
cally within a larger concentric rotatable plug. Each plug is supported
on heavy-duty bearings from the succeeding plug until the total supportis carried through the fixed cover.
The rotatable plug sections are cylindrical structures comprising
an outer ring beam and radially positioned inte rnal beams sandwiched
A-3
between top and bottom plates. The outer ring is shaped to provide aradiation shielding step, a shear ring-bearing step, and a load ringfastening step. The radial beams are positioned around all access
ports penetrating the plugs, and the liners for all of these ports are
double-stepped to mate with their respective components. From bottom
to top, the rotatable plugs are very similar to the fixed cover section;the main diffe rence is an additional thermal barrier that is added totheir bottom face in the form of plates separated by a gas space. Thisadditional barrier compensates for the higher temperature over thereactor core.
All three rotatable cover plugs are equipped with drive systems to
perform their rotational function. These units are located in the topvoid section of the cover structure section adjacent to the driven plugand are flush with the operating floor except for service connections.
In addition to the supporting, positioning, orientation, and alignmentfunctions that the primary tank's cover provides for mounting the variouscomponents attached to it, the cover also provides (1) the primary cool-ant and fission product barrier between the primary system and the
secondary containment during normal or abnormal reactor operation,(2) sufficient biological shielding to protect the health and safety of
operating personnel during periods of standard maintenance and refuel-
ing, and (3) sufficient structural and component mounting integrity toprevent impairment of the containment feature of the cover after anyrelease of energy during an accident up to and including the designbasis accident.
The conceptual design is completely described in section 2 of
re fe rence 1.
A-4
APPENDIX B
Core Vessel Upper ExtensionSumma ry De s c ription
B-1
The upper extension (flow divide r) of the core vessel is one of thefour basic sections of the core vessel assembly. Each section of thevessel is completely shop fabricated from 304 stainless steel and weldedinto an assembly. An internal step is accurately machined in the insidediamete r of the vessel's wall for positioning and attachment of the gridplate, which in turn positions and locates the reactor core. Thisarrangement permits the upper extension wall of the core vessel toextend up around the core and form the flow divider between the coreand the lower radial neutron shields. During normal reactor operationthe flow divider is subjected to a coolant temperature of 800 F at itslower end and a varying femperature profile up the wall to a maximumof approximately 1000 F at the top. The temperature of the wall ismaintained by dive rting coolant flow into the core blanket assembliesand the radial neutron shields. Thus, the upper extension wall of thecore vessel serves strictly as a flow divider between the referencecoolant areas and is subjected to essentially no pressure differentialor other loads in the reference design.
The conceptual design is fully described in refe rence 1, section 2.
B-2
APPENDIX C
Literature Survey
C-1
Introduction
The literature survey was limited to reviewing information onreactors that are currently operating or are proposed for operation inthe near future. This enabled us to evaluate accepted safety featuresand features that are being considered for acceptance. The followinglist does not contain all of the literature reviewed; rather, it presentsliterature containing primary containment safety features that could
be incorporated into the designof a 1000.-MWe LMFBR concept similarto B&W's reference design for the Follow-On Study.
List of Literature
1. Argonne National Laboratory, ANL-7618, Progress Report,September 1969.
2. Argonne National Laboratory, ANL-7577, Progress Report,April-May 1969.
3. Argonne National Laboratory, ANL-7553, Progress Report,February 1969.
4. Argonne National Laboratory, ANL-7214, Hydrodynamics of aNew Concept of Primary Containment by Energy Absorption,H. C. Sorensen and S. H. Festedis.
5. Oak Ridge National Laboratory, ORNL-NSIC-22, NuclearSafety Information Center, Missile Generation and Protectionin Light-Witer-Cooled Power Reactor Plants.
6. Battelle Northwest, BNWL-1090, Quarterly Report, July 1969,Reactor Response to the DBA (FFTF).
7. Battelle Northwest, BNWL-10:69, Sealing Mechanisms forFFTF Closed Loop and Open Test Position Closures Note(High Temperature Static Sealing Mechanisms).
8. Argonne National Laboratory, ANL-5719, Addendum to HazardSummary Report, January 7, 1964, Experimental BreederReactor II (EBR II), Notes, Blast Shield Information, pp 32and 128; Primary Tank Cover Deflection, p 72; RotatingPlug Freeze Seals, p 180.
9. Argonne National Laboratory, ANL-5719, Hazard SummaryReport, Experimental Breeder Reactor II (EBR II), ReactorCover Holddown, p 143.
10. Atomic Power Development Associates, APDA-124, EnricoFermi Atomic Power Plant, Core Holddown, pp 103, 108,109, 110; Rotating Shield Plug, pp 111-117 (Stainless SteelWool Used as Thermal Insulation Density 20 lb/ft3).
C-2
11. AEC Docket No. F-16, Section 1, Enrico Fermi Atomic PowerPlant, Part B Revised, License Application, TechnicalInformation and Hazards Summary Report Machinery Dome,p 102.36; Primary Shield Tank, p 102.34; Core Holddown,pp 102.17-102.34.
12. Experimental Breeder Reactor II, Containment of EBR II,H. 0. Mons on and M. M. Sluyte r.
13. Battelle Northwest, BNWL-941, Fast Flux Test FacilityCriteria, Periodic Technical Report.
14. Battelle Northwest, BNWL-607, Fast Flux Test FacilityCriteria for Containment.
15. Atomic Power Development Associates, APDA-303, Evolutionand Design of the Machinery Dome for the Enrico FermiAtomic Power Plant.
16. Experimental Breeder Reactor II, System Design Description,Volume II, Primary System, Chapter 4., Primary TankAssembly.
17. Argonne National Laboratory, ANL-7120, Proceedings of theConference on Safety, Fuels, and Core Design in Large FastPower Reactors, October 11-14, 1965:
(a) A Model Investigation of Explosion Containment inSingle Tank Fast Reactors, p 692, N. J. M. Rees,U. K. Atomic Energy Authority.
(b) Comparison of Pressure Loading Produced byContained Explosions in Water,and Sodium, p 720,presented by F. J. Walford of U. K. Atomic EnergyAuthority. -';
(c) Design Factors Influencing the Containment of InternalExplosion in Fast Reactors, p 734, D. E. J. Samuels,U.K. Atomic Energy Authority.
19. Battelle Northwest, BNWL-1166, ASPRIN-A Computer Codefor Predicting Reactor Vessel Response to Hypothe ticalMaximum Accidents on Fast Reactors.
20. Bohannon, J. R., and Baker, W. E., "Simulating NuclearBlast Effects, " Nucleonics, 16,3, March 1958, p 75.
C-3
t Z»"fll-/*.-R li·- ·• · . ..
.9
1 ' /,- trt.
¢ERGZ-*4,
ATOMIC ENERGY COMMISSION Post Office Box 62· Oak Ridge, Tennessee
. ),Ii). DIVISION OF TECHNICAL INFORMATION EXTENSION37830
' -·sfi "
In Reply Refer To: TDD:PWR October 20, 1970
Files
PROCESSING OF 1000-MWe LMFBR SAFETY STUDIES
We have been asked by L. W. Fromm, Manager, 1000-MWeStudies LMFBR Program Office, ANL, to print and dis-,tribute subject reports as a logical continuation ofour involvement in the Follow-on Study Program.
i- A total of 12 reports have been generated by BAW.There may also be a single report from each of 3
contractors, AI, GE and Westinghouse. Each reportwill be cleared for publication before being sent toDTIE.
»»S=»»/38.»-=to»'"','»lowsf '1UC-80 - 225 Copies l
iNTIS _ 25 extra cys. u'<
t .,\ 1/ r1 Stock - 50 copies s
[
1000 MWe Dist. - 165 copies 1 1
-35 copies i
t
Phillip W. Rosser !1
;
CC: Dreyer iMasters (12)
1 1
1 1
i
'
tl
1 -t,J . „ . f. .:;2. .1.. :i ...»i:·.:. .-,1-21....,;';·.· ..5 L . .·,':''i,(·. :.,· Sil.,)69'.'... :,.':.· 'p.. 6...'i!, 41.JK,:ilr'....:i.•·,. .. i ·-.'. ,·'.'.A'I ::'. f d.3..3 1
-*MI/,Twme E:k._trRIP• -· -
S.&.31-tttls :.'. --
. h. . ..1.IT Ii«0'3*11». 3#:*.
1+ '. . .. ....).1
..4 ..e..'.*- ... - ..: 1I f .... .:......=«* 1
1- /.6,\ 11161 LJ-ual'-Al,A USAIC
AIRGONNE NATIONAL LABOIRATOFRY
February 19, 1971
PRO:K:029
Mr. Robert L. Shannon, DirectorDivision of Technical Information ExtensionU.S. Atomic Energy CommissionP.O. Box 62Oak Ridge, Tennessee 27830
Reference: Letter, L. W. Fromm to R. L. Shannon, "1000-MWe LMFER iSafety Studies - Publication of Contractors' Phase and ' 1Topical Reports," October 16, 1970
Dear Mr. Shannon:
In the reference letter I advised you that we would be transmittingto you for publication a total of twelve Babcock & Wilcox Company Phase andTopical Reports, and possibly three reports from other contractors, generated ·
under the AEC-sponsored 1000-MWe LMFBR Safety Analysis Studies program. <With that letter I enclosed one B&W report (BAW-1344", which you have sincepublished), and advised that the remainder would be transmitted to you for
publication when received and patent-cleared.
I am enclosing herewith one copy of each of the eight B&W Topical Reportslisted below, all of which are now patent-cleared and ready for publication.The covers for these reports should be that used for the previously-issuedBAW-1344, except for the changes noted in the table below. The letters
heading the columns Of the table are 'keyed to the markings on the attached
4 Mr. R. L. Sh,innon, DirectorFabrunry 19, 1.971 "' 3
45 1. .4
I note that for BAW-1344 you used a two-piece cover with staple binding,and the "binding edge caption" actually appeared on the back of the report.
' If this is to be the case with the reports enclosed, then the binding edgecaptions may be omitted. However, if any of the reports will actually have
binding edges.upon which printing can appear (and be visible with the reports' on a library'shelf), then the above captions should be used.
The distribution of all. of these reports should be our "Distribution· A"plus Category UC-80, Reactor Technology, as before. For your convenience iam enclosing another copy of the "Distribution A" list previously suppliedto yoU.
in the raference letter I stated tliat there'would be twelve B&W reports,and possibly three from other contractors. This has now been revised downwardto eleven B&W reports· and ·one report from Atomics International. The singleremaining B&W report and the AI report will be transmitted to .you when receivedand patent-cleared.,
Thank you again for your excellent cooperation in publishing thesereports. If there are any questions, please contact me on FTS extension312/739-2971 or 312/739-4844.
Very truly yours,_
1
L. W. Fromm, Manager1000-MWe StudiesLMFBR Program Office . 1
LWF:elencls.
·
1cc: (w/0 encl.)AEC-RDT: Director
11
Asst. Dir. for Project Mgt. iChief, Liquid Metal Proj. Br.LMFBR Program ManagerSr. Site Representative - ANL
Manager, AEC-CH 'Director, LMFBR Program Office - ANL (2 copies)R. C. Dreyer, DTIEC. R. Bruce, DTIEP. W. Rosser, DTIE
