IEEE Transactions on Nuclear Science, Vol. NS-31, No. 1, February 1984

C-E IN-CORE INSTRUMENTATION - FUNCTIONS AND PERFORMANCER. M. Versluis

Combustion Engineering, Inc.P.O. Box 500

Windsor, Connecticut 06095203/688-1911, ext. 4135

Abstract

The middle and late seventies were a period ofmaturing of the existing PWR designs for in-coreinstrumentation and demonstration and exploitation ofits capabilities. While initially the in-coreinstrumentation system was primarily used for neutronflux mapping the later use of the system forcontinuous core limit monitoring has been verysuccessful in increasing plant productivity.

This paper discusses the uses of in-coreinstrumentation for monitoring and surveillance inPWRs as they have evolved over the years, reviews theperformance of C-E's in-core instrumentation, anddiscusses possible future developments.

I nt rodu ct i on

The middle and late seventies were a period ofmaturing of the existing designs for PWR in-coreinstrumentation and exploitation of itscapabilities. Nuclear Steam Supply Systems (NSSSs)of Westinghouse ( W) design rely on a movable-fission-chamber-based flux mapping system, whileCombustion Engineering (C-E) and Babcock & Wilcox(B&W) NSSSs rely primarily on a fixed in-coredetector system with replaceable core-resident self-powered neutron detectors (SPNDs). In some C-E NSSSsthere are supplementary movable in-core detectorsystems employing either fission chambers or SPNDs.In the meantime, development activities on primarilygamma-sensitive devices continued both inside andoutside the U.S. While in the mid-seventies thereappeared to be a market for in-core instrumentationthat could accommodate multiple vendors and justifythe development of any good device, the eightiespresent a very different picture. Due to plantcancellations the market for new and replacement in-core instruments is much more limited than originallyanticipated. Also, the good performance of rhodiumself-powered neutron detectors has dimiinished theincentive for development of other types of fixed in-core instruments. However, the success in achievingimproved productivity in C-E plants suggests thatreal-time core limit monitoring based on core-resident detectors could be justified in non-C-Eplants as well.The intent of this paper is to discuss the uses of in-core instrumentation for monitoring and surveillancein PWRs as they have evolved over the years, toreview the peformance of C-E's in-coreinstrumentation, and to discuss possible futuredevel opments.

Functions of In-Core Instrumentation

In PWRs in-core and ex-core instrumentation togetherserve to show that the core behaves as designedand predicted, and to help diagnose any anomaliesthat may occur. How this is accomplished, as well asthe split in functions between in-core and ex-coreinstrumentation, varies with PWR design and vintage.More specifically, the tasks to be performed by thecore instrumentation are:

1. Trip the plant, if deviation from expected be-havior is large or grows rapidly (Protection);

2. Warn the operators if certain limiting conditionsof operation (LCOs) are exceeded (Monitoring);

3. Verify from time to time thatdistribution-related variablesthei r Technical Speci f i cat i onveillance);

importantrema i nlimits

power-within( Sur-

4. Calibrate ex-core instruments from time to time(.al i brati on);

5. Measure detailed power distribution to providedata for Core-follow, Anomaly detection, CoreHistory Data Base.

In U.S. PWRs the Protection function is fulfilled byex-core instrumentation. The occasionly performedfunctions such as surveillance, calibration, anddetailed power-distribution measurement are fulfilledin some plants by movable in-core instruments and inothers by fixed in-core instruments.The standard real-time limit monitoring system inPWRs without fixed incores uses only ex-cores, asopposed to, for example, C-E PWRs where over theyears the fixed in-cores were increasingly reliedupon for monitoring. While originally the C-Ein-cores were used for power distribution informationonly (surveillance, etc.), in the mid-seventies theon-line kw/ft monitoring function was added. Withthe deployment of the Core Operating LimitSupervisory System (COLSS), starting with ArkansasNuclear One, IJnit 2 (ANO-2), the on-line kw/ft,DNBR, axial and azimuthal tilt monitoring functionswere added. The introduction of COLSS' real-time LCOmonitoring capability using fixed in-cores permitteda higher core power density and wider operating spacethan before. When the LOCA kw/ft limit was tightenedin the mid-seventies, some plants without fixed in-core instrumentation had to redefine the function oftheir movable detector flux mapping system to assistwith the monitoring functions. Several plants stilluse this Axial Power Distribution Monitoring System(APDMS) today.

Summary In-Core Instrument System Description

The fixed in-core instrument system hardware includesthe instrument assemblies, guide paths and othersupporting structures, field cabling and connectors,and signal conditioning and processing equipment. Anoverview is shown in Figure 1 for C-E's bottom-mounted, fixed plus moveable instrument system, whichis part of the 3800 MWt System 80. Figure 2 showsthe axial position of the five detectors in a System80 instrument assembly relative to the active core aswell as the locations of the instrumented fuelassemblies in the core. Approximately 25% of theassemblies are instrumented. Each instrumentassembly is inserted in a guide tube at the center ofthe fuel assembly. The bottom-mounted system permitsinstrumentation of both rodded and unrodded fuelassemblies. Earlier plants have a top-mounted design.

0018-9499/84/0002-0761$01.00 C 1984 IEEE

761

Figure 1OVERVIEW OF FIXED/MOVABLEIN-CORE INSTRUMENT SYSTEM

180_0_

'202 021 0,

02 92 01 010 ol7 02 2

0 0. -2 Ofl 0 0102 021 01

0 ol ol021 1O. 2OA 021 0d 0.1

02 ol021 102! 02 Od 0,

02 10 21 011 01-

02 O.j jo.0-2

-P 2=

C D EF oH I K oL AF G H J K L M

0.

CR-AL OUTLET THERMOCOUPLE

LOCAL-

FLUXT

DETECTORS 00%

70%

ACTIVECORE 50%

HEIGHT

30%

10%

Figure 2SYSTEM 80

FIXED IN-COREINSTRUMENTATION

LOCATIONS

270

ACCESSED BY TRANSFERMACHINE #1 (30)'ACCESSED BY TRANSFERMACHINE #2 (31)

The C-E fixed in-core instrumentation system employsself-powered neutron detectors (SPNDs) with rhodiumemitters 40 cm long and 0.045 cm in diameter. Eachdetector is an integral segment of a piece ofcoaxial, mineral-insulated cable having an inconelsheath and a 0.16 cm diameter central conductor. Astring of 4 or 5 SPNDs is contained in an instrumentassembly, which mechanically protects the detectorsby means of an outer sheath, and positions themaccurately. The assembly terminates electrically ina multi-pin electrical connector, which mates withthe field cable connector. Mechanically itterminates in a seal plug which brings out theelectrical leads through the primary pressureboLindary. This is shown in Figure 3 for C-F's bottom-mounted system. Also shown is the calibration tube.It is a dry thimble which provides access for amovable detector in the combined fixed/moveable in-core detector systems supplied on C-E's 3400 and 3800MWt NSSS designs. Individual detectors are wrappedaround the calibration tube within the outer sheath.

Rhodium-103 is the neutron-sensitive nuclide. It ismostly a thermal-neutron detector (it has a thermal-neutron cross section of about 100 barns) with anepithermal contribution of about 15%. Afterabsorbing a neutron, the rhodium-104 nucleus decays,emitting gamma and beta radiation. A fraction of theenergetic betas (electrons) escape the emitter andare collected in the detector sheath, thus creatingan "electron pump". If the circuit between the lead-wire and the sheath is closed, a current which isproportional to the neutron absorption rate ismeasured by the data acquisition system. About 6% ofthe betas are emitted promptly after neutronabsorption; the remainder decays with a half-life of42 seconds. For applications requiring a promptresponse, C-E has constructed a dynamic compensationfilter (Reference 8).

762

PLANTCOMPUTER

INSTRUMENTASSEMBLY

X

345678

90, 91011121314151617

A B N P R S T

CALIBRATION TUBEWITH MOVABLE IN-CORENEUTRON DETECTOR

TYPICALAXIAL FLUXDISTRIBUTION

DIRECTIONOF COOLANT

FLOW

I112 -.7

763

ELECTRICALCONNECTOR

BULLET OSEE

DRY DETECTOR TUBE

EPOXY SEAL

Figure 3IN-CORE DETECTOR ASSEMBLY

The small electrical current (about 2 micro-amps atnominal conditions) is conducted through well-shielded cable to the plant computer data acquisitionsystem. Specially designed, low-impedance, high-gaincurrent-to-voltage amplifiers, which also performnnoise filtering, produce a 0-10 Volt output. Theamplified signals are then multiplexed and convertedfrom an analog to a digital signal. The signal isthen further processed in the plant computer tocorrect for background, emitter burnup effects, andif needed, dynamically compensated for beta-decayeffects.

Further processing consists of signal-to-powerconversion, power distribution synthesis, andcalculation of margins to core operating limits.These calculations are performed continuously by theplant computer (COLSS) and occasionally by CECOR, amore detailed model.

The mechanical layout of the instrument system andits support structure is shown in Figure 4 for C-E'sbottom-mounted system. Prior to refueling andbeforethe water is raised above the vessel flange, theentire support frame including the moveable detectordrive and transfer machinery is moved out of the way,after disconnecting the dry thimbles. This allowsaccess to the seal table to dismantle the seals andwithdraw the instrument assemblies far enough (about25 feet) so the connectors and thimbles may be keptdry when the water level is raised to the normalrefueling level. This wet seal table arrangementduring refueling permits the removal and disposal ofinstrument assemblies under water.

Operati onal Experi enceIn terms of its most important performanceparameters, accuracy and availability, the C-E fixedin-core instrument system has an excellent record.By analyzing a sizable data base of measured powerdistributions, C-E has licensed the following 95/95uncertainties (Ref. 2):

Fq

FrFxy

6.2% (3-D peak)6.0% (integrated radial peak)5.3% (planar radial peak)

Figure 4MECHANICAL LAYOUT OF INSTRUMENT SYSTEIM

INSTRUMENTSEAL TABLE

-INSTRUMENTGUIDE TUBES(TYPICAL)

The rhodium emitters have shown good signal strengthand low background. The depletion law used tocompensate the signal for rhodium burnup effects iswell established and has been shown to be linear fordepletions up to about 70%, which corresponds roughlyto 3 cycles of operation (see Figure 5). It is notedthat the form of the depletion curve depends on howdetector sensitivity is defined, i.e., C-E'sdepletion law may not be compatible with signalinterpretation methodologies used by others. C-E'ssignal interpretation software (CECOR) andmethodology are described in Reference 3.

A1203

764

Figure 5CORfECTED RELATIVE SESITIVITY

0 100 200 300

ACCUILATED CMRGE (couLcms)

As for availability, the in-core detector system hasnot limited core power performance in any plant.When judging this availability performance, it mustbe borne in mind that in C-E reactors, fixed in-coresnot only fulfill the power distribution surveillancefunction, but also monitor the kw/ft limit and, indoing so, provide considerably more margin to theLOCA limit than an ex-core monitoring system couldobtain.

C-E's experience with the fixed in-core instrumentsystem spans 34 cycles of operation to date. Some ofthe early experience is found in Reference 4. As isto be expected, a number of design and manufacturingimprovements were made based on operating experienceto increase SPND lifetime. The primary causes ofdetector failure are breaks in the lead wire and lossof electrical resistance between lead wire and sheathdue to moisture.

Since the detector assemblies must be inserted andremoved through tightly curved guide paths in the top-mounted systems employed in the pre-ANO-2 plants,they must be flexible and yet strong. This hasresulted in a design for these plants with apermeable, flexible assembly sheath and an extrainternal member to increase tensile strength. Thisdesign has overcome earlier problems of breakinginstrument assemblies during removal.

Because the assembly sheath is permeable, theindividual detectors are in contact with the coolant,which increases the chances of corrosion of thedetector sheath and subsequent moisture ingress.While moisture ingress still can occur, it has beengreatly reduced by removing any welds, brazes orsplices from the detector sheath external surface("integral" detector) and careful materialsmanagement and QA by the vendor.

Starting with Arkansas Nuclear One Unit 2 (ANO-2),all top-mounted in-core systems have combinationfixed-moveable in-core detector assemblies and lesscomplex guide paths. The solid sheath instrumentassembly which is less flexible than the permeablesheath instrument assembly design has shown goodperformance in three cycles of operation at ANO-2.

Another improvement initiated in ANO-2 is toaccomplish the current-to-voltage conversion requiredfor signal multiplexing by active amplifiers, ratherthan dropping resistors. Dropping resistors aresimple devices and free of drift, but they offer ahigh terminating impedance to the detector.Amplifiers offer significantly lower impedance,thereby reducing leakage currents that may occur inolder detectors, and better noise filteringcapabilities.

FutureP

C-E PWRs

Further improvements in accuracy of C-E's system havebeen obtained by improving signal interpretationmethodology, such as using multi-level signal-to-power coefficients (see Reference 3), but little, ifany, accuracy improvement is foreseen from using adifferent type of sensor. Rhodium SPNDs have shown

4 superior signal strength and low background; thesomewhat delayed response of the rhodium SPNDfollowing a change in flux is not a problem forcurrent non-protective system applications.It is C-E's experience that a change of sensor or theuse of instruments that operate on a differentprinciple requires a long and costly developmentprocess involving examining the signal behavior undervarious conditions and long-term i rradi ation,developing the signal processing methodology, andfine-tuning the system until its accuracy at leastmatches that of the existing system. C-E completedthe fine-tuning process of its current system onlyrecently. It is, however, possible to economicallyjustify the development of longer-life detectors.C-E built into its later in-core instrumentationsystems the flexibility to use both fixed and movablesensors. The movable detector system, if suppliedwith the latest signal interpretation software andbackground-compensated detectors, could be used as acalibration device for fixed sensors. Thiscalibration capability could be used to extend thelifetime of depleted fixed rhodium detectors byanother cycle or more. It could further be used tointroduce slowly or non-depleting fixed detectors,e.g., vanadium SPNDs, which are compatible with theexisting signal interpretation methodology.

Gamma-sensitive detectors, though promising in somerespects, are as yet unproven in PWRs. With thecurrent methodologies for signal-to-power coefficientgeneration, a primarily gamma-sensitive detectorinvolves a major development effort. Such effort isunderway, primarily in France (Reference 5), butneither the final accuracy of signal interpretationnor the eventual cost of generating the library ofsignal-to-power coefficients are known at thispoint. It is clear, however, that in order to obtainthe coefficients an accurate calculation of the gammaas well as the neutron field in the core must beperformed each cycle. Detailed gamma fieldcalculations are not now a part of reload designcalculations.

765

Application of Fixed Jn-Core Instrumentati onSystems to Other PW'Rs

A clear economic incentive can be identified forintroducing fixed in-core detectors into some PW'Rswhich presently rely only on traveling instruments.A continuous core monitoring system based on fixed in-cores could permit an increased core thermal poweroutput by up to 6, (Reference 6), compared to the ex-core-detector-based monitoring system, withoutraising the maximum permitted linear heat rate(kw/ft) limit. This is possible because lessconservatism is required when the peak linear heatrate is measured using in-core instrumentation. Inaddition, fixed in-core detectors could be used tomonitor the higher flux peaks associated with moreefficient fuel managements without raising themaximum permitted linear heat rate limit.For PWR's originally outfitted with travellingdetectors alone, C-E has developed a real-time CoreOperating Limit Monitoring System (COLMS) usingstrings of core-resident flux detectors and real-timesoftware, which retains the current flux mappingcapability with movable detectors. The concept isbased on C-E's experience with real-time coremonitoring on its own plants, but has been adapted tothe other PWR designs to provide a needed capabilityat a reason-able cost, using as much of the existingmonitoring approach and Technical Specifications aspractical. This is done by placing ten strings ofcombination fixed/movable in-core detectors in thecore and deriving from the detector signals the powerdistribution information needed to calculate, on-line, the margins to the core operating limits. Bychoosing the ten instrumented positions correspondingto one movable detector drive machine and convertingthe drive machinery to handle an 0.118 inch 0nfission chamber, the ten combination assemblies canstill be accessed by a movable detector, therebyleaving the present flux mapping capability intact.Each of the ten instrument assemblies may include acore-exit thermocouple to provide, at little extracost, replaceable thermocouples over and above thepresent top-mounted, permanently installedthermocouples. These bottom-mounted thermocouplescan be furnished as LOCA-qualified instruments,provided they are terminated with qualified connector-cable assemblies for signal conduction inside thecontai nment.

Conclusi onIn light of the good accuracy and availability of theC-E Fixed In-Core Detector System and adequatesupplies of high-quality spare parts, the incentivesfor developing new types of flux or power sensors forC-E PWRs have diminished in time.There is little incentive for developing moreaccurate flux sensors; gains in accuracy are morelikely to result from signal interpretationimprovements. Incentives do exist for thedevelopment of longer-life sensors. C-E has apreference for sensors which are compatible with thecurrent signal interpretation methodology, but gamma-sensitive sensors are being investigated as well.An economic incentive can be identified forintroducing fixed in-core detectors into some PWRswhich presently rely only on traveling instruments.A continuous core monitoring system based on fixedincores could, for example, monitor the higher fluxpeaks associated with more efficient fuel managementswithout raising the maximum permitted linear heatrate limit.

Ack now I edgementThe author gratefully acknowledges the instrument-follow efforts of Dr. C. 0. Dechand and Dr. W. B.Terney which supplied operational performance data.

References:

1. L. A. Banda, B. I. Nappi, "Dynamic Compensationof Rhodium Self-Powered Neutron Detectors", IEEETransactions on Nuclear Science, Vol. NS-23,1975, pg. 311.

2. C-E Topical Report CENPD-153, Rev.of Uncertainty in the NuclearMeasured by the Self -Powered,Detector System", May 1980.

1, "EvaluationPower PeakingFixed In-Core

3. W. B. Terney, et al, "The C-E CECOR Fixed In-CoreDetector Analysis System", TIS7405, Trans ANS 44,p. 542, Detroit, June 1983.

4. L. A. Banda, "Operational Experience in RhodiumSelf-Powered Detectors"' IEEE Transactions onNuclear Science, Vol. NS-26, February 1979, pg.910.

5. G. Beraud, P. Guillery, "Le Thermometre Gamma,Nouvelle Instrumentation Fixed Pour La Mesure dela Puissance Locale d'un Reacteur PWR," IAEASymposium on Nuclear Power Plant Control andInstrumentation, Munich, FRG, October 1982.

6. To be published.