Regulatory Guide 1.105

Setpoints for Safety-Related Instrumentation

ML993560062
Person / Time
Issue date:	12/31/1999
From:	Office of Nuclear Regulatory Research
To:
Aggarwal S K (301)415-6005
References
RG-1.105, Rev. 3
Download: ML993560062 (7)

1ForthefulltextoftheGeneralDesignCriteriaandothersectionsoftheregulationscitedinthisguide,see10CFRPart50,"DomesticLicensingofProductionandUtilizationFacilities."RegulatoryguidesareissuedtodescribeandmakeavailabletothepublicsuchinformationasmethodsacceptabletotheNRCstaffforimplementingspecificpartsoftheNRC'sregulations,techniquesusedbythestaffinevaluatingspecificproblemsorpostulatedaccidents,anddataneededbytheNRCstaffinitsreviewofapplicationsforpermitsandlicenses.Regulatoryguidesarenotsubstitutesforregulations,andcompliancewiththemisnotrequired.MethodsandsolutionsdifferentfromthosesetoutintheguideswillbeacceptableiftheyprovideabasisforthefindingsrequisitetotheissuanceorcontinuanceofapermitorlicensebytheCommission.Thisguidewasissuedafterconsiderationofcommentsreceivedfromthepublic.Commentsandsuggestionsforimprovementsintheseguidesareencour agedatalltimes,andguideswillberevised,asappropriate,toaccommodatecommentsandtoreflectnewinformationorexperience.Writtencommentsmayb esubmittedtotheRulesandDirectivesBranch,ADM,U.S.NuclearRegulatoryCommission,Washington,DC20555-0001.Regulatoryguidesareissuedintenbroaddivisions:1,PowerReactors;2,ResearchandTestReactors;3,FuelsandMaterialsFacilities;4,EnvironmentalandSiting;5,MaterialsandPlantProtection;6,Products;7,Transportation;8,OccupationalHealth;9,AntitrustandFinancialReview;and10,Ge neral.Singlecopiesofregulatoryguides(whichmaybereproduced)maybeobtainedfreeofchargebywritingtheDistributionServicesSection,U.S.Nuclea rRegulatoryCommission,Washington,DC20555-0001,orbyfaxto(301)415-2289,orbyemailtoDISTRIBUTION@NRC.GOV.Manyregulatoryguidesare alsoavailableontheinternetatNRC'shomepageat<WWW.NRC.GOV>.U.S.NUCLEARREGULATORYCOMMISSIONRevision3December1999 REGULATORY

GUIDEOFFICEOFNUCLEARREGULATORYRESEARCHREGULATORYGUIDE1.105(DraftwasDG-1045)SETPOINTSFORSAFETY-RELATEDINSTRUMENTATION

A. INTRODUCTION

Criterion13,"InstrumentationandControl," 1ofAppendixA,"GeneralDesignCriteriaforNuclearPowerPlants,"to10CFRPart50,"DomesticLicensingofProductionandUtilizationFacilities,"requires, amongotherthings,thatinstrumentationbeprovidedtomonitorvariablesandsystemsandthatcontrolsbe providedtomaintainthesevariablesandsystemswithinprescribedoperatingranges.Criterion20,"ProtectionSystemFunctions,"ofAppendixAto10CFRPart50requires,amongotherthings,thattheprotectionsystembedesignedtoinitiateoperationofappropriatesystemstoensurethat specifiedacceptablefueldesignlimitsarenotexceeded.Paragraph(c)(1)(ii)(A)of§50.36,"TechnicalSpecifications,"of10CFRPart50requires,inpart,that,wherealimitingsafetysystemsettingisspecifiedforavariableonwhichasafetylimithasbeenplaced, thesettingbesochosenthatautomaticprotectiveactionwillcorrecttheabnormalsituationbeforeasafety limitisexceeded.Italsorequires,amongotherthings,thatthelicenseenotifytheNRCifthelicensee determinesthatanautomaticsafetysystemdoesnotfunctionasrequired.Thelicenseeisrequiredtothen reviewthematterandrecordtheresultsofthereview.

2CopiesmaybeobtainedfromtheInstrumentSocietyofAmerica,67AlexanderDrive,ResearchTrianglePark,NC

20779. 2ThisguidedescribesamethodacceptabletotheNRCstaffforcomplyingwiththeNRC'sregulationsforensuringthatsetpointsforsafety-relatedinstrumentationareinitiallywithinandremain withinthetechnicalspecificationlimits.TheguideisbeingrevisedtoendorsePartlofISA-S67.04-1994,

"SetpointsforNuclearSafety-RelatedInstrumentation." 2Thisstandardprovidesabasisforestablishingsetpointsfornuclearinstrumentationforsafetysystemsandaddressesknowncontributingerrorsinthe

channel.Theinformationcollectionscontainedinthisregulatoryguidearecoveredbytherequirementsin10CFRPart50,whichwereapprovedbytheOfficeofManagementandBudget,approvalnumber3150-

0011.TheNRCmaynotconductorsponsor,andapersonisnotrequiredtorespondto,acollectionof informationunlessitdisplaysacurrentlyvalidOMBcontrolnumber.

B. DISCUSSION

Instrumentsetpointuncertaintyallowancesandsetpointdiscrepancieshaveledtoanumberofoperationalproblems.Operatingexperienceindicatesthatsetpointsforsafety-relatedinstrumentationmay allowplantstooperateoutsidethelimitingconditionsofoperationspecifiedintheirtechnical specifications.Licenseeshavediscoveredconflictsbetweenexistingsetpointsandengineering calculations.Thecausesforthesesetpointdiscrepancieswereproblemswithindustrypracticesthatledto errorsincalibrationproceduresandalackofunderstandingoftherelationshipofthesetpointtothe allowablevalue.Additionalproblemsnotedincludedvaryingsetpointmethodologiesforengineering calculations,alackofaconsistentdefinitionofallowablevaluebetweendifferentsetpointmethodologies, andimproperunderstandingoftherelationshipoftheallowablevaluetoearliersetpointterminology, procedures,andoperabilitycriteria.Furtherproblemswerenotedwhenprocedures(thesetpointprocess)

(1)failedtoprovideanadequatemarginbetweentheinstrumentas-leftcriteriaandthevalues(tripset pointorallowablevalues)requiredperthetechnicalspecifications,(2)didnotalwaysreflectcurrent designcriteria,and(3)didnotensurethatrevisedinstrumentloopswereverifiedtotheoriginaldesign requirementsorthatinstrumentmodificationswereevaluatedfortheireffectonsetpointcalculations.It hasalsobeennotedthatlicenseesdonottypicallyverifywhethersetpointcalculationdriftassumptions haveremainedvalidforthesystemsurveillanceinterval.ISA-S67.04wasrevisedin1987toprovideclarificationandtoreflectindustrypractice.Theterm"tripsetpoint"wasmadeconsistentwiththeterminologyusedbytheNRCstaff.Thestandardwasrevisedfurtherin1994.Theeffectsofuncertaintyallowancesanddiscrepanciesinsetpoints,alongwithoperationalexperience,wereappropriatelyaddressedduringthisrevisionofISA-

S67.04.ThisrevisionofthestandardalsoreflectstheImprovedTechnicalSpecificationprogram(a cooperativeeffortbetweenindustryandtheNRCstaff)andreflectcurrentindustrypractice.Thisstandard providesabasisforestablishingsetpointsfornuclearinstrumentationforsafetysystemsandaddresses knowncontributingerrorsinaparticularchannelfromtheprocess(includingtheprimaryelementand sensor)throughandincludingthefinalsetpointdevice.Theterm"tripsetpoint"isretainedinISA-S67.04-1994.However,Figure1inISA-S67.04-1994(forconvenience,thisfigurehasbeenreproducedasFigure1inthisguide)hasbeenrevisedtodepict region"E,""aregionofcalibrationtolerance."Thecalibrationtoleranceuncertaintiesdepictedbyregion

3Singlecopiesofregulatoryguides,bothactiveanddraft,maybeobtainedfreeofchargebywritingtheOfficeofAdministration,Attn:ReproductionandDistributionServicesSection,U.S.NuclearRegulatoryCommission,Washington,DC20555,orbyfaxto(301)415-2289,orbyemailto<DISTRIBUTION@NRC.GOV>.CopiesarealsoavailableforinspectionorcopyingforafeefromtheNRCPublicDocumentRoomat2120LStreetNW.,

Washington,DC;thePDR'smailingaddressisMailStopLL-6,Washington,DC20555;telephone(202)634-3273;

fax(202)634-3343.

3"E"shouldbedefinedandaccountedforinthelicensee'ssetpointmethodology.Atripsetpointvalueidentifiedtobeoutsideregion"E"regardlessofdirectionrequiresreadjustmenttosatisfythesetpoint methodologyanduncertaintiesidentifiedinFigure1(acceptableas-leftcondition).Itshouldbenotedthat thisstandarddoesnotdefine"nominal"tripsetpoint.ThetripsetpointasdepictedinFigure1isconsistent withtheterm"nominal"tripsetpointasshownaboutadefinedcalibrationtoleranceband.Figure1ofthestandardprovidessetpointrelationshipsfornuclearsafety-relatedsetpoints.Thefiguredenotesrelativepositionandnotdirection,butitshouldbenotedthattheuncertaintyrelationships depictedbyFigure1donotrepresentanyoneparticular method(direction,combination,orrelationshipofuncertaintygroupings)forthedevelopmentofatrip setpointorallowablevalue.Section4ofISA-S67.04-1994statesthatthesafetysignificanceofvarioustypesofsetpointsforsafety-relatedinstrumentationmaydiffer,andthusalessrigoroussetpointdeterminationmethodmaybe appliedforcertainfunctionalunitsandlimitingconditionsofoperation(LCOs).Asetpointmethodology canincludesuchagradedapproach.However,thegradingtechniquechosenbythelicenseeshouldbe consistentwiththestandardandshouldconsiderapplicableuncertaintiesregardlessofthesetpoint application.Additionally,theapplicationofthestandard,usinga"graded"approach,isalsoappropriate fornon-safetysysteminstrumentationformaintainingdesignlimitsdescribedintheTechnical Specifications.Examplesmayincludeinstrumentationreliedoninemergencyoperating procedures(EOPS),andformeetingapplicableLCOs,andformeetingthevariablesinRegulatoryGuide

1.97,"InstrumentationforLight-Water-CooledNuclearPowerPlantsToAssessPlantandEnvirons ConditionsDuringandFollowinganAccident." 3TheindustryconsensusstandardANSI/ANS-10.4-1987,"GuidelinesfortheVerificationandValidationofScientificandEngineeringComputerProgramsfortheNuclearIndustry,"provideshelpful informationonthequalificationofsetpointmethodologysoftware.ISA-S67.04-1982hasbeenusedbylicenseesforsetpointmethodologyandinstrumentdriftevaluations.ISA-S67.04-1994provideslimitedguidanceondriftevaluationsanduncertaintyterm developmentfortheevaluationofaninstrumentsurveillanceinterval.The

4 A D E B CSafetyLimitAnalyticalLimitNote:Thisfigureisintendedtoproviderelative positionandnotto implydirection.Allowancedescribedinparagraph4.3.1Allowancedescribedinparagraph4.3.1Regionwherechannelmaybedeterminedinoperable Plantoperatingmargin Regionofcalibrationtolerance(acceptableasleftcondition)

describedinparagraph4.3.1 A. B. C.

D.

E.AllowableValue(LSSS)TripSetpoint(LSSS)NormalFigure1.NuclearSafety-RelatedSetpointRelationships

5(ReproducedfromISA-67.04-1994)

6staffhasgenerallyaccepteddriftevaluationsbasedonstatisticalpredictiontechniques.However,significantvariabilityhasbeenobservedinlicensees'surveillanceintervalevaluationswithregardtodrift, setpointmethodology,andcompleteness.ThefollowingconcernswereidentifiedduringtheNRCstaff review,buttheyhavebeenresolvedduringthedevelopmentofISA-S67.04-1994.Limitedinstrumentdriftdatawereincludedinthelicenseesetpointstudy.Driftdataaccountforalldatapointsfromasurveillancecalibration(i.e.,nine-pointcheck)asindependentdata,butinadequatejustificationisprovidedforthisassumption.Driftdatapoints alsoincludedinterimcalibrations.Alargenumberofdatapointswasprovidedforalimitednumberofinstruments.Flawedoutlieranalysisresultedinvaliddatabeingremovedfromthedataset.Driftdependencyontimewasassumedtobenegligibleovertheintervalselected,andinadequatejustificationwasprovidedwhenextrapolatingtoanextendedsurveillanceinterval(e.g.,24

months).Setpointmethodologyassumesnormaldistributionofdatawhensuchanassumptionwasnot verified.Instrumentationevaluations(historical,maintenance,drift)wereincomplete.Driftprojections,includingthosebasedonregressionanalyses,maynotaccountforpenaltiesforuncertaintyprojection(extendedsurveillanceinterval-drift)beyondthetimerangeforthedata

collected.Instrumentapplicationandprocessorinstallationvariableswerenotevaluated.Theuncertaintiesassumedforinstrumentation,includingprimaryelements,weresubsequentlynotverifiedorcontrolledthroughsurveillancetesting,qualification,ormaintenanceprograms.Theacceptabilityofpoolinggenericdriftdatawithplant-specificdataorweighingthedataaccordingtothesourceofthedatawasnotjustified.Allavailableapplicabledatawerenotutilizedintheanalysis.Section4.3ofISA-S67.04-1994statesthatthelimitingsafetysystemsetting(LSSS)maybethetripsetpoint,anallowablevalue,orboth.Forthestandardtechnicalspecifications,thestaffdesignatedthe allowablevalueastheLSSS.Inassociationwiththetripsetpointandlimitingconditionsforoperation (LCOs),theLSSSestablishesthethresholdforprotectivesystemactiontopreventacceptablelimitsbeing exceededduringdesignbasisaccidents.TheLSSSthereforeensuresthatautomaticprotectiveactionwill correcttheabnormalsituationbeforeasafetylimitisexceeded.Alicensee,withjustification,may proposeanalternativeLSSSbasedonitsparticularsetpointmethodologyorlicense.Thestandardprovidesfortheaccountingofmeasurementandtestequipment(MTE)uncertainties,butMTEcriteriaarenotspecificallyidentifiedwithinthestandard.CriteriaXIandXIIinAppendixBto

710CFRPart50providerequirementsforqualityregardingtesting.RegulatoryGuide1.118,"PeriodicTestingofElectricPowerandProtectionSystems," 3providesguidanceonperiodicsurveillancetesting.PartII,"MethodologiesfortheDeterminationofSetpointsfortheNuclearSafety-RelatedInstrumentation,"ofISA-S67.04-1994isnotaddressedbythisregulatoryguide.C.REGULATORYPOSITIONConformancewithPart1ofISA-S67.04-1994,"SetpointsforNuclearSafety-Related Instrumentation," 2withthefollowingexceptionsandclarifications,providesamethodacceptabletotheNRCstaffforsatisfyingtheNRC'sregulationsforensuringthatsetpointsforsafety-related instrumentationareestablishedandmaintainedwithinthetechnicalspecificationlimits.

1.Section4ofISA-S67.04-1994specifiesthemethods,butnotthecriterion,forcombininguncertaintiesindeterminingatripsetpointanditsallowablevalues.The95/95tolerancelimitisan acceptablecriterionforuncertainties.Thatis,thereisa95%probabilitythattheconstructedlimitscontain

95%ofthepopulationofinterestforthesurveillanceintervalselected.

2.Sections7and8ofPart1ofISA-S67.04-1994referenceseveralindustrycodesandstandards.IfareferencedstandardhasbeenincorporatedseparatelyintotheNRC'sregulations,licensees andapplicantsmustcomplywiththatstandardassetforthintheregulation.Ifthereferencedstandardhas beenendorsedinaregulatoryguide,thestandardconstitutesamethodacceptabletotheNRCstaffof meetingaregulatoryrequirementasdescribedintheregulatoryguide.Ifareferencedstandardhasbeen neitherincorporatedintotheNRC'sregulationsnorendorsedinaregulatoryguide,licenseesandapplicants mayconsiderandusetheinformationinthereferencedstandardifappropriatelyjustified,consistentwith currentregulatorypractice.

3.Section4.3ofISA-S67.04-1994statesthatthelimitingsafetysystemsetting(LSSS)maybemaintainedintechnicalspecificationsorappropriateplantprocedures.However,10CFR50.36states thatthetechnicalspecificationswillincludeitemsinthecategoriesofsafetylimits,limitingsafetysystem settings,andlimitingcontrolsettings.Thus,theLSSSmaynotbemaintainedinplantprocedures.Rather, theLSSSmustbespecifiedasatechnical-specification-definedlimitinordertosatisfytherequirementsof

10CFR50.36.TheLSSSshouldbedevelopedinaccordancewiththesetpointmethodologysetforthin thestandard,withtheLSSSlistedinthetechnicalspecifications.

4.ISA-S67.04-1994providesadiscussiononthepurposeandapplicationofanallowablevalue.Theallowablevalueisthelimitingvaluethatthetripsetpointcanhavewhentestedperiodically, beyondwhichtheinstrumentchannelisconsideredinoperableandcorrectiveactionmustbetakenin accordancewiththetechnicalspecifications.Theallowablevaluerelationshiptothesetpointmethodology andtestingrequirementsinthetechnicalspecificationsmustbedocumented.

D. IMPLEMENTATION

ThepurposeofthesectionistoprovideinformationtoapplicantsandlicenseesregardingtheNRCstaff'splansforusingthisregulatoryguide.

8ExceptinthosecasesinwhichanapplicantorlicenseeproposesanacceptablealternativemethodforcomplyingwithspecifiedportionsoftheNRC'sregulations,themethodsdescribedinthisguidewillbe usedintheevaluationofsubmittalsinconnectionwithapplicationsforconstructionpermits,operating licenses,andcombinedlicenses.Itwillalsobeusedtoevaluatesubmittalsfromoperatingreactor licenseeswhovoluntarilyproposetoinitiatesystemmodificationsifthereisaclearnexusbetweenthe proposedmodificationsandthisguidance.

9VALUE/IMPACTSTATEMENTAdraftvalue/impactstatementwaspublishedwiththedraftproposedRevision3ofthisguidewhenitwaspublishedforpubliccomment(DG-1045,October1996).Nochangeswerenecessary,soa separatevalue/impactstatementforthefinalguidehasnotbeenprepared.Acopyofthedraftvalue/impact statementisavailableforinspectionorcopyingforafeeintheNRC'sPublicDocumentRoomat2120L

StreetNW.,Washington,DCundertaskDG-1045.