Press Release-95-080, the Nuclear Regulatory Commission Has Received Two Reports from Its Independent Advisory Committee on Reactor Safeguards
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Issue date: | 06/22/1995 |
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No.95-80FORIMMEDIATERELEASETel.301/415-8200(Thursday,June22,1995)NOTETOEDITORS:TheNuclearRegulatoryCommissionhasreceivedtworeportsfromitsindependentAdvisoryCommitteeonReactorSafeguards.
Theattachedreports,intheformofletters,commenton:1)Aproposedfinalpolicystatementontheuseofprobabilisticriskassessmentmethodsinnuclearregulatory activities;and2)ProposedfinalrevisionstoAppendixJoftheNRC'sPart50regulation,"PrimaryReactorContainmentLeakageTestingfor Water-CooledPowerReactors."Inaddition,theNRC'sExecutiveDirectorforOperationshasreceivedtwoACRSletterreports.Theycommenton:1)Aproposedfinalruleandregulatoryguideforfracturetoughnessrequirementsforlightwaterreactorpressurevessels;
and2)AproposedCommissionpaperonstaffpositionsontechnicalissuespertainingtotheWestinghousestandardized nuclearreactordesigndesignatedAP600.
Attachments:Asstated June15,1995Mr.JamesM.TaylorExecutiveDirectorforOperations U.S.NuclearRegulatoryCommission Washington,D.C.20555-0001
DearMr.Taylor:
SUBJECT:
PROPOSEDCOMMISSIONPAPERONSTAFFPOSITIONSONTECHNICALISSUESPERTAININGTOTHEWESTINGHOUSE AP600STANDARDIZEDPASSIVEREACTORDESIGNDuringthe422ndmeetingoftheAdvisoryCommitteeonReactorSafeguards,June8-10,1995,wediscussedthesubjectCommission paper.OurSubcommitteeonWestinghouseStandardPlantDesigns metonMay31,1995,toreviewthismatter.Duringthese meetings,wehadthebenefitofdiscussionswithrepresentatives ofthestaffandWestinghouse.Wealsohadthebenefitofthe documentsreferenced.
TheintentoftheproposedCommissionpaperistorecordthe staffpositionsontenseparateissues.Insomecases,however, thereviewshavenotprogressedtothepointthatthestaffcan recommendaposition.Insuchcases,thepaperdescribesthe approachthatWestinghouseisproposinginitsapplicationwith littlestaffcomment.Thestaffiscontinuingitsreviewof thesematters.Ourcommentsfollowthesameorganizationfoundintheattachmenttothepaper.I.Leak-Before-BreakApproachWestinghouseproposesthatanydynamiceffectsassociatedwithpostulatedpiperupturesinabroadrangeofpipesizes cansafelybeexcludedfromtheAP600pipingdesignbasisby virtueofthecurrentunderstandingofleakageandflaw sizes,andtheproposedleakageratelimitof0.5gpm.The rangeofpipesizes(4inchdiameterandgreater)thatwould becoveredbytheleak-before-break(LBB)approachis broaderthanthatallowedincurrentlyoperatingpressurized waterreactorsforwhichtheusualplantleakageratelimit issetat1.0gpm.
Thestaffagreedthattheleakageratelimitof0.5gpmisachievableintheAP600designbutwishestoadd conservatisminapplyingtheLBBapproachatthedesign certificationstagebyrequiringthatallloadsusedinthe pipingdesignbemultipliedbyafactorof1.4.Thestaff considersthisprudentbecausethedetaileddesignofpiping configurationandtheas-builtstresslevelswillnotbe availableforreviewatthecertificationstage.
Westinghousearguedthatthisaddedconservatismisnot neededandwillacttolimitthegainsinplantarrangement, economy,andsafetythatapplicationoftheLBBapproach couldprovide.Webelievethatthestaffishardpressedtojustifyaddingconservatismonallthepipingloadsabovethatwhichhas beenappliedtootherplants.Althoughitistruethatthe detailsofthepipingdesignaresomeyearsaway,thestaff andWestinghouseshouldnowbeabletocombinethestandard pipingdesignprotocolswithwhatisknownaboutthe performanceofflawedpipesintoadesigncriterionwithout excessiveconservatism.II.SecurityDesignTheproposedAP600plantarrangementincludesavehiclebarrierata"stand-offdistance,"butthepersonnelaccess controlwillislocatedwithinthenuclearislandofthe plant.Thevitalareasoftheplantarecoterminous.This featureisnotspecifictothepassivenatureoftheplant designandmightbeofferedinotherplantdesignsaswell.
Thestaffcontinuestoreviewtheproposeddesign,butseems receptivetotheidea.Thestaffbelievesthatinspections, tests,analyses,andacceptancecriteria(ITAAC)maybe requiredforthissecuritydesign.Webelievetheproposedsecuritydesigncouldmeetthesafetyandsecurityrequirementswhenimplemented,andwe areinterestedinthecontinuingstaffreviewofthe proposeddesign.Wealsonotedthatthedesignseemsto offerlessflexibilityforthemanyworkaccesspointsthat operatingplantsneedduringoutageperiods.III.TechnicalSpecificationsWestinghouseproposesthathotshutdown,ratherthancoldshutdown,beconsideredthesafeshutdownendstate.The staffevaluationhasnotprogressedtothepointwherethe staffcouldmakesubstantialcomment.Wealsowillwithhold commentatthistime.Weexpectthatreviewofthe probabilisticriskassessmentregardingthisissuewillbe
instructive.IV.InitialTestProgram Westinghouseandthestaffhavebeendiscussingthecontentoftheinitialtestprogramtobeperformedbythefirst plantbuiltunderthedesigncertification,andtest programstobeperformedbysubsequentplants.Webelieve thatthestaffisapproachingthematterappropriately.
Whenthediscussionshaveresultedinnewsubmittalsfrom Westinghouse,wemayhavemoreinformationonwhichto
comment.V.PassiveSystemThermal-HydraulicPerformanceReliabilityThestaffbelievesthatthemagnitudeofthenaturalforcesreliedonforthepassivesafetysystemsleadstolarge uncertaintiesinthethermal-hydraulicperformance.It statedthatonecouldquantifytheseuncertainties,butonly with"aprohibitivelylargenumberofcomputations."The staffproposedinsteadthatasurrogateconservativerisk-basedmarginsapproachbedevelopedtoeliminatetheneedto quantifythermal-hydraulicuncertaintyformost,ifnotall, accidentsequences.Thisapproachmaybeexpedient,butwebelieveeffortsshouldcontinueonthequantificationoftheuncertaintyfor useinprobabilisticriskassessments.VI.RegulatoryTreatmentofNon-SafetySystemsWestinghouseandthestaffhavebeenmeetingtoreviewtheneedforsomelevelofregulatorytreatmentforsystemsand componentsthatarenotsafetygrade,butthathave importantsupportandbackupfunctions.Akeyissue identifiedbythestaffinthisregardisthereliancethat Westinghouseplacesonequipmentormaterialsthatmaybe requiredbeyond72hoursfollowinganaccidentbutwhichare nottobestoredonsite.Thestaffreviewofthisissueis currentlyunderway,andthestaffhasnotstatedaposition beyondidentifyingconcerns.Accidentscenariosforexistingplantsreachapointwhenreliancemustbeplacedonoffsitematerials.Weexpect thatthestaffwillneedtobesatisfiedthattheAP600 designcanbebroughttoastableconditionusingonsite equipment,andthatanyadditionalneededresourcesare reasonablyavailable.VII.ContainmentPerformanceThestaffintendstousebothdeterministicandprobabilisticcontainmentperformancegoalsinreviewingthe AP600.ThisisconsistentwiththeCommissiondirection givenintheJuly21,1993StaffRequirementsMemorandum relatedtoSECY-93-087.Webelievethatthestaffposition isappropriate.
VIII.ExternalReactorVesselCoolingWestinghouseproposesasevereaccidentmitigationstrategyfortheAP600thatincludestheabilitytofloodthecavity underthereactortoalevelthatiseffectiveincooling thelowerreactorvesselshellandpreventingreactorvessel melt-throughfollowingcoremelt.Thestaffstatedthat thiswouldbeadesirablefeatureifthetechnicalissues canberesolved.Thestaffispursuingthoseissueswith Westinghouse.Webelievethatthestaffisfollowingan appropriatepath,butwewillcloselyfollowtheresolution ofthetechnicalissues.IX.PassiveHydrogenControlMeasuresTheproposedAP600designincludesunpoweredcatalyticrecombinerstocontrolhydrogengeneratedinadesign-basis accident(DBA).Thisisconsistentwiththeoverallconcept ofcontrollingdesign-basisaccidentswithpassivemeasures.
(Theplanistouseigniterstocontrolsevereaccident hydrogen.)Therearetechnicalquestionsinvolvingthe qualificationandeffectivenessofcatalyticrecombinersin anaccidentenvironment.Thestaffproposestoapprovethe useofpassiverecombinerscontingentontheresolutionof theseissues.Webelievethatthestaffpositionis
appropriate.X.DBAandLong-TermSevereAccidentRadiologicalConsequencesWhilethepassivenatureoftheAP600safetyfeaturesisveryattractive,thedesignhassomedownside characteristics.Post-accidentpressureinthecontainment willremainpositivelongerthanaplantdesignedwith activecooling.Further,followingsevereaccidents,the removalofradioactivespeciesfromthecontainment atmosphereisexpectedtobelessefficientwithpassive meansthanitwouldbeusingactivespraysorfilters.
Thus,thereisthepotentialforradioactiveleakageforan extendedperiod,comparedtothatoftheexistingplants.
Thestaffbelievesthatthissituationcallsfor considerationofadditionalmeans,suchasanonsafety-grade containmentspray,toreducecontainmentpressureand suspendedradionuclidesfollowingasevereaccident.The staffhasaskedWestinghousetoreconsideritsproposed positioninthisregard.Inaddition,Westinghouseproposesasourcetermsomewhatdifferentfromwhatthestaffwouldusewithrespecttoboth timingandreleasefractions.Thestaffindicatesthatthe technicaldifferencesherewouldnotbeofmuchconcernif thestaffcanbesatisfiedthattherewouldbeanactive systemavailabletoreducethecontainmentleakage
potential.
Webelievethattheissuesassociatedwiththepotentialforradioactiveleakageandthesourcetermshouldbetreated separately.Webelievethatthestaffpositiononthe sourcetermisappropriate.Theradioactiveleakagefrom theproposedcontainmentdesign,however,shouldbe consideredwithrespecttopublicriskandthesafetygoals.Inthecourseofthisreview,ithasoccurredtousthatthecertificationofadvancedlight-waterreactorsprovidesan importantopportunitytocontinuetheevolutiontoward performance-basedregulation.Currentplans,unfortunately,do nottakecompleteadvantageofthisopportunity,perhapsbecause ofscheduleconstraints.Thedebateovertheproceduretoimpose unquantifiedlevelsofconservatismonanalysesofleak-before-breakforsmall-diameterpipingreflectsacontinuationofpast practice.TheaspirationsofboththeindustryandtheNRCwould bebetterservedbyaperformance-basedcriterion.Similarly, argumentsonthetimeframeforanalysesofradionuclide concentrationsincontainmentwouldbeunnecessaryifa performance-basedcriterionwerederived.Ingeneral,such performance-basedcriteriawouldbemoreconsistentwiththe state-of-the-artengineeringbeingemployedinthedesignof advancedlight-waterreactorsthanthecontinueduseof traditionalcriteriadevelopedinthepastwhentherewasa poorerunderstandingofsafety-relatedprocessesandphenomena.Dr.DanaA.PowersdidnotparticipateintheCommittee'sdeliberationsregardingthesevereaccidentsourceterm.Dr.
ThomasS.KressdidnotparticipateintheCommittee's deliberationsregardingexternalreactorvesselcooling.
Sincerely,T.S.Kress Chairman References
- 1.MemorandumdatedMay15,1995,fromJ.Taylor,NRCExecutiveDirectorforOperations,totheCommissioners,
Subject:
AdvanceInformationCopyofForthcomingCommissionPaper-StaffPositionsonTechnicalIssuesPertainingtothe WestinghouseAP600StandardizedPassiveReactorDesign2.SECY-93-087datedApril2,1993,fromJ.Taylor,NRCExecutiveDirectorforOperations,totheCommissioners,
Subject:
Policy,Technical,andLicensingIssuesPertaining toEvolutionaryandAdvancedLight-WaterReactorDesigns3.SRMdatedJuly21,1993,fromS.Chilk,SecretaryoftheCommission,toJ.Taylor,NRCExecutiveDirectorfor Operations,
Subject:
SECY-93-087-Policy,Technical,and LicensingIssuesPertainingtoEvolutionaryandAdvancedLight-WaterReactor(ALWR)Designs June16,1995TheHonorableIvanSelin Chairman U.S.NuclearRegulatoryCommission Washington,D.C.20555-0001
DearChairmanSelin:
SUBJECT:
PROPOSEDFINALPOLICYSTATEMENTONTHEUSEOFPROBABILISTICRISKASSESSMENTMETHODSINNUCLEAR REGULATORYACTIVITIESDuringthe422ndmeetingoftheAdvisoryCommitteeonReactorSafeguards,June8-10,1995,wereviewedtheproposedfinal PolicyStatementontheUseofProbabilisticRiskAssessment MethodsinNuclearRegulatoryActivities.Wehadthebenefitof presentationsbytheNRCstaffconcerningtheresolutionof publiccommentsaswellascommentswemadeonadraftversionof thePolicyStatement.Wealsohadthebenefitofpresentations byrepresentativesoftheNuclearEnergyInstituteconcerninga draftPSAApplicationsGuide.Finally,wehadthebenefitofthe referenceddocuments.Wesupportapolicystatementthatencouragestheuseofprobabilisticriskassessment(PRA)methodsinnuclearregulatory activities.Apolicystatementthatextendstheuseofsuch methodsbeyondtheregulationofnuclearpowerreactorsinto otherareaswithinthejurisdictionoftheNRCprovidesawelcome opportunitytoimproveboththeefficiencyandtheeffectiveness ofthebodyoftheNRCregulations.RevisionsmadetothePolicy Statementaccommodatecommentswemadeonanearlierdraft.We feelitusefultoissueapolicystatementtoupdatepositions adoptedinthepastbytheNRCconcerningtheuseofPRA.WeareinterestedinthechallengesthatwillhavetobemettoimplementthePolicyStatement.Technicallydefensible,risk-basedregulatoryactivitieswillrequiretheavailabilityofPRAs thatareadequatelycompleteandofacceptablequality.
Uncertaintiesintheresultsoftheseriskassessmentswillhave tobecharacterizedadequately.Thestaffindicatedthatitis awareoftheseneeds.Welookforwardtohearingmoreabout staffeffortstodefinestandardsforPRAsandstrategiesthat willbeadoptedtoauditandtoreviewPRAssubmittedby
licensees.
ThestaffisnowconsideringthedecisioncriteriathatwillbeusedinconjunctionwiththeapplicationofPRAs.Thestaffhas statedthatitfeelsinhibitedfromusingtheNRCsafetygoalsin decisionsconcerningspecificplants.Weencouragetheuseof technicallydefensiblePRAmethodsforriskmanagementof individualplantsconsistentwiththeNRCsafetygoals.Wenote that,insuchapplications,thesegoalsshouldnotbetreatedas safetycriteria.Webelievethatplant-specificriskmanagement isanimportantsubjectwhichweplantopursue.Wewillreport onourfindingsinthefuture.ThewidespreaduseofPRAmethodswithintheNRCwillnecessitateaculturalchangewithintheagency.Thestaffwillhavetobe receptivetodifferentapproachestogivenissuesbydifferent licensees.Trainingforthestaffmayneedtobeonmorethan PRAapplicationsandmethods.Forinstance,traininginformal decisionanalysismethodsmayalsoassisttheneededchangein cultureattheNRC.Weareinterestedinthefullscopeofthe trainingprograminPRAbeingdevelopedfortheNRCstaff.We plantoreviewthistrainingprogramandthePRAresearchprogram thatNRCsupports.ThePolicyStatementcallsfortheconsiderationoftheuseofPRAmethodsinareaswherethesemethodshavenotheretoforebeen extensivelyused.Consequently,themethodsforthesenew applicationsarenotaswelldevelopedastheyarefor applicationtonuclearpowerplants.TheNRCmayneedtosupport anexpandedresearcheffortinthedevelopmentofPRAmethodsfor applicationinthesenewareas.
Sincerely,T.S.Kress Chairman References
- 1.SECY-95-126datedMay18,1995,fromJamesM.Taylor,ExecutiveDirectorforOperations,NRC,fortheCommissioners,
Subject:
FinalPolicyStatementontheUseofProbabilisticRisk AssessmentMethodsin NuclearRegulatory Activities(Draft
Predecisional)2.ACRSReportdatedMay11,1994,fromT.S.Kress,Chairman,ACRS,toJamesM.Taylor,ExecutiveDirectorforOperations, NRC,
Subject:
DraftPolicyStatementontheUseof ProbabilisticRiskAssessmentMethodsinReactorRegulatory
Activities 3.LetterdatedJanuary17,1995,fromWilliamH.Rasin,Nu-clearEnergyInstitute,toAshokC.Thadani,OfficeofNu-clearReactorRegulation,NRC,transmittingfinaldraftof PSAApplicationsGuide4.ACRSReportdatedMay13,1987,fromWilliamKerr,Chairman,ACRS,toTheHonorableLandoW.Zech,Chairman,NRC,
Subject:
ACRSCommentsOnAnImplementationPlanForThe SafetyGoalPolicy June16,1995Mr.JamesM.TaylorExecutiveDirectorforOperations U.S.NuclearRegulatoryCommission Washington,D.C.20555-0001
DearMr.Taylor:
SUBJECT:
PROPOSEDFINALRULEANDREGULATORYGUIDEFORFRACTURETOUGHNESSREQUIREMENTSFORLIGHTWATERREACTORPRESSUREVESSELSDuringthe422ndmeetingoftheAdvisoryCommitteeonReactorSafeguards,June8-10,1995,wediscussedthesubjectruleandregulatoryguide.Wehadthe benefitofdiscussionswithrepresentativesoftheNRCstaff.Wealsohadthe benefitofthedocumentsreferenced.Theneedforthetimelydevelopmentofguidanceandrequirementsforthethermalannealingofreactorpressurevessels(RPVs)becameapparentduring considerationofthefracturetoughnessoftheRPVattheYankeeNuclearPower Station.TherecentreviewofthedataforthePalisadesRPVsuggeststhat variabilityinthecompositionofweldsand,hence,theuncertaintyinthe estimationofpressurizedthermalshockreferencetemperature(RT PTS)isgreaterthanpreviouslyconsidered.Theresultofthisreviewaddsgreaterweightto theneedforappropriateregulatoryguidanceonthermalannealing.WereviewedadraftversionoftheruleandtheregulatoryguideforfracturetoughnessrequirementsduringourSeptember1993meeting.Anumberofchanges havebeenmadeintheruleandregulatoryguideasaresultofpublic comments.Thesechangesdonotaffectourtechnicalassessmentthattherule andregulatoryguideshouldproveusefultothelicenseesandtheNRCstaff, andwebelievetheyshouldbeissued.Wealsosupporttheproposedchangesto AppendixHof10CFRPart50andthepressurizedthermalshockrule(10CFR
50.61).WehavenoobjectiontothechangesinAppendixGthatareintendedtoclarifyandrestructurethecurrentrequirements.Webelieve,however,thatthe prohibitionagainstusingnuclearheattoconductASMESectionXIpressureand leaktestsofboilingwaterreactorpressurevesselsmeritsre-examination.
Itisnotatallapparentthatthisprohibitioncanbejustifiedintermsof risk.Indeed,thereisreasontobelievethattherecouldbeareductionin riskinviewoftheincreasedrequirementsforcontainmentandemergencycore coolingforcriticalreactors.Werecommendthataprobabilisticassessment beperformed.Sincethepracticeofusingnuclearheat iscurrentlyprohibited,anexplicitstatementinAppendixGisunnecessaryandwouldrestrictfutureactionbasedupontheresultsoftheprobabilistic assessment.However,wedonotwishthisreassessmenttodelaypublicationof thethermalannealingrule,theamendmenttoAppendixH,ortheamended pressurizedthermalshockrule.
Sincerely,T.S.Kress Chairman References
- 1.LetterdatedSeptember20,1993,fromJ.Wilkins,Jr.,Chairman,ACRS,toJ.Taylor,ExecutiveDirectorforOperations,NRC,
Subject:
ProposedRule andRegulatoryGuideforFractureToughnessRequirements2.MemorandumdatedMay23,1995,fromL.Shao,Director,DivisionofEngineeringTechnology,RES,toJ.Larkins,ExecutiveDirector,ACRS,
Subject:
RequestforACRSReviewofFinalRuleandRegulatoryGuidefor FractureToughnessRequirementsforLightWaterReactorPressureVessels, withthefollowingattachments:
ÿAmendmentto10CFR50.61,"FractureToughnessRequirementsforProtectionAgainstPressurizedThermalShockEvents"ÿAmendmentto10CFRPart50,AppendixG,"FractureToughness Requirements"ÿAmendmentto10CFRPart50,AppendixH,"ReactorVesselMaterialSurveillanceProgramRequirements"ÿFinalRule(10CFR50.66),"RequirementsforThermalAnnealingoftheReactorPressureVessel"ÿProposedRegulatoryGuide1.XXXX,"FormatandContentofReportforThermalAnnealingofReactorPressureVessels" June16,1995TheHonorableIvanSelin Chairman U.S.NuclearRegulatoryCommission Washington,D.C.20555-0001
DearChairmanSelin:
SUBJECT:
PROPOSEDFINALREVISIONSTOAPPENDIXJOF10CFRPART50,"PRIMARYREACTORCONTAINMENTLEAKAGETESTINGFORWATER-COOLEDPOWER REACTORS"Duringthe422ndmeetingoftheAdvisoryCommitteeonReactorSafeguards,June8-10,1995,weconsideredthechangesmadetotheproposedfinalrevisions toAppendixJinresponsetopubliccomments.Thesechangesdidnotalterour viewsexpressedinthereportdatedSeptember19,1994.Wefindnoneedtomeet againwiththestaffonthissubjectandstandbyourpreviouslyexpressed position.Sincerely,T.S.Kress Chairman References
- 1.MemorandumdatedJune6,1995,fromJosephA.Murphy,ExecutiveAssistant totheDirector,RES,toJohnT.Larkins,ExecutiveDirector,ACRS,
Subject:
ACRSReviewofFinalAmendmenttoAppendixJof10CFRPart50(DraftPredecisionalAttachment) 2.ACRSReportdatedSeptember19,1994,fromT.S.Kress,Chairman,ACRS,toTheHonorableIvanSelin,Chairman,NRC,
Subject:
ProposedRevisionstoAppendixJof10CFRPart50, "PrimaryReactorContainment LeakageTestingforWater-Cooled PowerReactors" TheHonorableIvanSelin15