Risk-informeddesignguidanceforfuturereactorsystemsOriginalResearchArticle

Futurereactordesignsfaceanuncertainregulatoryenvironment.ItisanticipatedthattherewillbesomelevelofprobabilisticinsightsintheregulationsandsupportingregulatorydocumentsforGeneration-IVnuclearreactors.Centraltocurrentregulationsaredesignbasisaccidents(DBAs)andthegeneraldesigncriteria(GDC),whichwereestablishedbeforeprobabilisticriskassessments(PRAs)weredeveloped.Theseregulationsimplementastructuralistapproachtosafetythroughtraditionaldefenseindepthandlargesafetymargins.Inarationalistapproachtosafety,accidentfrequenciesarequantifiedandprotectivemeasuresareintroducedtomakethesefrequenciesacceptablylow.Bothapproacheshaveadvantagesanddisadvantagesandfuturereactordesignandlicensingprocesseswillhavetoimplementahybridapproach.Thispaperpresentsaniterativefour-steprisk-informedmethodologytoguidethedesignoffuture-reactorsystemsusingagas-cooledfastreactoremergencycorecoolingsystemasanexample.Thismethodologyhelpsdesignerstoanalyzealternativedesignsunderpotentialrisk-informedregulationsandtoanticipatedesignjustificationstheregulatormayrequireduringthelicensingprocess.Theanalysisdemonstratedtheimportanceofcommon-causefailuresandtheneedforguidanceonhowtochangethequantitativeimpactofthesepotentialfailuresonthefrequencyofaccidentsequencesasthedesignchanges.Deliberationisanimportantpartofthefour-stepmethodologybecauseitsupplementsthequantitativeresultsbyallowingtheinclusioninthedesignchoiceofelementssuchasbestdesignpracticesandeaseofonlinemaintenance,whichusuallycannotbequantified.Thecasestudyshowedthat,insomeinstances,thestructuralistandtherationalistapproacheswereinconsistent.Inparticular,GDC35treatsthedouble-endedbreakofthelargestpipeinthereactorcoolantsystemwithconcurrentlossofoffsitepowerandasinglefailureinthemostcriticalplaceastheDBAfortheemergencycorecoolingsystem.Seventeenoutofthe45variationsthatweconsideredviolatedthisDBA,butpassedtheprobabilisticscreeningcriteria.UsingPRAtechniques,wefoundthatthemeanfrequencyofthisaccidentwasverylow,thusindicatingthatdeterministiccriteriasuchasGDC35mustbereassessedinthelightofriskinsights.