UncertaintiesandprobabilitiesinnuclearreactorregulationOriginalResearchArticle

NuclearreactorregulationsintheUSAoriginallyspecifieddeterministiccriteriatobemetbythermal/hydrauliccodes.Asetofresultsfromasinglerunofthecodewascomparedwithasetofcriteria.Codesarenowbecoming“realistic”withuncertaintiesincorporatedintotheprocess,usuallyrequiringmanyrunsofthecode.Theresultisarangeofresultswithassociatedprobabilities.Regulationsdonotcurrentlyspecifyhowsuchresultsshouldbeevaluatedagainstacceptancecriteria.Asuggestionhasbeenmadetousealevelofconfidence(e.g.,0.95)thatthecriteriaaresatisfiedwithatleastsomespecifiedprobability(e.g.,0.95).Somevalidandinvalidwaysofdoingthisandtheirconsequencesarepresented.Itissuggestedthatanimprovedprocesswouldrelatethethermal/hydraulicoutputstomeasuresofcoredamageorsomeothersuitablemeasureofrisk.Furthermore,theuncertaintiesmodeledinthecodeshouldbecombinedwithuncertaintiesandprobabilities,e.g.,aboutfunctioningofequipmentandoperatoractions,thatarenowrepresentedintheprobabilisticriskassessment(PRA).Thetaskofcombiningthermal/hydrauliccodeswithPRAtechniquestoproduceaneffectiveandfeasibletechnologypresentsachallengetothetechnicalcommunity.