Level2analysistoestimateLERFriskfromathermallyinducedruptureofasteamgeneratortubedefectOriginalResearchArticle

ThispaperoutlinestheLevel2portionofamethodologyfordeterminingtheincrementalinducedsteamgeneratortuberupturelargeearlyreleasefractioncausedbyanactualthrough-walldefect.ThisdefectwasresponsiblefortheminorsteamgeneratortubeleakthatoccurredinSeptember2002attheComanchePeakSteamElectricStationUnit1.Inordertoquantifytheperformanceofthedefectovertheoperatingcycle,arangeofdefectlengthswereinputtothePROBFAILcomputercode[Kenton,M.,2001.PROBFAIL:AComputerCodeforEvaluatingtheLikelihoodofSteamGeneratorTubeRuptureinSevereNuclearPowerPlantAccidents,CREARETM-2138],usingappropriateboundaryconditionsderivedfromMAAP4[Henry,R.,etal.,May1994.MAAP4—ModularAccidentAnalysisProgramforLWRPowerPlants,ComputerCodeManual,EPRIResearchProject3131-02]runs.Fromtheanalysisofthecalculatedtimesofburstforeachassumeddefectlength,theminimumthrough-walldefectlengthnecessaryfortubebursttooccurpriortohotlegorsurgelinecreeprupturewascalculated.Theprobabilitythatthedefectwouldactuallyhavethislengthwasthenestimatedbydeterminingthefractionofthecycleforwhichthedefectwouldbeatleastthatlong.ThemethodologydevelopmentandimplementationreliedonMAAP4runs,whicharediscussedextensivelyinconnectionwiththeirrolein:(1)guidingtheconstructionoftheaccidentprogressioneventtree,(2)generatingrelevantinformationforprobabilityassignmentsinthevariousunderlyingfaulttreesand(3)obtainingboundaryconditionsofpressureandtemperatureforuseinPROBFAIL.TheoverallincrementinLERFduetotheexistenceofthedefectwascalculatedtobeapproximately4E−08.