Accidentmanagementfollowinglossofresidualheatremovalduringmid-loopoperationinaWestinghousetwo-loopPWROriginalResearchArticle

Thepossibilityofanaccidentorcomponentfailureduringmid-loopoperationhasbeenidentifiedinprobabilisticstudiesasamajorcontributortocoremeltfrequencyandsourcetermriskduringshutdownconditions.Thewiderangeofplantstatesencounteredandtheunavailabilityofcertainsafetyfeaturesmakeitdifficulttoguaranteethatsafetysystemsoperationwillalwaysbesufficienttoterminatetheaccidentevolution.InthiscontextanalysesareperformedusingMELCOR1.8.5forlossofresidualheatremoval(RHR)atvarioustimesduringmid-loopoperationofaWestinghousetwo-loopPWR.IntheabsenceofrecoveryofRHRorotheraccidentmanagement(AM)measures,thesequencesnecessarilyleadtoalongtermcoreuncovery,heat-upanddegradation,lossofgeometryandeventualfailureofthereactorpressurevessel(RPV).Theresultsshowanextensivetimewindowbeforeuncoveryandadditionallybeforecoredamage,whichincreaseprogressivelywithincreasingtimeaftershutdownatwhichlossofRHRoccurs.Significantoxidationofthecladdingmayresultinconcentrationsofhydrogensufficientfordeflagration.TheslowevolutionimpliesanopportunityfortheplantoperatorstoinitiateAMmeasuresevenaftercoreuncoveryhasstarted.Theanalysesindicateasubstantialtimewindowduringtheuncoverywithinwhichtheinjectioncanrecoverthecorewithoutdamage.Theupperendofthewindowisdeterminedbythetemperatureatwhichheatfromcladdingoxidationbecomesadominantfactor,markingacriticalpointfortheeffectivenessofthisrecoverymode.Theresultsprovideconfidenceintheinherentrobustnessoftheplantwithrespecttoaccidentsequencesofthistype.