Late-phasemeltconditionsaffectingthepotentialforin-vesselretentioninhighpowerreactorsOriginalResearchArticle

Ifcoolingisinadequateduringareactoraccident,asignificantamountofcorematerialcouldbecomemoltenandrelocatetothelowerheadofthereactorvessel,ashappenedintheThreeMileIslandUnit2accident.Insuchacase,concernsaboutcontainmentfailureandassociatedriskscanbeeliminatedifitispossibletoensurethatthelowerheadremainsintactsothatrelocatedcorematerialsareretainedwithinthevessel.Accordingly,in-vesselretention(IVR)ofcoremeltasakeysevereaccidentmanagementstrategyhasbeenadoptedbysomeoperatingnuclearpowerplantsandplannedforsomeadvancedlightwaterreactors.However,itisnotclearthatcurrentlyproposedexternalreactorvesselcooling(ERVC)withoutadditionalenhancementscanprovidesufficientheatremovaltoassureIVRforhighpowerreactors(i.e.,reactorswithpowerlevelsupto1500MWe).Consequently,ajointUnitedStates/KoreanInternationalNuclearEnergyResearchInitiative(I-NERI)hasbeenlaunchedtodeveloprecommendationstoimprovethemarginofsuccessforin-vesselretentioninhighpowerreactors.ThisprogramisinitiallyfocussedontheKoreanAdvancedPowerReactor—1400MWe(APR1400)design.However,recommendationswillbedevelopedthatcanbeappliedtoawiderangeofexistingandadvancedreactordesigns.TherecommendationswillfocusonmodificationstoenhanceERVCandmodificationstoenhancein-vesseldebriscoolability.Inthispaper,late-phasemeltconditionsaffectingthepotentialforIVRofcoremeltintheAPR1400wereestablishedasabasisfordevelopingtheI-NERIrecommendations.Theselectionof‘bounding’reactoraccidents,simulationofthoseaccidentsusingtheSCDAP/RELAP5-3D©code,andresultinglate-phasemeltconditionsarepresented.Resultsfromthiseffortindicatethatboundinglate-phasemeltconditionscouldincludelargemeltmasses(>120,000kg)relocatingathightemperatures(∼3400K).Estimatedlowerheadheatfluxesassociatedwiththismeltcouldexceedthemaximumcriticalheatflux,indicatingadditionalmeasuressuchastheuseofacorecatcherand/ormodificationstoenhanceexternalreactorvesselcoolingmaybenecessarytoensurein-vesselretentionofcoremelt.