Consequencesofmaterialeffectsonin-vesselretentionOriginalResearchArticle

In-vesselretention(IVR)consistsincoolingthecoriumcontainedinthereactorvesselbynaturalconvectionandreactorcavityflooding.Thisstrategyofsevereaccidentmanagementenablesthecoriumtobekeptinsidethesecondconfinementbarrier:thereactorvessel.ThegeneralapproachwhichisusedtostudyIVRproblemsisa“bounding”approachwhichconsistsinassumingaspecifiedcoriumstratificationinthevesselandthendemonstratingthatthevesselcancopewiththeresultingthermalandmechanicalloads.Thermalloadingonthevesseliscontrolledbytheconvectiveheattransferinsidethemoltencoriuminthelowerhead.Ifthereisnowaterinthevesselandifthecoriumpoolisoverlaidbyaliquidsteellayer,thentheheatfluxmightfocusonthevesselinfrontofthesteellayer(“focusingeffect”)andexceedthedry-outheatflux(CHForDHF).Oneofthecriticalpointsofthesestudiesislinkedtothedeterminationoftheheightofthemoltensteellayerthatcanstratifyabovetheoxidicpool.TheMASCAexperimentshavehighlightedthatpartofmoltensteelmaystratifyundertheoxidiccoriumwhichreducesthethicknessofthesteellayerontopofthepool.Thisbehaviorcanbeexplainedbychemicalinteractionbetweentheoxideandmetallicphasesofthepoolwhichconfirmsthatthesematerialscannotbetreatedasinertspecies.Followingtheseconclusions,amethodologywhichcouplesphysicochemicaleffectsandthermalhydraulicshasbeendevelopedtoaddresstheIVRissue.Themainpurposeofthispaperistopresentthismethodologyanditsapplicationforgivencoriummassinventories.AttentionfocusesontheinfluenceofparameterssuchastheratioU/Zrandoxidationratioofzirconia.Fora1000MWPWR,approximately10tofsteelstratifyatthebottomofthevesselfor40%Zroxidation,and25tfor30%Zroxidation.Thisleadstoa25–50%increaseofthemassofmoltensteelthatisrequiredforavoidingvesselmelt-through.