Analysisoflikelihoodoflowerheadfailureandex-vesselfuelcoolantinteractionenergeticsforAP1000OriginalResearchArticle

Aone-dimensionalmodelisformulatedtoassessthethermalresponseoftheWestinghouseAdvancedPlant(AP1000)lowerheadbasedontwoboundingmeltconfigurations.MeltConfigurationIinvolvesastratifiedlightmetalliclayerontopofamoltenceramicpool,andmeltConfigurationIIrepresentstheconditionsthatanadditionalheavymetallayerformsbelowtheceramicpool.Theapproachconsistsofthespecificationofinitialconditions;determinationofthemode,thesizeandthelocationoflowerheadfailurebasedonheattransferanalyses;computersimulationofthefuelcoolantinteractionprocesses;andfinally,anexaminationoftheimpactoftheuncertaintiesintheinitialconditionsandthemodelparametersonthefuelcoolantinteractionenergeticsthroughaseriesofsensitivitycalculations.TheresultsofthecalculationsformeltConfigurationIshowthattheheatfluxremainsbelowcriticalheatflux(CHF)inthemoltenoxidepool,buttheheatfluxinthelightmetallayercouldexceedCHFbecauseofthefocusingeffectassociatedwithpresenceofthethinmetalliclayers.Thethinmetalliclayersareassociatedwithsmallerquantitiesofthemoltenoxideinthelowerplenumfollowingtheinitialrelocationintothelowerhead.Thecalculationsshowthatthelowerheadfailureprobabilityduetothefocusingeffectofthestratifiedmetallayerrangesfrom∼0.04to∼0.30.Ontheotherhand,thethermalfailureofthelowerheadatthebottomlocationformeltConfigurationIIisassessedtobehighlyunlikely.Basedonthein-vesselretentionanalysis,thebasecasefortheex-vesselfuelcoolantinteraction(FCI)isassumedtoinvolveasidefailureofthevesselinvolvingametallicpourintothecavitywater.TheFCIsensitivitycalculationsintendedtoassesstheimplicationsoftheuncertaintiesininitialconditionsandtheFCImodelingparametersshowthattheFCIloadsrangefromafewMPatoupwardof1000MPa(maximumpoolpressure)withcorrespondingimpulseloadsrangingfromafewkPastoafewhundredkPas.