Semi-analyticalmodelingofaPWRlowerheadfailureundersevereaccidentconditionsusinganaxisymetricalshelltheoryOriginalResearchArticle

TheUSNRC/SNLOLHFprogramwascarriedoutwithintheframeworkofanOECDproject.Thisprogramconsistedoffourone-fifthscaleexperimentsofareactorpressurevessel(RPV)lowerheadfailure(LHF)underwellcontrolledinternalpressureandlargethroughwalltemperaturedifferentials;theobjectivesweretocharacterizethemode,timingandsizeofapossiblePWRlowerheadfailureintheeventofacoremeltdownaccident.Theseexperimentsshouldalsoleadtoabetterunderstandingofthemechanicalbehaviorofthereactorvessellowerhead,whichisofimportancebothinsevereaccidentassessmentandthedefinitionofaccidentmitigationstrategies.Awell-characterizedfailureofthelowerheadisofprimeimportancefortheevaluationofthequantityofcorematerialthatcanescapeintothecontainment,sincethisdefinestheinitialconditionsforallex-vesselevents.Alargequantityofescapingcoriummayleadtodirectheatingofthecontainmentorex-vesselsteamexplosion.Theseareimportantissuesduetotheirpotentialtocauseearlycontainmentfailure.Theexperimentsalsoprovidedataformodeldevelopmentandvalidation.Forourpart,asoneoftheprogrampartners,a2Dsemi-analyticalmodelhasbeendevelopedandusedtosimulatetheseexperiments.TheaimofthiseffortistodevelopasimplifiedbutwellpredictingcodethatcanbethenimplementedinEuropeanintegralsevereaccidentcomputercodes(ASTEC,ICARE/CATHARE).Thispaperpresentsthedetailedmathematicalformulationofthissimplifiedmethodwhichisusedtointerprettheexperimentalresults.Theaxi-symmetricshelltheoryunderinternalpressureproposedbyTimoshenkohasbeenutilised.Thesolutiontotheequilibriumequationsispresented,withparticularattentiontotheRabotnovanalyticalformula.Theradiusandthepolarangleofthedeformedstructurehavebeenwrittenasanalyticalexpressionsinordertotakethelargedisplacementsandlargestrainsintoaccountusingourmathematicalformulation.TheNortontypecreeplawandtheKachanovdamagelawhavebeenused.Severalfailurecriteriawereusedinthecalculationsandtheireffectonthenumericalresultsisdiscussed.This2Dsemi-analyticalmodelgivesverysatisfactoryresultswhencompared,withtheexperimentalandnumericalresultsthatwerepresentedrecentlyintheBenchmarkcalculationsbasedonthefirsttestoftheOLHFprogram.Theperformanceofthismodelisalsoillustratedbyitscapacitytoaccuratelysimulatethedeformationofthelowerhead,includingthevariationofwallthickness.