SimplifiedmodelingofaPWRreactorpressurevessellowerheadfailureinthecaseofasevereaccidentOriginalResearchArticle

Inordertocharacterizethetiming,modeandsizeofapossiblelowerheadfailure(LHF)ofthereactorpressurevessel(RPV)intheeventofacoremeltdownaccident,severallarge-scaleLHFexperimentswereperformedundertheUSNRC/SNLLHFprogram.Theexperimentsexaminedlowerheadfailureathighpressures(10MPainmostcases)andwithsmallthroughwalltemperaturedifferentials.AnotherUSNRC/SNLLHFprogram,calledtheOLHFprogram,hasbeenundertakenintheframeworkofanOECDproject.Thiswasanextensionofthefirstprogramanddealtwithlowandmoderatepressures(2–5MPa)butwithlargethroughwalltemperaturedifferentials.Theseexperimentsshouldleadtoabetterunderstandingofthemechanicalbehaviorofthereactorvessellowerhead,whichisofimportancebothinsevereaccidentassessmentandthedefinitionofaccidentmitigationstrategies.Awell-characterizedfailureofthelowerheadisofprimeimportancefortheevaluationofthequantityofcorematerialthatcanescapeintothecontainment,sincethisdefinestheinitialconditionsforallexternal-vesselevents.Thelargequantityofescapingcoriummayleadtodirectheatingofthecontainment.Thisisanimportantsevereaccidentissuebecauseofitspotentialtocauseearlycontainmentfailure.Theexperimentsalsoprovidedataformodeldevelopmentandvalidation.Forourpart,asoneoftheprogrampartners,numericalmodelingwasperformedtosimulatetheseexperiments.Thispaperpresentsadetaileddescriptionofthreeofournumericalmodelsusedforthesimulation.Thefirstmodelisasimplifiedsemi-analyticalapproachbasedonthetheoryofasphericalshellsubjectedtointernalpressure.Thetwoothermethodsdealwith2Dfiniteelement(2D-FE)modeling:onecombinestheNorton–BaileycreeplawwithadamagemodelproposedbyLemaitre–Chabochewhiletheotherusesonlyacreepfailurecriterionbuttakesintoaccountthermo-metallurgicalphasetransformations.Oneoftheobjectivesofthesimulationisthedevelopmentofasimplifiedandvalidanalyticalcodethatcanbeimplementedinintegralsevereaccidentcomputercodes.Thenumericalresultsareconsistentwiththeexperimentalmeasurements.Theeffectonthenumericalresultsofthemultiphasetransformationoftheshellmaterialandofthetwofailurecriteriaused,oneinvolvingnecking(Considèreʹscriterion)andtheotherinvolvingcreepdamage(Lemaitre–Chaboche)isdiscussed.