Experimentsonin-vesselmeltcoolabilityintheEC-FOREVERProgramOriginalResearchArticle

Thispaperreportstheresultsfromtheexperimentsconductedonthecoolabilityofcoriummeltduringasevereaccidentscenariowhenthebottomheadisfullofthecoremelt,undergoingnaturalcirculation.TheseexperimentsarepartoftheEC-FOREVERPrograminwhichvesselfailureexperimentshavealsobeenperformed.Theexperimentsareperformedina1/10thscalevessel(≅400mmdiameterand15mmwallthickness)andtheoxidicmeltemployedisthemixtureCaO+B2O3at≅1400K,representingthecoriummeltmixtureofUO2+ZrO2. Theexperimentsemployedaninitialphase,duringwhichuniformvolumetricheatingofthemeltwasprovidedandthevesselwaspressurisedto≅25bar,forseveralhours,togeneratemaximumcreepdeformationof≅5%,inordertoprovidetheconditionsfortheformationofagapbetweenthemelt-poolcrustandthebottomheadwall.Afterthisphase,thevesselwasfloodedwithwater. DatawereobtainedononlythevesselandthemeltpooltemperaturesinoneoftheEC-FOREVERexperimentsreportedhere.Inthesecondexperiment,however,besidesthetemperaturedata,additionaldatawereobtainedonthesteamflowrateandtheheattransfertothewater,attheupperfaceofthemeltpool,asafunctionoftime. Itwasfoundthatthegapcoolingmechanismwasnoteffectiveinreducingthevesselwalltemperaturesafterwaterflooding.Post-testexaminationsrevealedthatthewateringressionextendedtothedepthofonly≅60mminthemeltpool.ThecharacteroftheheattransfertothewaterfromthemeltpooluppersurfacewasfoundtobesimilartothatobservedintheMACEtestsforthecoolabilityofanex-vesselmeltpoolfloodedbywateratthetop.