Interpretationoffission-producttransportbehaviourinthePhébusFPT0andFPT1testsOriginalResearchArticle

PhébusFPstudiesthephenomenologyofsevereaccidentsinwater-coolednuclearreactors.Testscoverfuelroddegradationandbehaviouroffission-productsreleasedviathecoolantsystemintothecontainment.Analysisusingcomputercodesaimstoidentifymodellingweaknesses.Regardingfission-productbehaviourinthecoolantcircuit,analysesoftestsFPT0andFPT1arepresentedthatusedastandardversionofacode,SOPHAEROS,withinputdatabasedsolelyonmeasuredboundaryconditions.Disagreementsbetweencalculatedandexperimentalresultsareexploredandplausibleexplanationspresented.Itisshownthatinlaminarconditionsforasupersaturatedvapourwithcompetinghomogeneousnucleation,heterogeneousnucleationandcondensationonstructures,codescansignificantlyunderestimatethelatterifentranceeffectsareignored.Wherethermophoresisdominatesinhydrodynamicallydeveloped,weaklyturbulentflow,codescanoverestimatedeposition;thelikelyexplanationisunderestimatedmechanicalresuspension.Concerningthetransportofvapourspecies,itisshownthatobservationsarecompatiblewithpassageofnon-negligibleamountsofthegashydrogeniodidethroughthecircuit.Thefinalaspectoftheexerciseconcernsdepositremobilizationwherethiswasdifferentinthetwotestsandtheunderstandingofwhichremainsmorespeculative.ExplanationinvokesvibrationoftheapparatusproducingmechanicalresuspensioninFPT0andsteamreactingwithcaesiumdepositsproducingcaesiumhydroxideinFPT1.