IodinebehaviourunderLWRaccidentconditions:LessonslearntfromanalysesofthefirsttwoPhebusFPtestsOriginalResearchArticle

TheInternationalPhebusFissionProductprogramme,initiatedin1988andperformedbytheFrench“InstitutdeRadioprotectionetdeSûretéNucléaire”(IRSN),investigatesthroughaseriesofin-pileintegralexperiments,keyphenomenainvolvedinlightwaterreactor(LWR)severeaccidents.Thetestscoverfuelroddegradationandthebehaviouroffissionproductsreleasedviatheprimarycoolantcircuitintothecontainmentbuilding. Theresultsofthefirsttwotests,calledFPT0andFPT1,carriedoutunderlowpressure,inasteamrichatmosphereandusingfreshfuelforFPT0andfuelburnedinareactorat23GWdt−1forFPT1,wereimmenselychallenging,especiallywithregardtotheiodineradiochemistry.Someofthemostimportantobservedphenomenawithregardtothechemistryofiodinewereindeedneitherpredictednorpre-calculated,whichclearlyshowstheinterestandtheneedforcarryingoutintegralexperimentstostudythecomplexphenomenagoverningfissionproductbehaviourinaPWRinaccidentconditions.Thethreemostunexpectedresultsintheiodinebehaviourrelatedtoearlydetectionduringfueldegradationofaweakbutsignificantfractionofvolatileiodineinthecontainment,thekeyroleplayedbysilverrapidlybindingiodinetoforminsolubleAgIinthecontainmentsumpandtheimportanceofpaintedsurfacesinthecontainmentatmospherefortheformationofalargequantityofvolatileorganiciodides. TosupportthePhebustestinterpretationsmall-scaleanalyticalexperimentsandcomputercodeanalyseswerecarriedout.Theformer,helpingtowardsabetterunderstandingofoveralliodinebehaviour,wereusedtodeveloporimprovemodelswhilethelattermainlyaimedatidentifyingrelevantkeyphenomenaandatmodellingweaknesses.Specificeffortsweredevotedtoexploringthepotentialoriginsoftheearly-detectedvolatileiodineinthecontainmentbuilding.Ifaclearexplanationhasnotyetbeenfound,thenon-equilibriumchemicalprocessesfavouredintheprimarycoolantcircuitandtheearlyradiolyticoxidationofiodidesinthecondensedwaterfilmsareatpresentthemostlikelyexplanations.Modelsthatweremodifiedordevelopedandembodiedinthecomputercodesfororganiciodideformation/destructioninthegasphaseandAg–Ireactionsinthesumplead,inagreementwiththePhebusfindingsrespectivelytogreatlyenhancedorganiciodideformationkineticsandlongtermconcentrationinthecontainmentatmosphereononehandand,intheconditionsofPhebusexperiments,tosignificantlylimitedmoleculariodinevolatilisationfromthesumpinsofarassilverwasinexcesscomparedtoiodine,ontheotherhand.Organiciodidesthenquicklygaininimportanceandbecomethepredominantvolatileiodinespeciesatlongterm.