FailurecharacteristicsofcladdingtubesunderRIAconditionsOriginalResearchArticle

Intheframeofactionsforimprovingthesafetyofitsnuclearpowerplants,ElectricitédeFranceneedstobuildthemechanicalcriteriaensuringthecladintegrityforseveraloperatingconditions. Thispaperpresentsanalyticalmechanicalmodelsusedtoderivefailurecriteriaforreactivityinsertionaccidents(RIA)andinterpretationoftheCABRIREP-Naexperimentaltests. Buildinganalyticalcriteriarequiresanexperimentaldatabase.Mechanicaltestsperformedonnon-irradiatedandirradiatedcladdingtubeshavebeenprovidedfromFrenchandinternationalprogrammes(PROMETRA,EPRI,…).Thesetestsconsistoftubeburstandaxialtension,andringtension.Severalstrainbiaxialityratiosarethusavailable:purecircumferentialtension(fromringtension),pureaxialtension(fromtubeaxialtension),andplanestrainconditions(fromtubebursttests).Severalstrainrates,temperatures,irradiationconditionsarealsoavailable. Themajorfeatureofourstudyhasbeentomakeitpossiblethattheseseveralthermomechanicalconditionsberepresentativeof“standard”RIAloadingconditions. Tothisaim,wehavederivedsomebiaxialityandstrainratecorrectionstobeappliedtotheresultsofexperimentaltests,insuchawaythattheycouldberepresentativeofRIAbiaxialityconditions(whichareassumedtobestrainequibiaxiality),andalsorepresentativeofRIAstrainrateconditions(whichareassumedtobe5s−1). Thecorrectionsthatwederivearebasedonthefracturepropertiesofhydridedzirconiumalloys(especiallyintermsofanisotropy),andalsoonanassumedformofthematerialconstitutiveequations. Eachtestofthe“homogenized”databasehasthusbeenusedtocalculateastrainenergydensity,representativeofitsfracture(thestrainenergydensityisdefinedastheintegralofthestresstimesstrainratestates,overthedurationofthemechanicaltest).TheSEDvaluesareplottedagainstthesampleʹsoxidethickness,andalowerboundlimitcanbeestablished,withrespecttooxidethickness. Inordertoaddresstheproblemofrepresentativenessofthelaboratorydatabase,anexperimentalset-uphasbeendevelopedthataimsatcharacterizingthefailurebehaviorofcladdingtubesunderRIAconditions.Thedevelopedexperimentalset-upisbasedonelectromagneticforming. ThedevelopmentofthetestandinparticularofthedieisdelicatebutleadstorepeatableresultswithacontrolledstrainbiaxialityratiohigherthanthoseobtainedthroughconventionaltestssuchasringtestsorPSUringtests.Theuseofelectromagneticformingprocessallowstestingthespecimenwithveryhighstrainrates.Forthenexttestseriesotherzircaloyalloysatthereceptionstateandinhydridedconditionswillbetestedinordertolookathydridesinfluenceonfracturestrains. Afirstfiniteelementsimulationofthetestwasengaged.Thesimulationandtheexperimentalresultsareinquitegoodagreement.Infuture,theconsistencyofthepreviouslydevelopedanalyticalmechanicalcriterionwiththeelectromagneticformingexperimentalresultswillbeverified.