Università degli studi di Roma “Tor Vergata”

Ente Nazionale Energia e Ambiente

CFDAnalysisofLossOfVacuumAccidentforSafetyApplica:oninExperimentalFusionFacility

C.Bellecci1,P.Gaudio1,I.Lupelli1,A.Malizia1,M.T.Porfiri2,R.Quaranta1,M.Riche@a11EURATOM‐FacultyofEngineering,UniversityofRome“TorVergata”;ViadelPolitecnico1,00133Rome,Italy

2ENEANuclearFusionTechnologies,ViaEnricoFermi,45I‐00044FrascaU,Italy

LOVAinITERTheaimofthefusionresearchistodevelopaprototypefusionpowerplantthatissafeandreliable,environmentallyresponsibleandeconomicallyviable.ITERwillbetheworld'slargestexperimentalfacilitythataimstodemonstratethescienUficandtechnicalfeasibilityoffusionpower.InafusionexperimentalreactorthedustisgeneratedduringnormalmachineoperaUonsandbymacroscopicerosionoftheplasmafacingmaterialsduetointensethermalloadsthatoccurduringEdgeLocalizedModes(ELMs),plasmadisrupUons(PD)andVerUcalDisplacementEvents(VDEs).Thisdust,mainlyaccumulatedclosenessthedivertorzone,canbemobilizedoutsidetheVacuumVessel(VV)incaseofLOVAthreateningpublicsafetybecauseitmaycontaintriUum,mayberadioacUveandmaybechemicallyreacUveand/ortoxic.ALOVAisa Design Basis Accident (DBA) event in fusion reactors [1]. Several studies on fundamental phenomena have beencarried out in order to invesUgate the features of LOVA events both for thewet and dry case [2,3,4,5,6,7] and alsoreferenceeventsforITERwerepostulated[8].OneofthemainissueistodevelopmethodsofcomputaUonalanalysisfor the accident scenarios. An experimental facility, STARDUST, has been developed and uUlized to validate newcomputaUonalmodelscapableofpredicUngtheflowfieldfordustresuspensionandtransport infusionreactorLOVAscenarios.

STARDUSTDescrip:onSTARDUSTisastainlesssteelhorizontalcylinder(internalvolume0.17m3)closedbytwolids[10](Fig1).STARDUSTisequippedwithanautomaUcdataacquisiUonsystemthatallowsthecontrolofinternalpressure,walltemperatureandair flow inlet in order to carry out the experiments at the desired iniUal condiUons (see table 2).When the iniUalcondiUonsarereachedthecontrolsystemopenstheflowmeterinletvalveandthentheairflowsinsidethetankwithaflowrateof27 l/min inorder toachieveapressurizaUon rateof300Pa/s typical in the ITERLOVAreferenceevents(EX_A).InordertotesttheCFDmodelbyanalyUcalandexperimentalcomparisonsanexperimentisbeingperformedinwhichtheingressoccursbypressuredifferencebetweentheatmosphericcondiUonandtheinternalcondiUonofthetank (EX_B)withoutan inletflowrate imposed. Air inlet simulates the lossof vacuumevent . TheairflowpassesthroughtwovalvesposiUonedinthebacklidrespecUvelyatthemiddleofthetank,ifanequatorialportfailureoccurs(A),andat thebo@omof the tank (B), if adivertorport failureoccurs [10,11,12,13,14].Theexperimentshavebeencarriedoutintroducingasemicylindricalobstaclemadeofstainlesssteel,whichsimulatesthepresenceofstructures,inparUcularthedivertor,insidetheITERVV[13,14].Inordertotakeintoaccounttheinfluenceofthespacebetweenlimiteranddivertor,asintheVVofITER,aslithasbeenrealizedontheobstacle(Fig2).Intheexperimentalcampaignhasbeenevaluatedpunctualflowvelocity values, at 20cm from the inlet secUon,obtained in twodifferent internalcondiUons: 1)Experimentswithoutobstacle (VA_WO forA inlet andVB_WO forB inlet valve); 2)Experimentswithobstacle,inordertounderstandbe@ertheinfluenceofstructures(likedivertor)presentinsideSTARDUSTfacility(VA_OandVB_O);Thepressuretransducermeasuresthedifferenceofpressure(PΔ)betweenapressurereferencetube(staUcpressurePs)andthepressuremeasuredbytheheadofsensor(totalpressurePt).WhentheiniUalcondiUonsinsidethetankhavebeenreachedthecontrollerallowstheacquisiUon.ForthecalculaUonofvelocitymagnitudetheequaUon(1)

hasbeenusedwhereγistheraUoofthespecificheatofthefluidatconstantpressuretothespecificheatofthefluidatconstantvolume,Ristheuniversalgasconstant,TmeanismeantemperaturemeasuredbyinternalthermocouplesandMistheairmolecularmass.ThepressurizaUonrateismeasuredatthetopofthechamberwithaPiranisensor.

Figure1:STARDUSTfacility Figure2:Obstacle

Table1:InletandinternalcondiUons Table2:CondiUonandvesselfillUmes

VesselFillingAnalysisAclosedformsoluUonforpressurevs.Ume(tr)canbeobtainedunderthefollowingsimplifyingassumpUons:• Idealgaswithconstantspecificheat• Zero velocity and spaUally uniform properUes in thevessel• Isentropicflowthroughtheconstantareainlet• AdiabaUc or isothermal behavior depending on thelengthoftransientFlow is iniUally choked since thepressure is below thecriUcalvalue.TheUmetoreachthecriUcalpressureandthe pressurizaUon rate for choked and unchokedsoluUonfortheisothermal(I)andadiabaUc(A)caseare(Pr,iistheiniUalpressure)[2]:

IniUal (i)andfinal (f)pressure (p)and temperature (T),characterisUcUme(TcharwhichisgivenintermsofthevesselvolumeV,inletcrosssecUonalareaA,andsoundspeedbasedonthetemperatureofthesurroundingsc,tchar=V/Ac),Ume(Tc)toreachcriUcalpressureandthefillingUmefortheSTARDUSTfacilityareshowninTable2.

ModelDescrip:onSimulaUonofthethermalandflowfields intoSTARDUSThasbeencarriedon,withCOMSOLMulUphysics®(CM).Theimplemented model is based on the fully compressible formulaUon of the conUnuity equaUon and momentumequaUonsandonconducUon/convenUonequaUons

ρ:density(kg/m3)u:velocityvector(m/s)p:pressure(Pa)η:dynamicviscosity(Pa∙s)F:bodyforcevector(N/m3)

Cp:isthespecificheatcapacityatconstantpressure(J/(kg∙K))T:absolutetemperature(K)q:heatfluxbyconducUon(W/m2)τ:viscousstresstensor(Pa)S:strainratetensor(1/s):Q:containsheatsourcesotherthanviscousheaUng(W/m3)h:heattransfercoefficient(W/(m2∙K)).

ResultsA secUon of the tank for all experimentalconfiguraUonscharacterizingtheflowfieldisshowninfigures345and6.ThemodelmatchescloselytheexperimentaldatawithaslightlyfasterfillUme(fig7and8).SimulaUonsandexperimentsshowedthatthepresenceofanobstaclein STARDUST influences thedust’smobilizaUon solelyincaseofleakageatdivertorportslevel(valveB).Thisresult matches with experiments done in 2007experimental campaign [13,14] in which we havemeasuredthequanUtyofdustresuspendedincaseofLOVA. The percentage of dust resuspended, thatsimulates a LOVAat divertor level is lower in caseofobstacle inside the chamber [13,14]. These resultsmatch with experimental and numerical resultsobtained in the current work. In fact the velocityvaluesmeasuredwithoutobstacle(VB_WO)arehigherthanvelocityvaluesmeasuredwithobstacle(VB_O).

ConclusionComsolmodelsforshorttransients(~1,5s)havegivenreasonableresults.Futureworkwillfocusonobtainingaccurateresults for longer transients, resolving flow features such as shocks and boundary layers, implemenUng of dustresuspensionmodels, improvingmesh and geometry impact on cpuUme, including turbulencemodel behavior. Thedust resuspension models include most of the transport phenomena relevant for ITER. The 3D numerical codedescribingthedustmobilizaUoninSTARDUSTfacilitywillbeimprovedbymeansofCOMSOLsozwareandcomparisonwillbemadewithexperimentaldataacquiredduring2007,inordertovalidatethemodel.ThenthesamemodelwillbeadaptedtoatoroidalshapeandanopUcaldiagnosUcformeasurementofdustmobilizaUonevoluUonwillbechosenandstudied.TheresultswillbeusefulforthedesignofopUcalportsinanewexperimentalfacilitywithageometricalshapesimilartoITERforeseentoberealized.

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Figure3:FlowfieldVA_WO Figure4:FlowfieldVA_O Figure5:FlowfieldVB_WO 6:FlowfieldVB_O 7:Velocity@20cmfrominlet Figure8:Pressurevs.Time

Università di Roma “Tor Vergata” Dipartimento di Ingegneria dell’Impresa Via del Politecnico 1 00133 Roma Tel +39 06 7259 7201 E-mail: bellecci@uniroma2.it

Presented at the COMSOL Conference 2009 Milan