Choice of a low operating temperature for the DEMO EUROFER97 divertor cassette

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FusionEngineeringandDesign124(2017)655–658

ContentslistsavailableatScienceDirect

Fusion

Engineering

and

Design

j ou rn a l h o m epa g e : w w w . e l s e v i e r . c o m / l o c a t e / f u s e n g d e s

Choice

of

a

low

operating

temperature

for

the

DEMO

EUROFER97

divertor

cassette

Giuseppe

Mazzone

a,∗

_,

_J.

_Aktaa

b

_,

_C.

_Bachmann

c

_,

a_Department_of_Fusion_and_Technology_for_Nuclear_Safety_and_Security,_ENEA_C._R._Frascati,_via_E._Fermi_45,₀₀₀₄₄_Frascati,_Roma,_Italy

b_Karlsruhe_Institute_of_Technology,_Institute_for_Applied_Materials,_{Hermann-von-Helmholtz-Platz}_1,₇₆₃₄₄_{Eggenstein-Leopoldshafen,}_Germany

c_EUROfusion,_PPPT,_Boltzmann_Str._2,₈₅₇₄₈_Garching,_Germany

d_CREATE_{Consortium/University}_of_Naples_Federico_II,_Department_of_Industrial_Engineering_(DII),_Piazzale_Tecchio_80,₈₀₁₂₅_Napoli,_Italy

e_Department_of_{Astronautics,}_Electrical_and_Energetics_Engineering,_Sapienza_University_of_Rome_Palazzo_Baleani,_Corso_Vittorio_Emanuele_II,_244,₀₀₁₈₆

Rome,Italy

f_Karlsruhe_Institute_of_Technology,_IAM-AWP,_POB_3640,₇₆₀₂₁_Karlsruhe,_Germany

g_Max_Planck_Institute_for_Plasma_Physics,_Boltzmann_Str._2,₈₅₇₄₈_Garching,_Germany

h

i

g

h

l

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g

h

t

s

•InthisarticlethelowerlimitofallowedoperationtemperaturewindowwasdeﬁnedforreducedactivationsteelEUROFER97asstructuralmaterialof DEMOdivertorbody.

•Theunderlyingrationaleandsupportingexperimentaldatafromanumberofirradiationtestswerepresented.

•ThemotivationofthisstudywastoexplorethepossibilitytouseEUROFER97forwater-cooleddivertorcassetteattemperaturesbelow350◦_C.

•LiteraturedatashowedthattheFTTTofEUROFER97at6dpa(theexpectedmaximumdoseofcassettebodyafter2fpy)isabout180◦_C.

•Thisresultimposestheguidelineonthecoolantinlettemperatureofthedivertorcassette.

a

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i

c

l

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f

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Articlehistory:

Received22September2016

Receivedinrevisedform19January2017

Accepted5February2017

Availableonline10February2017

Keywords: DEMO Divertorcassette EUROFER97 Neutronﬂuence

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b

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Oneofthefundamentalinputparametersrequiredforthethermohydraulicandstructuraldesignofa divertorcassetteistheoperationtemperaturerange.Inthecurrentdesignactivitiestodevelop Euro-peanDEMOdivertorintheframeofEUROfusion,reducedactivationsteelEUROFER97waschosenas structuralmaterialforthedivertorcassettebodyconsideringitslowlong-termactivationandsuperior creepandswellingresistanceunderneutronirradiation(Youetal.,2016)[1].Forspecifyinganoperation temperaturerange(i.e.coolingcondition)various,oftenconﬂictingrequirementshavetobeconsidered. InthisarticlethelowerlimitofallowedoperationtemperaturewindowisdeﬁnedforEUROFER97for structuraldesignofDEMOdivertorcassettebody.Theunderlyingrationaleandsupportingexperimental datafromanumberofpreviousirradiationtestsarealsopresented.Themotivationofthissurveystudy istoexplorethepossibilitytouseEUROFER97forwater-cooleddivertorcassetteattemperaturesbelow 350◦_C_which_has_been_regarded_as_limit_temperature_to_preserve_ductility_under_irradiation._Based_on_the literaturedataofFTTT(FractureToughnessTransitionTemperature)calibratedbyMasterCurvemethod, itisconcludedthatEUROFER97attheenvisagedmaximumdoseof6dpawillhavetobeoperatedabove 180◦_C_taking_the_{embrittlement}_due_to_helium_production_into_account.

1. Introduction

ThedesignofaDemonstrationFusionPowerReactor(DEMO)to followITER,withthecapabilityofgeneratinghundredsofMWnet

∗ Correspondingauthor.

E-mailaddress:giuseppe.mazzone@enea.it(G.Mazzone).

electricity,isviewedbyfusioncommunityasaremainingcrucial steptowardstheexploitationoffusionpower.TherecentEUfusion roadmapHorizon2020[2]advocatesforapragmaticapproachand considersapulsed“lowextrapolation”DEMO.Thisshouldbebased onmaturetechnologiesandreliableregimesofoperation,asmuch aspossibleextrapolatedfromtheITERexperienceandontheuse ofmaterialsandtechnologiesadequatefortheexpectedlevelof

http://dx.doi.org/10.1016/j.fusengdes.2017.02.013

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656 G.Mazzoneetal./FusionEngineeringandDesign124(2017)655–658

neutronﬂuence.IntheConsortiumEUROfusionacorresponding pre-conceptualdesignactivityhasnowbeenlaunched.

AnimportantcomponentofDEMOistheDivertor(CADcontour modelillustratedinFig.1).TheessentialfunctionoftheDivertor istocontrolthecontentofheliumashandotherimpuritiesinthe burningplasmacorebyremovingplasmaparticlesfromthe scrape-offlayeroftheplasmaboundary.Theremovaloftheedgeplasma isachievedby intenseparticlebombardment ontotheDivertor Targetplateproducing neutralgasbeingpumpedout.This pro-cessgenerateshighheatﬂuxontotheTargetsurfacethathasto becontinuouslyremovedfromtheDivertorTarget.Therequired powerexhaustamountstoroughly20%ofthetotalthermalpower ofthefusionplasma(includingalphapower).Asoneofthemajor in-vesselcomponentsinterfacingbetweentheplasmaandthe Vac-uumVessel,theDivertorshalltoleratetheresultingheatﬂuxloads whileprovidingneutronshieldingtotheVacuumVesselandthe superconductingmagnetsinthevicinityatthesametime.

Inthepre-conceptualdesignactivitiesfortheEuropeanDEMO divertor,manymaterialshavebeenproposedasforPlasmaFacing componentsasforthedivertorcassettebasingononeofthe funda-mentaldesignparameterssuchastheoperationtemperaturerange ofthedivertorcassette.

Ingeneralformaterialselection thestarting pointwasbeen tryingtousethesameITERmaterialifpossible.FortheITER diver-torcassettetheausteniticstainlesssteelAISI316L(N)IGhasbeen usedasstructuralmaterial.Whenthenuclearﬂuencesincrease,as inITERTBM(TestBlanketModule)orinDEMOin-vessel compo-nents,itisnotpossibletouseAISI316,becauseofhighcontentof nickel,itissubjecttostrongactivation.Alsotheswellingbehaviors offerriticandausteniticsteelsunderneutronirradiationarevery different(seeFig.2).

9CrsteelEUROFER97iscurrentlyconsideredasthestructural materialforthecassettebodyasisthecaseforthebreeding blan-ket[1].ThisuseofEUROFER97 steelhassigniﬁcantadvantages owingtobeneﬁcialpropertiessuchasreducedlong-term

activa-Fig.1.CADofDEMOdivertorcassetteasof2015.

Fig.2. Comparativeswellingbehaviorunderﬁssionneutronirradiationforferritic

andausteniticsteels[6].

Fig.3.YieldStress(Rp0,2)behaviourofirradiatedEUROFER97ondependenceof

testtemperaturecomparedtounirradiateddata(thetemperatureinthelegend

indicatestheirradiationtemperature)[9].

Fig.4. IrradiationhardeningvsirradiationdoseforEUROFER97andotherRAFM

steels[3].

tionandstrongresistanceagainstcreepandswellingunderintense neutronirradiation.Theoptimaloperationtemperature(thusthe coolingcondition)forthecassetteisidentifiedconsidering differ-entandoftenconflictingrequirementssuchasthetypeandallowed pressureofcoolant,possibleconsequencesof LOCAevents(loss ofcoolant accident),limitationby designcoderules andpower conversionefficiency.

Inthispaper,amaterial-basedrationaletoidentifythe allow-ableoperationtemperaturerangeisdiscussedfocusingonfracture mechanicalproperties.

2. EUROFER97swellingandtensileproperties

Asdiscussed,reducedactivationEUROFER97andRAFMsteels aretheprimarychoicematerialsforﬁrstwallandbreedingblanket forfuturefusionpowerplants[3–5].Thismainlybecausemetals andalloys with“Body-centredcubic(Bcc)”crystal lattice struc-ture,includingironandferriticsteels,showbetterresistanceto prolongedirradiationthanmetalswith“Face-centred-cubic(Fcc)” lattices[6].Furthermore,relevantadvantagesintermsofswelling behaviorhavebeendemonstratedunderﬁssionirradiationfor fer-riticsteel(Fig.2).

Lot’sinformationonEUROFER97canbefoundinRefs.[5,7]and

[8].

Asregardstensileproperties,EUROFER97YieldStress(Rp0.2) showsdependencesfromtemperatureandirradiationcondition.

Fig.3[9]showsYieldStressvstesttemperatureforEUROFER97 intheunirradiatedconditionandafterneutronirradiationin dif-ferentmediumandhighdoseEuropeanirradiationprogrammesat targetirradiationtemperature(Tirr)between250and350◦C.

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G.Mazzoneetal./FusionEngineeringandDesign124(2017)655–658 657

Fig.5.DBTTvsirradiationtemperatureforselectedRAFMsteelsfromSPICEtests

(averagedamagedoseinSpicewas16.3dpa)[3].

YieldStressofRAFMsteelswiththedamagedose.TheYieldStress increaseisrathersteepatdosesbelow10dpa.Thehardeningrate appearstobesigniﬁcantlydecreasedattheachieveddamagedoses andacleartendencytowardssaturationisidentiﬁed.Forthe anal-ysisofhighdoseirradiationbehaviorofEUROFER97differentiation hastobedonebetweendifferentproductformsaswellas differ-entheattreatmentconditions.Infactthereisastrongsensitivity ofmaterials’mechanicalpropertiesandirradiationperformanceto metallurgicalparameters.

Thehatchedareamarksthescatteringbandofhighdose hard-eningfordifferentRAFMsteels.

3. EUROFER97fracturemechanicalpropertiesandhelium

effect

For deﬁning the allowableoperation temperature range for DEMOdivertorcassette,irradiationembrittlementhastobetaken intoaccount.

Inparticulartheeffectsoftemperature,irradiationandHelium production onDuctileto BrittleTransition Temperature (DBTT) andFractureToughnessTransitionTemperature(FTTT)havebeen investigated.

TheDBTTisdeﬁnedasthetemperatureatwhichthefracture energypassesbelowapredeterminedvalue(Charpyimpacttest).

Fig.5[3]showstheDBTTvsirradiationtemperatureforEUROFER97 andotherRAFMsteels.

TheDBTTisinﬂuencedmostatlowirradiationtemperature(T irr<330).Theevolutionoftheneutronirradiationinduced embrit-tlementwithdoseatdifferentirradiationtemperaturesisshown inFig.6[3].AllRAFMsteelsshowincreaseintheDBTTwithdose below15dpa.

TheresultsonF82Hand F82H-mod areplotted togetherfor different heat treatments and material compositions. The pre-irradiationheattreatment(HT)ofEUROFER97leadstoconsiderable improvementoftheirradiationresistanceatdosesupto30dpa.At theachieveddamagedoses,however,theembrittlementof EURO-FER97 HT becomes comparable to that of Eufofer97. AllRAFM steelsshowsteepincreaseintheDBTTwithdosebelow15dpa. Withfurtherincreasingthedamagedosetheembrittlementrate decreasesandacleartendencytowardssaturationisobservedat theachieveddamagedoses.

TheFTTTisdeﬁnedasmidpointtemperaturebetweencomplete brittlefractureandcompleteductiletearingbehavior.Fig.7shows theneutronirradiationinducedshiftinFTTT(FractureToughness TransitionTemperature)andKLST(specimenaccordingtoDIN50 115)andISO-VDBTT forEUROFER97vsirradiationdose. Irradi-ationinducedshiftsinFTTTaresigniﬁcantlylargerthanshiftsin

Fig.6. IrradiationshiftsoftheDBTTvsdoseforEUROFER97andotherRAFMsteels.

Fig.7.IrradiationinducedshiftinFTTTandKLSTandISO-VDBTTforEUROFER97

[3].

CharpyDBTTwhichindicatesanon-conservativeestimationson theembrittlementbyCharpytest.

There is signiﬁcantuncertainty regarding the magnitude of additionalembrittlementthatmightbeintroducedduring fusion-relevantneutronirradiationthatwouldgenerate∼10appmHe/dpa insteelsduetohelium-inducedhardening.

Experimentsbasedonneutron-irradiatedB-dopedRAFMsteels (whereadditionalHegenerationiscontrolledbyboron transmu-tation)indicate theincrease in DBTT fromHe canapproach or exceedtheDBTTincreaseassociatedwithradiationhardeningat 250–350◦C.

Fig. 8 [3] shows the additional increment of DBTT increase attributabletoHeproductionfollowingﬁssionneutronirradiation ofB-dopedEUROFER97steels.

4. DBTTandFTTTforDEMOdivertorcassetteinirradiation

condition

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658 G.Mazzoneetal./FusionEngineeringandDesign124(2017)655–658

Fig.8. Heliuminducedextraembrittlementvsextraheliumamountforirradiated

borondopedsteels.

thefracturetoughness(FTTT)transitiontemperaturesquantiﬁed inquasi-staticfracture-mechanicaltestsarecorrelatedbut experi-mentalresultsshowthatthetwotransitiontemperaturesdifferto somedegree.

TheDBTT of EUROFER97 varieswith thebatch number and productform.Forthe1stbatchofEUROFER97(EUROFER97-1)the averageDBTTisabout−80◦_C₍_Fig.₅_).

TheFTTT of EUROFER97also varies withthe batch number andproductform.Inaddition,thereisanadditionaluncertainty in FTTT imposed by application of the standard Master Curve methodology.FortheﬁrstbatchofEUROFER97theFTTTisabout −108◦_C._Application_of_the_modiﬁed_Master_Curve_procedure_[11]

yieldsconsiderablyhighertransitiontemperatureof−78◦_C.

How-ever,sincethemodiﬁedmastercurvemethodologyhasnotbeen validatedyetintheirradiatedstate,FTTTof−108◦_C _in_the

un-irradiatedconditionisconsideredhere.

Post-irradiationassessmentbothDBTTandFTTTconcludesthe followingregardingtheshiftoftheTransitionTemperatureafter irradiationat6dpa:

-AccordingtoFig.6theDBTTofEUROFER97shiftsfromthe un-irradiatedlevelat∼−80◦_C_by_∼123_K_to_∼43◦_C;

-AccordingtoFig.7theFTTTofEUROFER97shiftsfromthe un-irradiatedlevelat∼−108◦_C_by_∼220_K_to_∼112◦_C.

HoweversinceFigs.7and8arebasedonmaterialsamples irra-diatedinﬁssionreactors,Heliumproductioninthematerialthat willoccurduetoirradiationwithhighenergyneutronsgeneratedin thefusionreactionisnottakenintoaccount.InRef.[3]aDBTTshift intherange0.5–0.6K/appmHeisestimatedonthebaseofCharpy impactexperimentsonborondopedmodelsteels.Henceforour caseof100appmHeanadditionalshiftoftheDBTTof50–60Kis expected.Correspondingexaminationsofheliumeffectsonthe

FTTTshiftarenotknowntotheauthorsandareassumedhere tobeofsimilarmagnitude.Thisassumptionneedstobevalidated infuturebutisassumedtobeconservative.

Henceforanirradiationdamageof6dpatheDBTTof EURO-FER97isconsideredat∼100◦_C,_the_FTTT_at_∼180◦_C.

5. Conclusions

Inthis articlethelowerlimitof allowedoperation tempera-turewindowwasdeﬁnedforreducedactivationsteelEUROFER97 as structural material of DEMO divertor body. The underlying rationale and supporting experimental data from a number of irradiationtests were presented.The motivation of this survey studywastoexplorethepossibilitytouseEUROFER97for water-cooleddivertorcassetteattemperaturesbelow350◦Cwhichhas beenregarded aslimit temperatureto preserveductilityunder irradiation.Literature databasedonthestandard MasterCurve methodologyshowedthattheFTTTofEUROFER97at6dpa(the expected maximum doseof cassette body after 2fpy) is about 180◦Ctaking theembrittlementdue toheliumproductioninto account. Thisresult imposes theguideline on thecoolant inlet temperatureofthedivertorcassette.Attentionshouldbepaidto off-normalshutdowneventswherethetemperaturepathmayfall belowFTTTafteralong-termnuclearoperation.

Acknowledgement

This work has been carried out within the framework of theEUROfusionConsortium and hasreceivedfundingfromthe Euratomresearchandtrainingprogramme2014–2018undergrant agreementNo633053.Theviewsandopinionsexpressedhereindo notnecessarilyreﬂectthoseoftheEuropeanCommission.

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