The preparation of the Shutdown Dose Rate experiment for the next JET Deuterium-Tritium campaign

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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

The preparation of the Shutdown Dose Rate experiment for the next JET Deuterium-Tritium campaign

N. Fonnesu

, R. Villari

, S. Loreti

, M. Angelone

, R. Pilotti

, A. Klix

, P. Batistoni

, JET contributors

aENEA,DepartmentofFusionandNuclearSafetyTechnology,I-00044Frascati,Rome,Italy

bDepartmentofIndustrialEngineering,UniveristyofRome‘TorVergata’,ViadelPolitecnico1,00133Rome,Italy

cKarlsruheInstituteofTechnology,76344,Eggenstein-Leopoldshafen,Karlsruhe,Germany

h i g h l i g h t s

•Theassessmentoftheshutdowndoserateisamajorsafetyissueforfusiondevices.ThefutureDTE2campaignatJETwillprovideauniqueopportunity tocheckthecapabilitiesofthenumericaltoolsforSDRpredictions.

•DetectorsforSDRexperimentarecharacterizedbyexcellentreproducibility,long-termstabilityandflatenergyresponse.

•DifferentENEAfacilitiesandlaboratorieshavebeenusedforcalibratingandtestingthedosimetryequipmentselectedfortheexperiment.

•Flatenergyresponseintermsofairkermawithin4.1%hasbeenobservedforboththeionizationchambers.

a r t i c l e i n f o

Articlehistory:

Received30September2016

Receivedinrevisedform9January2017 Accepted18January2017

Availableonline26January2017

Keywords:

JET

Shutdowndoserate

Occupationalexposureinfusion experiments

DTE2

a b s t r a c t

TheassessmentoftheShutdownDoseRate(SDR)duetoneutronactivationisamajorsafetyissuefor fusiondevicesandinthelastdecadeseveralbenchmarkexperimentshavebeenconductedatJETduring Deuterium-DeuteriumexperimentsforthevalidationofthenumericaltoolsusedinITERnuclearanaly- ses.ThefutureDeuterium-TritiumcampaignatJET(DTE2)willprovideauniqueopportunitytovalidate thecodesunderITER-relevantconditionsthroughthecomparisonbetweennumericalpredictionsand measuredquantities(C/E).Forthispurpose,anovelSDRexperiment,describedinthepresentwork,is inpreparationintheframeoftheWPJET3-NEXPsubprojectwithinEUROfusionConsortium.Theexper- imentalsetuphasbeenaccuratelydesignedtoreducemeasurementuncertainties;sphericalair-vented ionizationchambers(ICs)willbeusedforon-lineex-vesseldecaygammadosemeasurementsduring JETshutdownfollowingDToperationsandactivationfoilshavebeenselectedformeasuringtheneutron fluencenearICsduringoperations.Activedosimeters(basedonICs)havebeencalibratedoverabroad energyrange(fromabout30keVto1.3MeV)withXandgammareferencebeamqualities.Neutronirra- diationtestsconfirmedthecapabilityofactivedosimetersofperformingon-linedecaygammadoserate measurements,tofollowgammadosedecayattheendofneutronirradiationaswellasinsignificant activationoftheICs.

©2017TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

∗ Correspondingauthorat:ENEA,DepartmentofFusionandNuclearSafetyTech- nology,I-00044Frascati,Rome,Italy.

E-mailaddress:nicola.fonnesu@enea.it(N.Fonnesu).

1 SeetheAppendixofRomanelliF.etal2014Proc.25thIAEAFusionEnergyConf.

2014(St.Petersburg,Russia)EUROfusionConsortium,JET,CulhamScienceCentre, Abingdon,OX143DB,UK.

1. Introduction

Neutrons produced during Deuterium-Deuterium (DD) and Deuterium-Tritium(DT)plasmaoperationsinducetheactivation ofthematerialsconstitutingthefusionmachinecomponents.

Theassessmentoftheshutdowndoserate(SDR)isamajorsafety issueforfusiondevices,toguaranteetherespectofdoselimitsto externalexposureduringmaintenanceandinterventions.Radia- tiondoselimitsarebasedonprotectionquantitiesthatarenot directlymeasurableandtheneedforreadilymeasurablequantities thatcanberelatedtoeffectivedoseandequivalentdose[1]hasled

http://dx.doi.org/10.1016/j.fusengdes.2017.01.030

0920-3796/©2017TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.

0/).

tothedevelopmentofoperationalquantitiesfortheassessmentof externalexposure.Forareamonitoring,theoperationalquantity forstronglypenetratingradiationasgammaraysduetoneutron activation,istheambientdoseequivalentata10mmdepthofthe ICRUsphereH*(10)[1,2].

SDRexperimentsrepresentalsoakeyissueforthedesignand forplanningthemaintenanceoperationsoffuturefusiondevices, in particular for ITER. In order to ensurethe reliability of SDR predictionsforITER,theuseofqualifiedandvalidatedcodesand nucleardataisfundamental.ThefutureDeuterium-Tritiumcam- paign(DTE2)atJETwillprovidea uniqueopportunitytocheck thecapabilitiesofthenumericaltoolsforSDRpredictions[3–6]

inacomplexfusiondeviceandtovalidatethecodesunderITER- relevant conditions, exploiting the significant 14MeV neutron production(upto1.7·10²¹neutrons)[7].

Thepresentworksummarizesthemainexperimentalactivities inpreparationoftheSDRexperimentduringthenextDTE2:the selectedmeasuringequipmenttobeinstalledatJETisdescribedin Section2,calibrationofactivedosimetersandneutronirradiation tests,respectively,inSections3and4.Conclusionsaregivenin Section5.

2. Experimentalassembly

ThemeasuringequipmentatJETconsistsofthreeactivedosime- ters,formeasuringthedoserateattheshutdownandduringsome inter-shots,andapassivesystemformeasuringneutronfluence (i.e.,twoactivationfoilassemblies)duringJETpulsesnearactive dosimeters[8].Thermo-luminescentdetectors(TLDs)forpassive in-vesselmeasurementsand aportableHigh-PurityGermanium spectrometer(HPGe)forgamma-rayspectrameasurementsatthe shutdown(whenthehumanaccesstothetorushallisallowed), completetheequipment.

Theactivegammadosimetersarebasedontwo140mmdiam- eterair-ventedsphericalionizationchambers(ICs),PTW model 32002,andononesmallerIC,44mmdiameter,PTWmodel32005 [9].ICs32002havebeenprocuredbyENEAandKIT(henceforth, namedrespectivelyENEAIC32002andKITIC32002);thesmaller IChasbeenprocuredbyENEA(herelabeledENEAIC32005).ICs 32002and32005 aredesigned forradiationprotectionand are characterizedbyexcellentreproducibility,long-termstability of thesensitivevolumeandaboveall,flatenergyresponse(interms ofairkerma[1]),whichisessentialinthisapplication.Thespherical constructionensuresanearlyuniformresponsetogammaradia- tionfromeverydirection.Thesedetectorshavebeenselectedto coveradoseraterangefrombackgroundto30mSv/h,aspredicted bycalculationsreportedin[10].

ICsareoperatedincurrentmodeandtheoutputsignalisana- lyzedbytwosuitable electrometersfordosimetry,onefor both ENEAICsandtheotheronefortheKITIC.Theseelectrometers, modelPTWUNIDOS[9],areequippedwithanEthernetinterfacefor integratingtheminthelaboratorylocalnetwork(LAN)forremote access.Auserwrittensoftwarehasbeenimplementedforremote controlofelectrometersanddataacquisition.Highqualitytriaxial cables,100mlong,serveaslownoiseconnectionofradiationdetec- tors,locatednearthetokamak,toelectrometers,locatedoutside thetorushallforlimitingradiationdamage.Theselectedcables, designedforprecisecurrentmeasurementsdownto10⁻¹⁵Aand withalowleakagecausedbyirradiation,provideinsulatedpoten- tialsforthemeasuringsignal,theguardelectrode,andhighvoltage (i.e.,400V)toICs.

Thetwoactivationfoilassembliesconsistofanaluminumholder (100mm×50mm×4.5mm)with7foils(Co,Ta,AgandNifoils,4 bare+3Cd-covered)[8].

Twoex-vessel positions,close totheJEThorizontal portsof Octants1and2,havebeenchosenforthelocationofICsandactiva- tionfoilassemblies,onthebasisofcalculationsreportedin[8].The positioninOctant1isclosetotheRadialNeutronCameraandthe positioninOctant2isonthetopoftheITER-likeAntenna.ENEAICs 32002and32005,togetherwithanactivationfoilassembly,willbe locatedonadedicatedsupportinOctant1;KITIC32002andthe othersetofactivationfoils,onadedicatedsupportinOctant2.The in-vesselpositionforTLDsisthesameasinthepreviousbenchmark experimentdescribedin[10].

3. Calibrationofactivedosimeters 3.1. ENEAdosimeters

Thetwodosimetrysystems,respectivelybasedontheENEAICs 32002and32005,connectedtotheassociatedPTWelectrometer, havebeencalibratedatENEA-INMRI[11]intermsofairkerma, usingtheselectedXandgammareferencebeamqualitiesatlow doses(therangeofinterestis<40mSv/h)reportedinTable1and accordingtoISO4037[12].NqualitiesrepresentfilteredX-rayspro- ducedwithanacceleratedelectronbeam(voltageofvacuumtube andaddedfiltersareindicatedinthesametable),whileSqualities aregamma-emitters.

Airkermaatthereferencemeasuringpoint(K_air)wasdeter- minedwithINMRInationalstandardreferenceionizationchambers (parallelplateandcylindricalchambers),eachdedicatedtoaspe- cificradiationquality.TheairkermacalibrationfactorN_Kair ata specifiedradiationquality,isthencalculatedastheratiobetween K_air andthedosimetrysystemreadingM.Acalibrationinterms ofH*(10)wouldbeuseless,sincethedetectorresponse forthis operationalquantityisstronglyenergy-dependentandthedecay gammaenergyspectrumatthedetectorsisvariableduringmea- surements.Forthesereasons,aweightedintegrationofcalibration coefficients atdifferent beamqualities overthe gammaenergy spectrum,fordeterminingthemeancalibrationcoefficienttobe usedforconvertinginstrumentreadingMintoH*(10),isprecluded.

Anenergy-independentresponseisneededinthiscaseand,asear- liermentioned,theselectedICsshowaflatenergyresponsewhen measuringairkerma.AmbientdoseequivalentH*(10)canbethen calculatedfromairkermabymeansofICRPconversioncoefficients [13].

ResultsofthecalibrationsarereportedinTable1andshown in Fig. 1, where diamond-shaped dots are N_Kair for the differ- entbeamqualities;circulardotsincorrespondenceoftheCo-60 arethecalibrationfactorsmeasuredbyPTW,consistentwiththe INMRIones.ExpandedmeasurementuncertaintyU(coveragefac- tor k=2), which gives a level of confidence of approximately 95%and calculated accordingtotheISOrecommendations [12]

isalsoindicated. Calibrationshave beencarriedoutunder con- trolledreferenceambientconditions(i.e.,T=20^◦C,P=1013.25hPa andrelativehumidity=50%,withoutsignificantvariations),since variationsofairdensityandhumidityaffecttheresponseofair ventedICs.AirdensityandhumiditywillbemonitoredduringSDR experiment anddepartures fromthereferenceconditions must be considered in order to apply appropriate correction factors totheelectrometersreadout.Toconverttheinstrumentreadout (Coulomb)intoairkerma(Gray),an‘equivalent’calibrationfac- tor(N_K^cal)isneeded;underthehypothesisofperfectflatenergy responseofthedosimeters,y=N_K^cal,themeasuredcalibrationfac- torsN_Kaircanbeconsideredenergyindependent.Inparticular,if

Table1

RadiationqualitiesusedforthecalibrationofENEAICs.

RadiationQuality HighVoltage(kV) Added Filtration (mm)

Average Energy (KeV)

Airkermarate (Gy/s)

IC32002 IC32005

IC32002 IC32005 NKair(Gy/C) U(%) NKair(Gy/C) U(%)

N-40 40 4Al+0.21Cu 32.5 – 2.0·10⁻⁴ – – 1.145·10⁶ 1.0

N-150 150 4Al+2.5Sn 116.6 5.6·10⁻⁵ 2.6·10⁻⁵ 2.442·10⁴ 2.4 1.090·10⁶ 2.3

N-300 300 4Al+3Sn+5Pb 249.6 2.8·10⁻⁵ 2.8·10⁻⁵ 2.526·10⁴ 2.5 1.113·10⁶ 2.4

S-Am 59.0 4.2·10⁻⁶ 2.8·10⁻⁵ 2.462·10⁴ 2.4 1.098·10⁶ 2.4

S-Cs 662.0 1.9·10⁻⁶ 1.9·10⁻⁶ 2.506·10⁴ 2.4 1.107·10⁶ 2.4

S-Co ENEAlab. 1253.0 1.3·10⁻⁴ 1.3·10⁻⁴ 2.402·10⁴ 1.4 1.087·10⁶ 1.1

PTWlab. min1.7·10⁻⁷/max5.0·10⁻³ 2.487·10⁴ 2.5 1.099·10⁶ 2.5

Fig.1.NKairfactorsfortheENEAICs32002(left)and32005(right)resultingfromtheINMRI(diamond-shapedpoints)andPTW(circle-shapedpoints)calibration.Errorbars representtheexpandedmeasurementuncertainty(coveragefactork=2).The‘equivalent’calibrationfactorNKcalisplottedassolidline;dashedlinesrepresentthelimitsof theexpandeduncertainty(k=2)associatedtoNKcal.

theirdistributionaboutNKcalfollowsaGaussianfunction,themost probablevalueofN_K^calistheweightedleastsquaresestimator[14]:

N^cal_K =



N_Kair(i)·(i)⁻²



(i)⁻²

(1)

InEq.(1),summationsareoverthencalibrationpointsandeach N_Kairvalueisweightedwiththeinversesquareoftheassociated uncertainty.Finally,theuncertaintyassociatedtoNKcaliscalculated asthesquarerootofthesamplevariance:

_cal²=







n−1 (2)

Wheren-1isthenumberofdegreesoffreedomofthesample(i.e., 5forIC32002and6forIC32005).

N_K^calvaluesareindicatedinFig.1forthetwodosimetersand plottedassolidhorizontallines;inthesamefigure,dashedlines representthelimitsofthe95%confidenceinterval.Bothdosimeters showflatenergyresponseintermsofairkermawithin4.1%.

3.2. KITdosimeter

To compare the response of the KIT dosimetry system (IC 32002+PTWUNIDOSelectrometer)withtheENEAidenticalsys- temcalibratedatINMRI,across-calibrationhasbeenperformedat thegammacalibrationlaboratoryofENEA-INMRI.Thetwoioniza- tionchamberswerealternativelyexposedto3differentstandard

Cs–137gammasources.Aschemeoftheexperimentalsetupisin Fig.2.

Thedoserateatthespherepositionforthe3standardgamma sources was respectively 0.2, 1.7 and 3.6mGy/h. The detectors had beenpreventively exposed for 5mintothe gammasource beforethemeasurementsstarted.Duringmeasurements,temper- ature(18.4^◦C–18.8^◦C),pressure(1011.1–1011.3hPa)andrelative humidity(42%)didnotundergosignificantvariations.

The collectedcharge wasrecorded every60sand measure- mentslasted900seach.Theslopeofthelinearfitofthecollected chargevs.acquisitiontimeshowninFig.3(bottom)istheionization currentmeasuredwiththeENEAdosimetrysystem,proportional (throughthecalibrationfactor)totheairkermarate.Normalized differencesbetweenmeasurementsofcollectedchargeperformed withtheKIT(C_KIT)andtheENEA(C_ENEA)systems,withrespectto C_KIT,areshownintheuppergraphofFig.3.Asystematicover- estimation of about0.5%of theKIT dosimeteris observed.The maximumdifferenceiswithin1%.

4. IrradiationtestsatFNG

A preliminary test of the ENEA IC 32002 was carried out atFNG (FrascatiNeutron Generator),theENEA 14MeV neutron sourcefacility,withtheaimofassessingthecorrectfunctioning ofthedetectorafterneutronirradiation.Thistestshowedthatthe dosimetrysystemcorrectlymeasuresthebackgrounddoseratein thelaboratoryattheneutronsourceshutdown,afterarunofirra- diationexperiment[8].Afurthertestwasperformedtocheckfor self-activationofthedetectorinducedbyneutrons.Thedetector waspositioned1mfromtheFNGtarget(whereneutronsarepro- duced)andirradiatedforabout3h.The14MeVneutronfluence

Fig.2.Sourceleadcollimator(a)andionizationchamberlocatedonthemovablecarriage(b)atENEAINMRIgammacalibrationlaboratory;(c)schemeoftheexperimental setup.

Fig.3.Bottom:CollectedchargemeasuredwiththeENEAdosimetrysystematdif- ferentdoserates(0.2,1.7and3.6mGy/h).Top:normalizeddifferenceofcollected chargemeasuredwiththeKITandENEAsystems.

Fig.4.AirkermaratemeasuredafterthesecondirradiationtestatFNGwiththe irradiatedIC(i.e.,ENEAIC32002)andthenon-irradiatedIC(i.e.,KITIC32002).

(atthesphereposition)was8.2·10⁸cm⁻².Theionizationchamber, afterbeingirradiated,wasimmediatelymovedtothecontrolroom andthenswitchedon.MeasuredairkermarateisshowninFig.4 duringthedifferentphasesofthetest.Thebackgrounddoserate incontrolroom,previouslymeasured,isalsoreported.Background intheFNGcontrolroomisnotconstantandstronglydependenton theRn-222concentrationinair.

Theacquisitionwiththeirradiateddetectorinthecontrolroom lasted 22h. An exponential decrease (see Fig. 4) is observable in this phase witha decay constant␭≈2.06·10⁻⁴s⁻¹ and half- lifeT_1/2≈0.9h.Thisindicatesthatonly short-termactivationis observedanditisrelatedtotheactivationofairinsidetheIC.Subse- quently,theKITIC32002,identicaltotheENEAchamberbutnever exposedtoneutrons,wasconnectedtotheelectrometer,inplace oftheirradiateddetector,for24h.Thebackgrounddoseratemea- surementsinthecontrolroomwiththetwodetectorsshowthat afteraboutonedaythetwoICsmeasurethesamecurrent;afterthis period,noresidualcurrentduetotheself-activationisdetectable.

Italsoconfirmsthatnodamageswereinducedintheirradiated ionizationchamber.

5. Conclusions

ThepreparationoftheShutdownDoseRate(SDR)experiment atJETrequiredthechoiceofsuitabledetectorsforradiationprotec- tion,characterizedbyexcellentreproducibility,long-termstability andflatenergyresponse.DifferentENEAfacilitiesandlaboratories havebeenusedforcalibratingandtestingthedosimetryequip- mentselectedfortheexperiment.Inparticular,thetwodosimetry systems,respectivelybasedontheENEAIC32002and32005,have beencalibratedatENEA-INMRIintermsofairkermawithXand gammasourcesatlowdoses.Flatenergyresponseintermsofair kermawithin4.1%hasbeenobservedforboththeionizationcham- bers.IrradiationtestsoftheENEAIC32002werecarriedoutatFNG withtheaimofassessing

thecorrectfunctioningofthedetectorafterneutronirradia- tionandforcheckingtheself-activationofthedetectorinduced byneutrons.Thesetestshaveshownthataftertheirradiationof thedetector,thedosimetrysystemmeasurescorrectlythecharac- teristicdoseratetrendintheFNGlaboratoryattheneutronsource shutdown.Afteraboutoneday,noresidualcurrentsignaldueto theself-activationisdetectableandnodamageswereinducedin theirradiatedionizationchamber.

Acknowledgments

This work has been carried out within the framework of theEUROfusion Consortiumand hasreceivedfundingfromthe EuratomResearchandTrainingProgramme2014-2018undergrant agreementNo633053.Theviewsandopinionsexpressedhereindo notnecessarilyreflectthoseoftheEuropeanCommission.

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