Experimental study of consistency degradation of different greases in mixed neutron and gamma radiation

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Contents lists available atScienceDirect

Heliyon

www.elsevier.com/locate/heliyon

Experimental

study

of consistency

degradation

of

diﬀerent

greases in

mixed

neutron

and

gamma

radiation

Matteo Ferrari

a,b,∗

_,

_{Aldo Zenoni}

a,b,∗

_,

_{Monika Hartl}

c

_,

_{Yongjoong Lee}

c

_,

_{Alberto Andrighetto}

d

_,

Alberto Monetti

d

_,

_{Andrea Salvini}

e

_,

_{Fabio Zelaschi}

e

a_Dipartimento_di_Ingegneria_Meccanica_e_Industriale,_Università_degli_Studi_di_Brescia,_Via_Branze_38,_I-25123_Brescia,_Italy b_Istituto_Nazionale_di_Fisica_Nucleare,_Via_Bassi_6,_I-27100_Pavia,_Italy

c_European_Spallation_Source,_{Odarslövsvägen}₁₁₃_SE-22484_Lund,_Sweden

d_Laboratori_Nazionali_di_Legnaro_dell’INFN,_Viale_{dell’Università}_2,_I-35020_Legnaro_(PD),_Italy

e_Laboratorio_Energia_Nucleare_Applicata_LENA,_Università_degli_Studi_di_Pavia,_Via_{Aselli 41,}_I-27100_Pavia,_Italy

A

R

T

I

C

L

E

I

N

F

O

A

B

S

T

R

A

C

T

Keywords: Materialsscience Aerospaceengineering Industrialengineering Nuclearengineering Materialschemistry Mechanicalengineering Nuclearreactorirradiation Polymerdegradation Neutrondamage Lubricatinggrease Radiationeﬀect Mixedradiationﬁeld

Manyofthemovingcomponentsinacceleratorandtargetenvironmentsrequirelubrication.Lubricantsinsuch environmentsareexposedtohighfluxesofsecondaryradiation,whichoriginatesfrombeaminteractionswith thetargetandfrombeamlosses.Thesecondaryradiationisamixofcomponents,whichcanincludesignificant fractionsofneutrons.Lubricantsareradiation-sensitivepolymericmaterials.Theradiation-inducedmodifications oftheirstructurereducetheirservicelifetimeandimposeadditionalfacilitymaintenance,whichiscomplicated bytheenvironmentalradioactivity.Thestudyofthelubricantsradiationresistanceisthereforenecessaryforthe constructionofnewgenerationacceleratorsandtargetsystems.Nevertheless,datacollectedinmixedradiation fieldsarescarce.NinecommercialgreaseswereirradiatedataTRIGAMarkIIResearchReactortoservefor theconstructionofnewacceleratorprojectsliketheEuropeanSpallationSource(ESS)atLund(Sweden)and SelectiveProductionofExoticSpecies(SPES)atLegnaro,(Italy).Mixedneutronandgammadosesrangingfrom 0.1 MGyto9.0 MGyweredeliveredtothegreases.Foranexperimentalquantificationoftheirdegradation, consistencywasmeasured.Twoofthegreasesremainedstable,whiletheothersbecamefluid.Post-irradiation examinationsevidencethecleavageofthepolymericstructureasthedominantradiationeffect.Doseandfluence limitsfortheuseofeachproductarepresented.Apartfromthescientificsignificance,theresultsrepresentan originalandusefulreferenceinselectingradiationresistantgreasesforacceleratorandtargetapplications.

1. Introduction

Polymericmaterialsusedinhighpoweracceleratorandtarget envi-ronmentsareexposedtomixedradiationfields.Neutronsandgamma canrepresentsignificantcomponentsofthesefields.Theabsorbeddose duetodifferentradiationscausesstructuralmodificationsofthese mate-rials.Radiationsaffectpolymersmainlythroughthebasicmechanisms ofcleavageandcross-linkingofmacromolecularchains[1,2,3]. There-fore,significantradiation-inducedmodificationsoftheirphysicaland mechanicalpropertiescanbeinduced,whichcouldleadtoapremature failureofthecomponents.

TheEuropeanSpallationSource(ESS)projectisunderconstruction inLund,Sweden,aimingatbecomingthebrightestneutronsourcein theworld[4].TheSelectiveProductionofExoticSpecies(SPES)facility

*

Correspondingauthorsat:DipartimentodiIngegneriaMeccanicaeIndustriale,UniversitàdegliStudidiBrescia,ViaBranze38,I-25123Brescia,Italy.

E-mailaddresses:matteo.ferrari@unibs.it(M. Ferrari),aldo.zenoni@unibs.it(A. Zenoni).

isbeingbuiltinLegnaro,ItalyaimingatproducingintenseRadioactive IonBeamsviafissionreactionsoccurringin itstarget[5,6].Bothof theirtargetswillproduceintensemixedfieldsofneutronsandgamma radiation.ElastomericO-rings,lubricatingoilsandgreasesare neces-sarilyemployedintheirtargetareas.Astheyareexposedtosecondary neutronsandgammas,theirmodificationsmustbeevaluatedforsafe andreliablefacilityoperations.Unfortunately,radiationdamagedata onpolymersinneutronfieldsandinmixedradiationfieldsarescarce intheliterature.

Mostoftheavailabledataoriginatefromtechnicalreportsproduced byscientiﬁcorganisationsandbycompaniesmanufacturingproducts certiﬁedtoberadiation-resistant.TheYellowReportsfromCERNare oneofthewidelyusedreferencesconcerningtheradiationresistance ofpolymericmaterials.ResultsonseventypesofelastomericO-rings

https://doi.org/10.1016/j.heliyon.2019.e02489

Received18February2019;Receivedinrevisedform10April2019;Accepted27August2019

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availableontheCERNstoreshelvesandtwotypesoflubricatingoils irradiateduptoseveral MGyofgammadosearereportedinRef. [7]. Greaseswerenotanalysed.

InanITERreport [8],twocommercialelastomericgasketsandfour greasesdeclaredbytheproducer asradiation-hardweretestedupto highgammadosesfrom10 MGyto100 MGy.Arecentwork[9] iden-tiﬁesO-rings,lubricantsandcableinsulatorsamongthemostcritical componentsintheITERdesign.Radiationeﬀectsonoilsandgreases evaluatedintheviewpointoftheirchemicalcompositionarereported inRefs. [1] and[10].Neutronandgammairradiationdataontwelve solidlubricantsarereportedin [11].

Theavailableliteratureontheradiationeffectsinpolymersisnot satisfactoryfortheconstructionofnewacceleratorsandtarget facili-ties.Infact,mostofthestudieswereperformedmorethanadecade ago.Consideringtherapidevolutionoftheindustrialproduct develop-ment,mostofthetestedproductsarenowoutdated.Furthermore,most oftheliteratureanddataavailablebymanufacturerslackdetailsabout theirradiation conditionssuch asradiation type,doserate,presence ofoxygenandlinearenergytransfer(LET),whichinfluenceradiation effectsinpolymers [12,13].Lastly,most ofthestudiesusedgamma sources,althoughacceleratorandtargetenvironmentsgeneratemixed radiationfields.Insomeofthemneutronsrepresentthedominant com-ponent.

Gammasourceshavebeenpreferredtoneutronsourcesfor materi-alstestingbecauseoftheirwideravailabilityandbecausetheirradiated samplesmaintainamuchlowerresidualactivation.Ithasbeenbelieved thattheradiation-induceddamagedependsonthetotalamountof ab-sorbed doseonly,irrespectively of thetypeofradiation [7, 14, 15]. Accordingly,thepolymersradiationresistancehasbeenexpressedin termsofgammadoseanddirectlytranslatedtoneutronandmixedﬁeld dose.Nevertheless,theequivalenceofgammaandneutrondoseshas notbeentheoreticallyveriﬁednorexperimentallyvalidated.

Contrarytotheassumedequivalencebetweengammaandneutron doses,itisreportedinRef. [1] thattheeffectsofmixedreactor radia-tioninpolymersmightdifferfromthatofgammaalone.Thedatahere presentedchallengethislongbelievedequivalenceaswell.InaUKAEA report [16],theneedforestablishingmorepreciselytherelativeeffects of similar dosesof neutrons andgammas isaddressed. In aseminal studyperformedatORNL,theradiationdamageinducedinelastomers byreactorradiationiscomparedwiththedamageproducedby differ-entgammasources.Itwasreportedthatsimilardamagesareinduced bydosesofthetworadiationsthatcandifferofafactortwo[17].In amorerecentwork,theunderstandingofradiationeffectsinpolymers andtheirdependenceontheradiationtypologyareclearlystatedasone ofthemostimportantproblemstobesolved[18].Accordingto Riva-tonandArnold,fastneutronseffectsonorganicmaterialsarenotfully understooddespitetheirrelevanceinthenuclearengineeringandnew investigationsareneeded[19].Thefullcomprehensionof theeffects ofintenseneutronfieldsinpolymersisreportedasarequirementfor theirapplicationinnucleartechnologybyBoninetal. [20].Theneed forresearchonfastneutroneffectsonpolymersisreportedbySeguchi etal. [3].Theyreportaswellthelackofsuchstudiesintheliterature, whichisdominatedbygammaradiationstudies.

Forthementionedreasons,radiationdamageonpolymeric materi-alshastobefurtherinvestigated.Inparticular,mixedradiationdataare necessaryforthedesignandconstructionofnewfacilitiesproducing in-tenseneutronfields.Wededicatedapreviousstudytothedevelopment ofamethodologyfortestingelastomericO-ringsirradiatedinneutron andgammamixedfields [21].Afollowingstudywasdedicatedtothe experimental determinationof theend-of-lifeconditionsofaspecific EPDMO-ringtobeinstalledinagatevalveoftheSPESfacility.It rep-resentsanexampleofthedesignofacomponentwhichoperatesina mixedneutronandgammaradiationenvironment [22].The manage-mentof theSPESfacilityasanintenseneutronsourceis detailedin [23].The problemof theradiationresistanceof lubricants isbriefly mentionedtherein.

In this framework, the present study is dedicated to lubricating greases. A methodology for greases irradiation in a mixed neutron andgammareactorﬁeldwasdeveloped.Post-irradiationexaminations basedonconsistencyevaluationswerecompleted.Thechosen deﬁni-tionof dosethresholdsallowedtheselectedproductstobe compara-tivelyevaluated.

InSection2.1adescriptionofthecommercialproductsselectedfor thisstudycanbefoundaswellastheirmostrelevantproperties.Seven ofthemostimportantgreaseproducers intheworldhave been con-sidered.InSection2.2adescriptionofthemixedneutronandgamma irradiationfacilityof aTRIGAMarkIInuclearresearch reactorused forsampleirradiationisreported.InSection2.3adescriptionof the dosecomponentsabsorbedbythedifferentgreasesintheneutronand gamma irradiation mixed field is detailed. The methodology devel-opedtoirradiateandtestthegreaseevolutionwithdoseisreported inSection2.4.Consistencyischosenasthemostsignificantindicatorof radiation-inducedgreasedeterioration,asreportedinSection2.5. Re-sultsonninegreasesirradiatedinarangebetween0.1 MGyand9 MGy ofabsorbeddose areshown in Section3. Experimentalevidences of radiation-inducedcleavageofthegreasestructurearereportedin Sec-tion4.1whereasconsiderationsonthegreaseradiationresistanceasa functionoftheirchemicalcompositionsarereportedinSection4.2.In thediscussionsection,thegreasesarecomparedbasedontheir consis-tency.Doseandparticlefluencethresholdsaredefinedtoestimatethe usabilityofeachgrease,asreportedinSection4.3.Consistencyresults arecomparedwiththeradiationresistancedeclarationsofthe produc-ersinSection4.4.Thepossibilityofusingthepresentresultstoforesee theend-of-lifegreaseconditionsinspecificapplicationsisdiscussedin Section4.5.ConclusionsaredrawninSection5.

2. Productselectionandtestingmethodology

Greases aresemi-ﬂuid to solid systemsoriginating from the dis-persionofathickenerin aliquidoil[1].Astandardgreasecontains about85%base oil,10%thickenerand5%other additives.Greases arecomplexmulti-phasesystemswhosechemicalandphysical proper-tiesoriginatefromthecoexistenceandinteractionoftheircomponents [24].

Theradiationresistanceofbaseoils isreportedtopredominantly dependontheiraromaticcontent.Ahighercontentofaromatic struc-turesisassociatedtoahigherstabilityoftheoilviscosityasafunction oftheabsorbeddose.Forthisreason,greasesbasedonaromaticand polyetheroilsareconsideredmoreradiationresistantthanmineraloil basedones[1] [10].However,theexperimentaldataherepresented challengethisgeneralstatement.

Theacademicliteratureon theradiationresistanceof lubricating greasesisverylimited,asreportedbyR.M.Mortieretal. [24].Forthis reason,inthepresentpaperalltheavailableinformationprovidedby theproducersisusedasreferencetoo.Radiationeffectsingreasesare complicatedbytheinteractionofthebaseoilwiththethickenerand theadditivesandtheoveralleffectisexpectedtodependnotonlyon theeffectsonitsbulkcomponents,butontheircomplexinteractionas well.

Sincetheavailableinformationonthegreaseradiationresistanceis solimited,awideandqualifiedspectrumofproductstobetestedwas necessary.Amarketresearchhasbeencompletedtoselect:i) experi-encedcompaniescertifyingaqualitycontrolonthegreaseproduction; ii) greasesdifferinginthechemicalcompositionsoftheircomponents; iii) bothgreasesdeclared asradiation resistantbytheproducer and genericones;iv) greaseshavingdifferentfeatures:vacuum compatibil-ity,extremepressureresistanceandextremeduration.

Thesecriterialeadtoaselectionofhigh-qualitygreasessuitablefor diﬀerentapplicationsinacceleratorandtargetenvironments.

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2.1. Selectedlubricatinggreases

Ninecommercial greaseswereselected from themarket(see Ta-ble1).Indicationsabouttheirchemicalcompositionareprovidedby theproducerandreportedindatasheets.Consistency,whosevalueis reportedasaNLGIclass,isoneofthemostrelevantproperties charac-terizingthegrease [24].

Thenineselectedgreasesandtheirmajorpropertiesarelisted be-low:

• AFB-LFisproducedbyTHK(Japan).Itisageneral-purposegrease forbearingapplications,manufacturedusingareﬁnedmineraloil andaLibasedthickener.Itfeaturesextremepressureresistance andhighmechanicalstability.

• ApiezonM is producedby M&IMaterials(UnitedKingdom).1 _It

isdevelopedforvacuumuseanditisdeclaredasradiation resis-tantuptoabout1 MGyofdosedeliveredusinga4 MeVelectron beam.Apiezon Mismanufacturedwithahydrocarbonoilandit is halogenfree. This is an advantage for applications in radia-tionenvironment,becausehalogenscanevolveacids.Apiezon M is manufacturedwithout athickener. Forthis reason,itis more properlyreferredasaveryviscousﬂuidratherthanagrease. • FAGArcanolLOAD 220isproducedbySchaeﬄer(Germany).Itis

developedforballandrollerbearings.Itismanufacturedusinga mineraloilandamixedthickener.Itfeatureshighloadresistance anddeclareddurableperformance.

• GrizzlygreaseNo. 1is producedbyLubricant ConsultantGmbH Lubcon (Germany). It is manufactured using a mineraloil and a Li/Ca special soap. It is developed for gears application and declared as resistantto gammaradiation up toa dose level of 1.2 MGy.

• KlüberlubBE41-542isproducedbyKlüberLubrication(Germany). Itismanufacturedusingamineraloilandaspeciallithiumsoap. Itisdeﬁnedasheavy-dutyproduct,developedforhigh-loadrolling bearingsandfeaturingextremepressure(EP)resistance.

• Krytox240ACisproducedbyChemoursCompany(USA).2_Krytox

fluoropolymersarehigh-performancelubricantsinitiallydeveloped for the aerospace industry. Now they have a broader rangeof applications,fromautomotivetoelectronics.Krytoxisa perfluo-ropolyether(PFPE)whosepolymerchainiscompletelysaturated withfluorine.Accordingtotheproducerdeclarations,itcontains carbon,oxygenandfluorineonly. Krytox240ACgreaseis com-poundedusing polytetrafluoroethylene(PTFE) asthickener.Itis selected in thepresentwork in view ofits claimed outstanding performanceandofitshydrogen-freecomposition.Thegreaseand itsbaseoilwereirradiatedusinga60_Co_gamma_source_and_then testedbytheproducer.Aprogressivegreasesofteningisreported asafunctionoftheabsorbeddose.Reportedconsistencyvariations aregenerallynotexceedinga10%-15%comparedtothe unirradi-atedsamples.Theradiationstabilityofthebaseoilwasevaluated inanuclearreactor,uptoamaximumvalueof1 MGy.

• PetamoGHY133NisproducedbyKlüberLubrication.Itis manu-facturedusingamineraloilandapolyureathickener.Itisdeclared asalong-termandhigh-temperaturegreaseforrollingbearings. • RG-42R-1 is producedby MORESCO Corporation (Japan). It is

declared as radiation resistant up to high gamma doses. Tests areperformedbytheproduceronits baseoilonly. Itis consid-ered as representative of the whole grease in a simplistic way. Absorbeddoselevels up to30 MGy aredeliveredtotheoil us-inga 60_Co _gamma_source_with_a _dose_rate_of _{0.01 MGy/h.}_The viscosity evolution of theoil asa function of the dose is eval-uated. A dose threshold of 15 MGy is determined accordingly.

1 _Apiezon®is_a_registered_trademark. 2 _ChemoursTM_and_KrytoxTM_are_trademarks.

Table 1

Thelubricatinggreases selectedfromthemarket forirradiationandtesting arelisted.Thecompositionofthebaseoilandofthethickenerisspeciﬁed. ConsistencyexpressedinNLGIgradeisreported.Radiationresistance,when declaredbytheproducer,isreported.Theinformationoriginatesfromthedata sheetsoftheproductsprovidedbythesuppliers.

Producer and product Base oil Thick. Cons. NLGI

Rad-hardness

AFB-LF Mineral Li 2 Not declared

Apiezon M Hydrocarbon — — 1 MGy, electr.

FAG Arcanol LOAD 220 Mineral Mixed 1-2 Not declared Grizzlygrease No.1 Mineral Li/Ca 0 1.2 MGy,𝛾 Klüberlub BE 41-542 Mineral Li 2 Not declared

Krytox 240 AC PFPE PTFE 2 1 MGy, reactor

RG-42R-1 PPE PC 1 15 MGy,𝛾

Petamo GHY 133 N Mineral Polyurea 2 Not declared Turmopolgrease 2 Polyglicol Li 2 Not declared

Table 2

Elementalcompositionoftheselectedgreasesinweightpercentisobtained withCHNanalyses.ThepercentageofthemassnotmeasuredbytheCHN anal-ysisisreportedaswell.Themeasurementerrorsarecontainedinthesecond decimalﬁgureofthequotedresults.

Product C % H % N % Others % AFB-LF 82.75 14.13 0.00 3.12 Apiezon M 86.19 13.70 0.11 0.00

FAG Arcanol LOAD 220 80.57 13.15 0.25 6.03

Grizzlygrease No.1 79.23 11.63 0.18 8.96 Klüberlub BE 41-542 82.50 12.29 0.58 4.63 Krytox 240 AC 21.03 0.00 1.01 77.96 Petamo GHY 133 N 84.26 12.42 1.23 2.09 RG-42R-1 78.50 9.67 0.00 11.83 Turmopolgrease 2 62.97 10.71 0.00 26.32

The radiation resistance of the grease is supposed to be equal tothe radiation resistance of the oil by the producer. Most of theMORESCOradiationresistantproductsincludingRG-42R-1are manufacturedusingapolyphenyletherbaseoilanda polycarbon-atethickener.

• Turmopolgrease2producedbyLubricantConsultantGmbH Lub-conisagreasemanufactured withapolyglicoloilandalithium basedsoap.Itfeaturesaverylonglubricationservicecomparedto conventionalproductswiththesamecomposition.

Thenumberofgreasesdeclaredasradiationresistanceontheworld marketisverylimited.

As described in details in Paragraph 2.2, the dose absorbed by greasesinafastneutronﬁeldhighlydependsonitslightelement com-position.Inparticular,itisproportionaltothehydrogencontent.For thisreason,themassfractionofsomerelevantlightelementscomposing thegreasesismeasuredbyCHNchemicalanalysis.Thepercentcontent ofcarbon,hydrogenandnitrogenineachproductismeasuredand re-portedinTable2.

Thehydrogenconcentrationfortheselectedproductsrangesfrom 0.00%to14.13%,depending on their chemicalcomposition. Krytox 240ACishydrogen-free,asdeclaredbytheproducer.Itiscompletely saturatedwithﬂuorine,whoseamountisabout70%ofthetotalmass.

ThehydrogenmasspercentageofMORESCORG-42R-1productis 9.67%.Itiscompatiblewiththechemicalcompositionofitsoil,which is polyphenylether based. Thehydrogen massconcentration of min-eraloilbasedproductsrangesfrom11.63%and14.22%.Theresults oftheseanalysesareusedtosimulatethecompositionofthematerials fordosimetrycalculations

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Fig. 1. Thein-coreCentralThimbleirradiationfacilityoftheTRIGAMarkIInuclearreactoroftheUniversityofPavia.Left:apictureofthereactorcore.The aluminumpipeoftheCentralThimble,reachingthemiddleofthereactorcore,canbeseen.Right:ahorizontalcross-sectionofthereactorcoreasmodelledwith thesimulationprogramMCNP5.TheCentralThimblefacilityisindicated.

Table 3

IrradiationconditionsoftheCentralThimblefacilityoftheTRIGA Mark IInuclearreactoroftheUniversityofPavia.Parametersreferto thenominalworkingpowerof250 kW.Fluxesarecalculatedusinga reactormodelrealizedwiththesimulationMonteCarlocodeMCNP5. Thefastneutronfluxreferstoneutronshavingenergyhigherthan0.5 MeV.Thegammafluxconsidersthefissionpromptgammasandthe photonsfromradiativecaptureofneutronsonthereactormaterials. Sincethegammasproducedbythefissionproductdecayarenot sim-ulatedbyMCNP5,thegammafluxisunderestimated.

Parameter Value

Neutron ﬂux 1.72 1013_neutron/(cm2_s) Fast neutron ﬂux 3.80 1012_neutron/(cm2_s)

Gamma ﬂux 1.65 1013_photon/(cm2_s)

Temperature range 50◦_{C – 70}◦_C

Atmosphere Air at atmospheric pressure

2.2. Irradiationconditions,ﬂuxesandspectra

TheTRIGAMarkIInuclearreactoroftheUniversityofPavia,Italy isequippedwithirradiationfacilitiesforresearch.TheCentralThimble (seeFig.1)isanin-corefacilityreachingthecentre ofthefuelelements. Radiationcomponentsin theCentralThimbleoriginate from sev-eralreactionsoccurringinthereactorcore.Nuclearfissionisthemain sourceofneutronsandphotons.Mostofthegammasareproducedby fissionreactionsandbythedecayofradioactivefissionproducts.They arereferredtoaspromptgammasanddelayedgammasrespectively.In addition,aminorgammacomponentisduetoradiativecapture reac-tionsinthematerialsconstitutingthereactor[25].

Inthepresentwork,radiationfieldsanddosecomponentsare cal-culatedwithasimulationofthereactor[26] realizedwiththeMonte CarlocodeMCNP5[27].Inapreviousexperimentalcampaign,the Cen-tralThimbleneutronspectrumwasmeasured.Thecalculatedneutron spectrumagrees withthemeasuredone[28]. Accordingly,the calcu-latedneutronspectrumcanbeconsideredareliablerepresentationof themeasuredone.Onthecontrary,thecalculatedgammafluxis un-derestimatedbecauseMCNP5doesnotaccountforthefissionproducts decay.BasedonexistingmeasurementsonsimilarTRIGAMarkII reac-tors,themissinggammacomponentcanbeestimated.Itcanrepresent upto30%ofthetotalgammacomponent[29].Thesystematicerror onthegammadoseisevaluatedaccordingly.Table3summarizesthe CentralThimblecalculatedparameters.

Fig.2showsthecalculatedneutronenergyspectrumintheCentral Thimble. The spectrum is normalized tothe nominal reactor work-ing power of 250 kW, which is associated to a source intensity of 1.9⋅ 1016 _n/s._Two_main_components_are_present._The_fast_component refershere toneutrons withenergy higherthan 0.5 MeV.Fast neu-tronsoriginatefromﬁssionreactions.TheirdistributionintheCentral

Fig. 2. TheneutronenergyspectrumintheCentralThimblefacilitycalculated withMCNP5.Thespectrumisnormalizedtothenominalreactorpowerof250 kW.Thefastneutroncomponentandthethermal-epithermalonearedominant. Thimblefacilityis comparableto atypical ﬁssionspectrum.An im-portantcomponentofslowerneutronsinthethermalandepithermal energyrangeis presentaswell.They originatefrom themoderation offastneutronsinthereactormoderatorandstructures.Theneutron doseabsorbedbytheirradiatedgreasesdependsonthespeciﬁcneutron spectrum.

2.3. Dosimetrycalculations

Theneutronandphotondosecomponentsabsorbedbyeach irra-diatedgreasearecalculatedusingMCNP5.Thegreasecompositionin thesimulationisbasedontheCHNanalysesresults(seeTable2). Addi-tionalassumptionsaremadetomodelthemissingcompositionfraction. ForKrytox240AC,whichisafluorinatedmaterial,themissingmass isassumedtobefluorine(70.4%)andoxygen(7.56%).These assump-tionsarebasedonthebaseoilandthickenerchemicalcompositions,as declaredbytheproducer.Foralltheothergreases,theremainingmass isassumedtobeoxygen.Metallictracesarepresentintheproductsin mostcases.However,theirrelevanceintheoveralldosimetry calcula-tionisnegligible.Asafirstapproximation,metallictracesareneglected. Thesystematicerrorintroducedbythesehypothesesinthedosimetry calculationsisestimatedbychangingtheassumedcompositionsin dif-ferentways.However,sincetheneutrondoseismostlyrelatedtothe hydrogencontent,whoseamountismeasured,theerrorduetothe as-sumptionsonthecompositionisnotlargerthanfewpercent.

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Fig. 3. DoseratescalculatedwithMCNP5forgreasesirradiatedintheCentralThimblefacilityatthenominalworkingpowerof250 kW.Greasesareordered fromlefttorightaccordingtotheincreasinghydrogencontent,rangingfrom0.00%(Krytox 240 AC)to14.13%(AFB-LF).Theneutrondoseratecomponentisin darkgrey,thegammaoneinpalegrey.Theoverallsystematicerrorisestimatedtobelowerthan15%.Thestatisticalerroronthesimulatedresultsisnegligible comparedtothesystematicone.

The dose absorbed in a neutron and gamma mixed ﬁeld highly dependsonthecompositionoftheirradiatedmaterialsandonthe ra-diationenergyspectrum.Forthisreason,thedosimetryconsiderations herereportedspeciﬁcallyrefertotheselectedgreasesirradiatedinthe CentralThimble.Thecalculatedtotaldoseratesforthegreasesare re-portedinFig.3.Thedoserateisseparatedinitstwomainneutronand

gammacomponents.

Except forKrytox 240 AC,whose hydrogencontentisnegligible, fortheothergreases thetotaldoseraterangesfrom0.76 MGy/hto 0.94 MGy/h.Thediﬀerencedependsontheneutroncontribution, rang-ingfrom0.50 MGy/hto0.67 MGy/h.Itisproportionaltothehydrogen masscontent,rangingfrom9.67%to14.13%.Theneutroncomponents dominate,representing65%-71%ofthetotal.Thegammacomponent isabout0.26 MGy/hforallthegreases.Itisroughlyirrespectiveofthe speciﬁcmaterialcomposition.

Becauseofitshydrogen-freecomposition,Krytox240ACbehave dif-ferentlyinthisneutronfield.Itstotaldoserateis0.28 MGy/h,about threetimeslowercomparedtotheothergreases.Thisisduetothe neu-tron component,which ismore thanoneorder of magnitude lower. Thegammacomponentdominates,representingabout93%ofthe to-tal.Productscontaininghalogensareusuallynotemployedinradiation environmentsbecausetheycanproduceacidsduringirradiation. Never-theless,fluorinatedproductsareinterestingforapplicationsinneutron fieldsinviewoftheirlowhydrogencontent.Thisisthereasonwhya fluorinatedproducthasbeenselectedforthepresentstudy.

Theneutrondosecomponentisproportionaltothehydrogen con-tentforallthegreases.Thisisbecausethemainenergyrelease mecha-nismistheelasticscatteringoffastneutrononlightnucleicontainedin thegrease.Theaverageenergytransferredtoanucleusinthisprocess increaseswhenthemassofthetargetnucleusdecreases.Forthis rea-son,theprocessismostlyeﬀectiveonhydrogen.Moreover,theaverage energytransferredviaelastic scatteringisproportional tothe incom-ingneutronenergy.Forthisreason,theaverageenergytransferredby fastneutronsisseveralordersofmagnitudehigherthantheone trans-ferredbythermalneutronsbythesamemechanism[30].High-energy

secondaryprotonsoriginatingfromneutronscatteringonhydrogen de-liveralargefractionofthetotaldose.

Thesystematicerroronthetotalabsorbeddoseisestimatedtobe lowerthan15%.Itdepends ontheagreementbetween the measure-mentsandthesimulatedmodel.However,thetotalabsorbeddoseis underestimatedbecauseofthefractionofthegammacomponentthat isnotaccountedforbythecode.Thestatisticalerroronthesimulated resultsisnegligiblecomparedtothesystematicone.

Inthepresentwork,thetotal amountofabsorbeddoseischosen astheparametertowhichthegreaseconsistencyevolutionisreferred. Inaddition,radiationeffectsingreasesarereferredaswelltothetotal neutronandgammaparticlefluencetowhichthesamplesareexposed duringirradiation.Table4showsthedosevaluesabsorbedbythe irra-diatedgreasesandthetotalneutronandgammafluence.Thosevalues dependontheparticlefluxesintheirradiationfacility,asreportedin Table3andontheexposuretimeonly,irrespectiveofthegrease compo-sition.Infacttheirradiatedgreasesamplesdonotsignificantlyperturb theparticle fluxesinthefacilitybecauseoftheirsmallsize.

2.4. Experimentalmethodology

Theninegreaseswereirradiatedforexposuretimesrangingbetween 10minutesand10hours.Themoststablegreaseswereselectedfor5 h and10 hlongirradiations. Theirradiationsarerealizedin the Cen-tralThimblefacilityatthenominalreactorpowerof250kW.Table4

reportsthechosen irradiationtimes andthecorresponding absorbed doses.Absorbeddosesrangefromabout0.1MGytoabout9MGy.The samplesareirradiatedinairatatmosphericpressure.

Asingleirradiation wascompletedon Krytox240 ACgrease.An irradiationtimeof3 hand30 minuteswasselectedinviewofits ﬂuo-rinatedcomposition.Additionalirradiationswerenotpossiblebecause ofthelimitedamountofavailableproduct,whosecommercialpriceis extremelyhighcomparedtotheotherones.

Qualitativepreliminarytestswereperformedonirradiatedgreases todesigna safeaneﬃcientset-up. Onetestevaluates the radiation-inducedgreasemobility.Smallplasticcontainerswereﬁlledwith

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dif-Table 4

DosesabsorbedbytheselectedgreasesinMGy.CorrespondingirradiationtimesintheCentralThimblefacilityofthe TRIGAMarkIIreactorarereported.Krytox240ACisnotreportedhere.Sinceitsdoserateinthefacilityismuchlower comparedtotheotherones,asingleirradiationtimeof3 hoursand30 minuteswaschosen,correspondingtoatotal doseof0.99 MGy.Totalneutronandgammaﬂuencetowhichthesamplesareexposedisreportedaswellasafunction oftheirradiationtime.

Irradiation time 10 min 30 min 1 h 2 h 5 h 10 h

Product

AFB-LF (MGy) — 0.47 0.94 1.88 — —

Apiezon M (MGy) 0.16 0.47 0.94 1.88 4.70 —

FAG Arcanol LOAD 220 (MGy) 0.15 0.46 0.91 1.83 — —

Grizzlygrease No.1 (MGy) 0.14 0.42 0.85 1.70 — —

Klüberlub BE 41-542 (MGy) 0.15 0.44 0.88 1.75 4.39 —

Petamo GHY 133 N (MGy) 0.15 0.44 0.89 1.78 4.44 8.89

RG-42R-1 (MGy) — 0.38 0.76 1.52 3.80 7.60

Turmopolgrease 2 (MGy) 0.13 0.41 0.81 1.63 — —

Neutron ﬂuence (1016_{par cm}−2₎ _1.03 _3.1 _6.2 _12.4 _31.0 _62.0

Gamma ﬂuence (1016_{par cm}−2₎ _0.99 _3.0 _5.9 _11.9 _29.7 _59.4

Fig. 4. Left:asyringeﬁlledwithgrease,brokenduetotheradiation-inducedgas pressure.Right:asectionviewofthesyringeafterirradiation.The radiation-inducedgreasemobilitycanbeobserved.Thescatteringofthegreaseinsidethe syringevolumeisduetotheproductionofgasbubbles.

ferentamountsofgreaserangingfrom20%to80%ofthetotalvolume. Animportantradiation-inducedgasevolutionisobserved,promotinga highgreasemobilityinthecontainers(seeFig.4).Thedeveloped pres-sureduetothegasproductionintheset-upcan reachcriticallevels, sometimesdamagingandbreakingthecontainer.

Radioactivenuclidesareproducedintheirradiatedgreasesdueto neutroncapturereactions.Becauseoftheirresidualactivation, irradi-atedgreasesmustbehandledbyqualiﬁedoperatorsaccordingtothe existing radiation protection regulation. Seven daysafterirradiation thecontactdoseraterangesbetween0.1 μSv/hand0.3 μSv/hforall thegreases.Thisiscomparablewiththenaturalradiationbackground, thereforethegreasesamplescanbesafelytestedafterirradiation.

Basedonthepreliminarytests,a20 mLdisposableplasticsyringeis chosenasirradiationset-up.Amaximumof10 mLofgreaseis irradi-ated,toavoidcriticalgasevolutionandexcessivemobility.Thegrease is distributedasalayer on theinnersyringesurface. Thesyringeis placedinacylindricalaluminumcontainerroutinelyusedforsample irradiation.Fig.5showstheset-up.

2.5. Consistencytests

Consistencyiswidelyconsideredasthemostimportantpropertyof alubricatinggreaseandmarksthediﬀerencebetweenliquidoilsand greases [24]. The presenceof thethickener givesthegrease a solid characterandarigiditycommonlyreferredtoasconsistency.

Consis-Fig. 5. Theset-upusedtoirradiategreasesamplesintheCentralThimble facil-ity.Adisposableplastic20 mLsyringeisﬁlledwithabout10 mLofgrease.The syringeisplacedinanaluminumcontainer.

tencyismeasuredforbothirradiatedandunirradiatedgreasesamples accordingtoASTMD217-02andASTMD1403-02 standards. Consis-tencyisthereindefined asthe degreeof resistancetomovement under stress.Itismeasuredwithapenetrometerequippedwithaconewhose tippenetratestheflatgreasesurface(seeFig.6).Theconeis freeto fallintothegreaseundergravityfor5 seconds.Consistencymeasures theconepenetrationintenthsofmillimeters.Asemi-automatic preci-sionpenetrometerbyS.D.M.ApparecchiScientificis.r.l.equipped with aone-quarterscaleconeis usedin this work.The one-quarterscale equipmentasdescribedinASTMD1403 allowsaconsistencytesttobe completedonasampleofabout5mLonly.Thisisanadvantage con-sideringthelimitedavailablespaceintheirradiationfacilityandthe needtoreduceasmuchaspossibletheamountofproducedradioactive waste.

Accordingtothestandard,thegreasemustbeworkedusinga ma-nipulatorbeforebeing tested(seeFig.7).Thegreaseis subjectedto 60 fulldoublestrokesinone minute.This manipulationis necessary toperformaworkedpenetrationtest.Theconsistencymeasuredafter manipulationisreferredtoasworkedconsistency.

TheNationalLubricatingGreaseInstitute(NLGI)classifiesgreases based on their worked consistency. NLGI grades are universally ac-ceptedandusedbyproducersandfinalusers.NLGIgradesrangefrom 000to6. Grade000 indicatesa softand almostfluidmaterial con-dition,correspondingtohighpenetrations(445-475 mm/10).Grade6 indicatesahardsolid-likematerialcondition,correspondingtolow pen-etrations(85-115 mm/10).Forthegreasesselectedinthepresentstudy, a10%consistencyvariationapproximatelycorrespondstoavariation ofonegrade.

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Fig. 6. Thesemi-automaticpenetrometerbyS.D.M.Srlusedforconsistencymeasurements.Right:thepenetrationconepositionedovertheﬂatsurfaceofthegrease, beforethemeasurement.Theone-quarterscaleequipmentisused.

Fig. 7. Theone-quarter scaleequipmentusedtomanipulatethegrease be-forethemeasurementofconsistency.Thegreaseissubjectedto60fulldouble strokesinaboutone minute.

The one-quarterscale equipment used in this workcan measure NLGIgradesrangingfrom0to4.Asreported.Measuredvalueslower than0inNLGIclasswillbeherereferredtoasout-of-scale.Measured valueshigherthan4werenotobserved.

3. Results

Consistencyvariationsoftheirradiatedsamplesrelativetothe unir-radiatedonesarereportedasafunctionofthetotalabsorbeddose(see Fig.8).Therelativeconsistencyvaluesarecalculatedasfollows:

𝐶𝑟𝑒𝑙(𝐷) =𝐶(𝐷)_𝐶

0

(1) C(D)istheconsistencyvalue(inmm/10)ofthegreaseirradiatedat a totaldose D.𝐶0 is theconsistencyvalue(inmm/10) of the unir-radiated grease. The error associated to 𝐶𝑟𝑒𝑙(𝐷) is estimated to be

lowerthan5%,basingonrepeatedmeasurementsonunirradiated sam-ples.

Six out of the nine tested greases experience severe radiation-inducedconsistencyincreaseasafunctionofthedose.Theybecome almostﬂuidbetween0.4 MGyand5 MGy.Theconditionand appear-anceofthesegreasesamplesafterirradiationarecompletelydiﬀerent fromtheunirradiatedones.Theconsistencyincreaseisremarkable, ex-ceedingtherangeoftheinstrument.Theseexceedingvaluesareanyway

usedinEquation1toestimatearelativeconsistencyvariation.Theyare associatedtoaﬂuidgreasestateandtheyareinthepresentpaper re-ferredtoas out-of-scalevalues.Theyaremarked bydashedlinesin Fig.8.

Post-irradiationconsistencyresultscanbeanalysedwithreference totheirpercentagedeviationfromtheirunirradiatedsamples consis-tency,recallingthata10%consistencyvariationisapproximately asso-ciatedtoachangeinNLGIgrade.

Threegroupsof productscan be distinguishedaccordingtotheir consistencymodiﬁcationwithabsorbeddose:

1. HugeconsistencyincreasesarereportedforAFB-LF,Grizzlygrease No.1,Krytox240ACandTurmopolgrease2atdosevalueslower than1 MGy.

AFB-LF consistencyincreasesof 25%at 0.94 MGy.Itbecomes ﬂuidat1.9 MGy,showinga46%increaseinconsistency.

Grizzlygrease No.1 consistency increases of 25% at 0.4 MGy. This valueisout-of-scaleandthegreasebecomes ﬂuid (seeFig.9).Thegreaseremainssteadilyﬂuiduptoabout 1.7 MGy.

Krytox 240 AC becomesﬂuidat0.97MGy,showinga45% con-sistencyincrease.Acidgasesevolvedduringirradiation due totheﬂuorinecontentledtothecorrosion ofthe aluminumcontainer(seeFig.10).

Turmopolgrease 2 consistencyincreasesof37%at0.8 MGy.It becomesﬂuidat1.6 MGyofdose,showinga60% con-sistencyincrease.

2. ApiezonM,FAGArcanolLOAD220andKlüberlubBE41-542 re-mainratherstableup to1.0 MGy.Theirconsistencyremarkably increasesbetween1.5 MGyand5 MGy.

FAG Arcanol LOAD 220 isstableupto0.9 MGy.Itabruptly be-comesﬂuidat1.8 MGy,showinga40%consistency in-crease.

Klüberlub BE 41-542 exhibitsaprogressiveconsistencyincrease as a functionof thedose. Consistency valueincreases of 7%at 0.9 MGyandof10%at 1.8 MGyof dose.It becomes ﬂuidat4.4 MGy,showinga30%consistency increase.

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Fig. 8. Relativeconsistencyvalue𝐶𝑟𝑒𝑙(𝐷) asafunctionofthetotalabsorbeddoseforthenineselectedgreases.Dashedlinesindicateconsistencyvaluesexceeding

themaximuminstrumentrange,correspondingtoanalmostﬂuidmaterialstate.Highconsistencyvaluesareassociatedtosoftergreases,whichcorrespondtolower NLGIconsistencygrades.

Fig. 9. Post-irradiationconsistencytestofGrizzlygrease No.1.Topleft:grease irradiatedat0.15 MGy.Consistencyiscomparabletotheunirradiatedone. Bot-tomleft:GrizzlygreaseNo.1irradiatedat0.45 MGy.Aremarkableconsistency increasecanbeobserved.Themeasuredconsistencyvalueisoutofthe instru-mentscale.Theirradiatedgreasebecomesﬂuid.Right:thegreaseirradiatedat 0.45 MGyafterpenetrationmeasurement.Thegreasedripsfromthe penetrom-etertip.

Apiezon M doesnotexhibitsigniﬁcantconsistencyvariationsup to1.9 MGy.Itabruptlybecomesﬂuidat4.7 MGy, show-inga70%consistencyincrease.

3. PetamoGHY133N and RG-42R-1 donotexhibitsigniﬁcant con-sistencyvariationsuptothemaximuminvestigateddosevaluesof 8.9 MGyand7.6 MGyrespectively.

Thecolourofsomegreasesisdarkenedbyirradiation.Itis particu-larlyevidentforPetamoGHY133N(seeFig.11),whosecolour turns fromyellowish(unirradiatedsample)toblackatabout0.4 MGy.

Fig. 10. Left:Krytox 240 ACsampleirradiatedat0.97MGy(3 hand30min). Right:thealuminumset-upusedforirradiation.Thestopperhasbeencorroded bytheacidgasesevolved.

4. Discussion

4.1. Cleavageobservationinthegreasestructure

Greasesarecomplexmulti-phasesystems.Forthisreason,the con-sistencyevolutionwithdoseisexpectedtobecomplextoo.Radiation effectsingreasesresultfromtheinteractionofseveraldamage mecha-nisms.Twomaineffectscharacterizetheinteractionofradiationwith polymersatthestructurallevel:cleavageandcross-linkingofthe poly-mericchains.Asreported byR.O.Bolt andJ.G.Carrol [1],thefirst radiationeffectexpectedingreasesisthedamageofthegelling struc-ture of the thickener, causinga consistency increase.Radiation can inducefracturesinthesoapfibres,whichbecomeunabletomaintain agellingstructure.Athigherdoselevels,aconsistencydecreaseis ex-pectedbecauseoftheincreasedviscosityofitsbaseoilduetopolymeric chainscross-linking.Theformerbehaviourwasobservedformostofthe investigatedproducts,whereasthelatterwasnotobservedinthe con-sideredirradiationconditions.

Accordingtotheliterature,thepresenceofaromaticgroupsinthe baseoilshoulddeterminetheradiation-resistanceofagrease.However, theconsistencyresultsdon’tallowthegreasechemicalstructureandits radiationresistancetobeeasilycorrelated.Thecollecteddataconﬁrm thecomplexityoftheradiationinteractionwithmulti-phasesystems.

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Fig. 11. PetamoGHY133Ngreasesamples.Fromlefttoright:anunirradiatedsample,asampleirradiatedat0.15MGy(10minirradiation)andasampleirradiated at0.44MGy(30minirradiation).Aprogressivegreasedarkeningcanbeobserved,fromyellowishtocompletelyblack.Samplesirradiatedathigherdoselevels havethesamecolour oftheoneirradiatedat0.44MGy.

The tested greases feature either a stable consistency or a con-sistencyincreasewith dose.Sevenout ofthenineirradiated greases becomefluidanddripfromthetestinginstrumentundergravity(See Fig.9).Bydefinition,agreaseshouldremaininplaceasasolidbody undergravity [24].Greasefluidizationisanindicatorofsevere mate-rialdamage,probablyrelatedtoacompletedamageof thethickener structure.Thethickenerisaradiation-sensitivecomponentandforthis reasonitisrelevantindeterminingtheoverallradiation-resistanceof thegrease.

ApiezonMbehaviourisdiscussedseparatelybecauseitis manufac-turedwithoutathickener.Forthisreason,itsﬂuidizationat4.7 MGyis notrelatedtothethickenerdisruptionandmightberelatedtotheoil structurecleavage.

Thequalitativepreliminarytestsprovideindirectinformationonthe greaseradiationeﬀectstoo.Greasemobilityishighlypromotedby ir-radiationforallthetestedproducts.Theintensegasevolutioncould originatefromthecleavageofthepolymericchains.Thegasevolved duringirradiationleadstotheformationofbubbles,whichmodifythe distributionofthegreaseintheset-up.AcidgasesareevolvedbyKrytox 240ACbecauseofitsﬂuorinatedcomposition.

Inconclusion, cleavagedominates asdamage mechanismfor the greases here analyzed. Further understanding of the mechanismsof radiationinteractionatthestructuralscaleisnotpossiblebasedon con-sistencyresultsonly.Greasesarecomplicatedsystems,whosechemical andphysicalcharacteristicscannotbeeasilymodelled.Thelackof aca-demicbaseresearchintothegreasefundamentalsevidencestheneed forfurtherinvestigation[24].

4.2. Correlationbetweenradiationeﬀectsandchemicalcomposition

According to the literature,the grease radiation stability should be predominantly determined by its base oilchemical composition. Forexample,polyphenyletherbasedproductsarereportedtobemore radiation-resistantthanmineraloilbasedones[1,10].The outstand-ingstabilityofRG-42R-1complieswiththisconsideration.However, therearereasonstobelievethatradiationstabilitydependsonamore complexinteractionbetweenthechemicalnatureof thebaseoil,the thickenerandtheadditives.Thisis supportedbysomeof theresults herereported.

Fiveof thetestedgreases(AFB-LF, FAGArcanolLOAD 220,

Griz-zlygrease No.1, Klüberlub BE 41-542 and Petamo GHY 133 N) are

manufactured withmineral oilsbutdifferent thickeners.Despite the similar composition of their base oil, they show very different be-haviour,maybethankstothepresenceofdifferentthickeners.Petamo GHY133Nhasastableconsistencyuptoabout9 MGy,whereasthe othersbecomefluidbelow5 MGy.

PetamoGHY133NandRG-42R-1haveacomparablestabilitywith dose,despitetheirdiﬀerentchemicalcomposition.Theformeris

real-Table 5

Thresholdsforthetestedgreasesassociatedtothefunctionalendpoint deﬁni-tion,correspondingtoa10%variationfromtheoriginalconsistency. Thresh-oldsaredeterminedusingalinear interpolation.Whenvariationsarelower than10%forallthetesteddosevalues,consistencyvariationatthemaximum doseisreported. Product Dose MGy Cons.var % Fluence(n) parcm−2 Fluence(𝛾) parcm−2 Turmopolgrease 2 0.1 +10% 7.6 1015 _{7.3 10}15 Krytox 240 AC 0.2 +10% 4.1 1016 _{4.0 10}16 Grizzlygrease No. 1 0.3 +10% 2.2 1016 _{2.1 10}16 AFB-LF 0.4 +10% 2.7 1016 _{2.6 10}16

FAG Arcanol LOAD 220 1.1 +10% 7.5 1016 _{7.2 10}16 Klüberlub BE 41-542 1.8 +10 % 1.2 1017 _{1.2 10}17

Apiezon M 2.3 +10% 1.5 1017 _{1.5 10}17

RG-42R-1 7.6 +5% 6.2 1017 _{5.9 10}17

Petamo GHY 133 N 8.9 −3% 6.2 1017 _{5.9 10}17

izedwithamineraloilandapolyurea thickener,whilethelatter is realizedwithpolyphenylether oilandapolycarbonatethickener.

Therefore,itisconcludedthatthechemicalcompositionoftheoil aloneisnotrepresentativeofthegreaseradiationsensitivity.Specific additivesareexpectedtoinfluenceradiationstabilityaswell.Forthose reasons,generalconclusionsbasedonthechemicalcompositioncanbe hardlydrawnandspecificproductsmustbeindividuallytested.

4.3. Thresholdsofradiationdamage

Theconceptofradiationdamagethresholdingreasesisscarcely de-ﬁned.However,itisofutmostimportanceintheengineeringviewpoint andforend-userslike theESS andtheSPESfacilities.Inthepresent work,twoendpointsaredeﬁnedtoevaluatetheradiation-resistanceof thetestedproducts.

A10%deviationfromthenon-irradiatedconsistencyisconsidered inthepresentstudyasafunctionalendpoint.Consistencycanbe ex-pressedaswellinNLGIgrades,rangingfrom000(soft)to6(hard).The NLGIgradeisarelevantparameterforthegreasespecificapplication. Adeviationofonegradefromtheoriginalconsistencygrade approx-imatelycorrespondstoa10%consistencyvariation.Table5contains thethresholdsassociatedtothis functionalendpoint. Thresholdsare expressedintermsoftotalabsorbeddoseandintermsofexposureto thetotalneutronandgammafluenceinthemixedirradiationfieldof thefacility.

Aseverestructuralendpointisassociatedtotheradiation-induced ﬂuidizationofthegrease.Itcorresponds toconsistencyvaluesbeing outofthemaximumpenetrometerrange.Thisendpointisevidenced bydashedlinesinFig.8andmarksanendof lifeconditionforthe irradiatedgrease.

The deﬁned endpoints provide useful dose and particle ﬂuence thresholdstocomparetheproductsradiation-resistance.PetamoGHY

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133NandRG-42R-1areidentiﬁedasthemostradiationresistant prod-uctscomparatively.Infact,theirconsistencydeviationfromtheoriginal valueislowerthan10%foralltheinvestigateddosevalues.Forthese products,theconsistencyvariationatthemaximumabsorbeddoseis reportedin Table 5. Fluorinatedproducts, despitethelower amount ofneutrondoseabsorbedincomparisonwithhydrogenatedones,are notrecommendedforapplicationinradioactiveenvironments.Infact, theacidgasesevolvedduringirradiationcoulddamagethesurrounding metalliccomponents.

4.4. Comparisonwithproducers’declarations

Consistencyresultsobtainedinthepresentworkcanbecompared withtheradiationresistancedeclarationsoftheproducers.Theresults achievedonGrizzlygreaseNo.1areparticularlyintriguing.Its consis-tencyabruptlyincreasesat0.4 MGyofmixedneutronandgammadose. Thisvalueismuchlowerthanthe1.2 MGygammadosethreshold de-claredbytheproducer.Theequivalenceoftheeffectsofgammadose andneutrondoseinpolymershasbeenlonglybelieved.However,the differencesreportedforGrizzlygreaseNo.1couldbeattributedtothe differentirradiationconditionadoptedinthetests,inparticulartheuse ofpuregammaormixedradiationfields.

Inaddition,itisinterestingtonotethatthetwomoststablegreases have diﬀerent producer declarations concerning the radiation resis-tance.RG-42R-1isdeclaredbytheproducerasradiationresistantup to15 MGyofgammadose,whilePetamoGHY133Nhasnoradiation resistancedeclaration.

Itcanbeconcludedthattheproductslackingaradiationresistance declaration do not necessarily featurescarce radiationresistance. In fact, only a limited number of companies in the world tested their productswithradiation.Asherereported,somegreasesfeaturing excel-lentperformancebutnottestedwithradiationshowexcellentradiation stability.Onthecontrary,productshavingadeclareddosethreshold donotnecessarilyhaveacomparableradiationresistancewhen irradi-atedatsamedosesindifferentradiationfields.Thisevidencesupports theneedforfurthertestingonspecificproducts,especially whenitis neededtoknowtheirradiationresistanceinspecificirradiation condi-tions.Theproducer’sdeclarationbasedongammairradiationcannot beconsideredassatisfactoryfortheuseofaproductinadifferent radi-ationfield.

4.5. Predictiveabilityoftheresults

Inthepresentstudy,greasesareirradiatedinaneutronandgamma mixedreactorfield.Thechosenirradiationconditionsaresignificantfor applicationsinfacilitiesthatwillproducesimilarradiationfields,asthe SPESandtheESSsystems.However,theradiationfieldsinoperation willdifferfromtheoneusedinthepresentstudybyseveralparameters. Firstly,thedoserateexpectedinoperationwillbeingeneralordersof magnitudelowerthantheoneusedfortestinginthereactor.Testsin thereactorarenecessarilyaccelerated.Greaseshavebeenirradiatedin reactoruptoseveralMGyoftotaldose,correspondingtoneutronand gammafluencesoftheorderofmagnitudeof1017_particle_cm−2_._Similar exposureswillbecumulatedinmuchlongeroperationtimes,ranging from weekstodecadesin theSPES andin theESSfacilities.Testing conditionscannoteasilyreplicatesolongexposuretimes,especiallyin caseofextensiveexperimentalcampaignswhichcountseveralproducts irradiatedatvariousdoselevels.Tocompleteirradiationina reason-abletime,quicker radiationdamagetests,referredtoasaccelerated, arenecessary.

Moreover,greasesaretestedinairatmosphereatatmospheric pres-sure,whileinsomeapplicationstheywillworkinvacuumorinabsence ofoxygen.Thetemperatureintheirradiationfacilityishigherthatthe temperatureexpectedformostapplications.Theexpectedneutron spec-truminoperationwilldiﬀerfromthereactorone.Neutronswithhigher

energywillbeproducedbyspallationneutrontargetsandbyfacilities fortheproductionofradioactiveionbeams.

Damagemechanismsinpolymersareexpectedtodependonseveral parametersastheradiationfields,thedoserate,theoxygendiffusion inthematerialandthetemperature,inasynergisticway.Forthis rea-son,thepresentresultscannotbeusedtoexactlypredicttheexpected greasebehaviourinspecificoperatingconditions.Nevertheless,the con-sistencybehaviourofseveraldifferentcommercialgreasesirradiatedin thesameconditionsinaneutronandgammamixedfieldoveratotal absorbeddoseofabout10 MGyareherereported.Thesetofcollected dataisuniqueinthepresentliterature.

5. Summaryandconclusions

Thepresentworkapproachestheproblemof radiationeffectson lubricatinggreases from an experimentalviewpoint. A methodology forgreaseirradiationin aneutronandgammamixedfieldand post-irradiation examination is developed. Nine commercially available productsareselectedfromsomeofthemostimportantproducersinthe world.Greasesamplesareirradiatedinanin-corefacilityofaresearch nuclearreactoratabsorbeddosesrangingbetween0.1 MGyand9 MGy. Dosimetryintheneutronandgammamixedfieldiscalculated. Consis-tencyis testedafterirradiation asthemostsignificantfunctional pa-rametercharacterizingagrease.Extremeradiation-inducedconsistency modificationsareobservedforsevenoutof theninetestedproducts, leadingtothegreasefluidization.Twooutoftheninetestedproducts exhibitstableconsistencyasafunctionofthedose.

Itcanbeconcludedthatthecleavageofthegreasestructureisthe dominantprocessinthechosenirradiationconditions.Thisisevidenced bythegreaseﬂuidizationandbytheevolutionofgas,whichdepends onthethickenerdisruptionandontheproductionoffragments respec-tively.

Someofthegreaseshavingbaseoilswithasimilarchemical com-positionshowverydiﬀerentradiationeﬀects.Forthisreason,itisnot possibletodirectlycorrelatethechemicalstructureofgreaseswiththeir radiationresistance.Thepresentstudycontributestoprovide experi-mentalevidencesofcleavageasthedominantprocessinducedby radi-ationonthegreasestructureuptothetestedvaluesofabsorbeddose. However,thecollecteddataevidencetheneedforafurther understand-ingoftheinteractionbetweenradiationsandthecomplexmulti-phase chemicalstructureofgreases.

Endoflifeusabilityconditionsarepresentedtoproposeradiation thresholdsforthe greases.Twogreases arecomparatively evaluated as the most radiation resistant ones. Someof the results allow the commonlyuseddefinitionsofradiation-resistancetobequestioned.In fact,someofthecommercialproductswhicharedeclaredbythe pro-ducerasradiationresistant,show extremedegradationinthepresent study.Theseproducerdeclarationsrefertogammaradiationonly.The observeddifferencesareprobablyduetothedifferentirradiation con-ditions,whichinfluencetheradiationdamage.Forthisreason,gamma radiationdatashouldnotbeusedtodrawgeneralconclusionsonthe radiationresistanceofgreasesandtodefineuniversallyaccepteddose thresholds.Inaddition, thepresentedresults challengethegenerally believedassumptionclaimingthatequaldosesofdifferent radiations areequallyeffectiveindamagingpolymers.Theuseofmixedradiation fieldsisnecessarytomoreaccuratelytesttheresistanceofpolymeric materialsthatmustworkinthepresenceofmixedradiations.

Theresultscollectedinthispaperrepresentasetofdataachieved inirradiationconditionsthatdifferfromtheonescommonlyemployed totestpolymers.Thedevelopedmethodologyheredescribedallowsthe radiationresistanceofthetestedgreasestobecomparativelyevaluated. Apartfromthescientificsignificance,thedatacollectedinmixed neu-tronandgammaradiationareparticularlyrelevantfornewgeneration facilitiesinwhichtheusedgreasesareexposedtoamixedneutronand gammaenvironment.Discussionswithsomeofthegreaseprovidersare ongoingaswellfor adeeperinvestigation andunderstandingof the

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basicmechanismsthatdeterminetheobservedphenomena.Chemical analysesonselectedirradiatedgreaseswillbeperformedforadeeper understandingoftheexperimentalresultspresentedinthispaper.

Declarations

Authorcontributionstatement

M.Ferrari:Conceivedanddesignedtheexperiments;Performedthe experiments;Analyzedandinterpretedthedata;Contributedreagents, materials,analysistoolsordata;Wrotethepaper.

A. Zenoni,Y.Lee: Conceivedanddesignedtheexperiments; Ana-lyzedandinterpretedthedata;Contributedreagents,materials,analysis toolsordata.

M.Hartl:Analyzedandinterpretedthedata.

A. Andrighetto: Conceived and designed the experiments; Con-tributedreagents,materials,analysistoolsordata.

A.Monetti:Conceivedanddesignedtheexperiments;Contributed reagents,materials,analysistoolsordata.

A.Salvini:Contributedreagents,materials,analysistoolsordata. F.Zelaschi:Conceivedanddesignedtheexperiments;Performedthe experiments;Analyzedandinterpretedthedata;Contributedreagents, materials,analysistoolsordata.

Fundingstatement

Thisresearchdidnotreceiveanyspeciﬁcgrantfromfunding agen-ciesinthepublic,commercial,ornot-for-proﬁtsectors.

Competingintereststatement

Theauthorsdeclarenoconﬂictofinterest.

Additionalinformation

Noadditionalinformationisavailableforthispaper.

Dataavailability

Therawdatarequiredtoreproducetheseﬁndingscannotbeshared atthistimeasthedataalsoformspartofanongoingstudy.The pro-cesseddatarequiredtoreproduce theseﬁndingscannotbesharedat thistimeasthedataalsoformspartofanongoingstudy.

Acknowledgements

Theauthorsaregratefulforthecontributiontothestudygivenby Prof.S.PandiniandbyDr.C.ReminooftheUniversityofBrescia.The authorsthankDr.F.BorgnaoftheSPESprojectofINFNforassistance withelementalanalysisperformedongreasesandDr.F.D’Agostiniof theSPESprojectofINFNforhelpinginpurchasingsomeoftheproducts.

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