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Journal of Hazardous Materials
j ou rn a l h om epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t
Research paper
Analysis of contaminated nuclear plant steel by laser-induced breakdown spectroscopy
Adam Lang
a,b, Dirk Engelberg
b, Nicholas T. Smith
c,d,e, Divyesh Trivedi
c,e,
Owen Horsfall
c,e, Anthony Banford
c,e, Philip A. Martin
e, Paul Coffey
e, William R. Bower
a, Clemens Walther
f, Martin Weiß
f, Hauke Bosco
f, Alex Jenkins
g, Gareth T.W. Law
a,d,∗aCentreforRadiochemistryResearch,SchoolofChemistry,TheUniversityofManchester,OxfordRoad,Manchester,M139PL,UnitedKingdom
bCorrosionandProtectionCentre,MaterialsPerformanceCentre,SchoolofMaterials,TheUniversityofManchester,OxfordRoad,Manchester,M139PL, UnitedKingdom
cNationalNuclearLaboratory,ChadwickHouse,WarringtonRoad,BirchwoodPark,Warrington,WA36AE,UnitedKingdom
dReseachCentreforRadwasteDisposal,SchoolofEarthandEnvironmentalSciences,TheUniversityofManchester,OxfordRoad,Manchester,M139PL, UnitedKingdom
eSchoolofChemicalEngineeringandAnalyticalSciences,TheUniversityofManchester,OxfordRoad,Manchester,M139PL,UnitedKingdom
fInstituteforRadioecologyandRadiationProtection,Leibniz-UniversitätHannover,HerrenhäuserStraße2,D-30419Hannover,Germany
gDecontaminationCentreofExpertise,SellafieldLtd.,Sellafield,Cumbria,CA201PG,UnitedKingdom
h i g h l i g h t s
•StandoffLIBSanalysisofSrandCscontaminatednuclearplantsteelisdemonstratedatmillimetredistances.
•StandoffLIBShasthepotentialtoallowsurveyingofcontaminationatmuchlargerdistances(metres).
•MultipulseLIBScanalsoprovidedepthresolvedinformationoncontaminantdistributioninsteel.
a r t i c l e i n f o
Articlehistory:
Received24April2017
Receivedinrevisedform30October2017 Accepted31October2017
Availableonline7November2017
Keywords:
LIBS
Contaminatedstainlesssteel Decommissioning Strontium Cesium
a b s t r a c t
LaserInducedBreakdownSpectroscopy(LIBS)hasthepotentialtoallowdirect,standoffmeasurement ofcontaminantsonnuclearplant.Here,LIBSisevaluatedasananalyticaltoolformeasurementofSr andCscontaminationontype304stainlesssteelsurfaces.Sampleswerereactedinmodelacidic(PUREX reprocessing)andalkaline(spentfuelponds)SrandCsbearingliquors,withLIBSmulti-pulseablation alsoexploredtomeasurecontaminantpenetration.TheSrII(407.77nm)andCsI(894.35nm)emission linescouldbeseparatedfromthebulkemissionspectra,thoughonlySrcouldbereliablydetectedat surfaceloadings>0.5mgcm−2.DepthprofilingshoweddecayoftheSrsignalwithtime,butimportantly, elementalanalysisindicatedthatmaterialexpelledfromLIBScratersisredistributedandmayinterfere inlaterlasershotanalyses.
©2017TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
LaserInducedBreakdownSpectroscopy(LIBS)isaquasi-non- destructivetechniquethatoperatesonthefundamentalprinciple oftheablationofasmallamountofsamplebyalaserpulsethat is focusedonto the surface. Theablated materialthen forms a plasmaofexcitedatoms,ions,andfreeelectronsthat,oncooling, emitsradiationatcharacteristicwavelengthsdependentuponthe
∗Correspondingauthor.
E-mailaddress:gareth.law@manchester.ac.uk(G.T.W.Law).
elementalcompositionofthesample.Increasingly,LIBSisbeing recognisedasapromisingtechniqueforelementalanalysis,includ- inginindustry[1].Examplesofuseinclude:elementalanalysis inspaceexplorationprograms[2],qualitycontroluseinpharma- ceuticals[3],andforensicandarchaeologicalsampleanalysis[4].
ThisdiverserangeofLIBSapplicationsislargelydrivenbythetech- nique’scapabilitytoperformfast,multi-elementalanalysisofsolids [5],liquids[6],andgases[7],withvirtuallynosamplepreparation, andwithlowppmsensitivity.
LIBSisalsousedinthenuclearindustryasthetechniqueper- mitsstandoffanalysisofradioactivesamplesatmillimetreto10’s ofmetresofdistance.Therein,a rapidlyemergingapplicationis https://doi.org/10.1016/j.jhazmat.2017.10.064
0304-3894/©2017TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
decontaminate.Itisthereforehighlydesirabletodevelopanalytical techniquesthatpermitrapid,standoffassessmentofcontaminated plantcomponentsinordertolimithumanexposureandtoallow evaluationofmaterialsneedingdecontaminationand/orsentenc- ingwhilstalsominimising dosetoon-siteworkersastheycan operateLIBSequipmentawayfromthematerialbeinganalysed.
LIBS presents an attractive option to meet this task as it only requiresopticalaccesstosamples.Accordingly,analysismaybe completedinradiologicallyandchemicallyhazardousareassuch ashotcellswithoutincurringanadditionaldosepenalty.Thisis insharpcontrasttowettechniquescurrentlyemployedforthe quantitativeanalysisofnuclearmaterials[16,17]whichbycom- parisonrequirecomparativelylonganalysistimes.Further,these methods areoften commerciallyexpensiveand hence not eco- nomicallysuitedfortheanalysisofindustrialvolumesofmaterial.
SuchissuesareovercomewithLIBS,whichcanperformreal-time quantitativeanalysisofmultipleelementswithminimalrestric- tionsonsamplecondition.AnadditionalpossibilityofLIBSisits potentialtoperformhigh-resolutiondepthprofilingofmaterials bymulti-pulselaserexcitation[1].Inturn,thiscouldyielduseful informationofcontaminantpenetrationintonuclearplantsteels orothercontaminatedmaterials.
ForsuccessfulapplicationofLIBStonuclearplantsteels,thedis- criminationofanalyteemissionlinesfromthesupportingmatrix spectrummustbepossible.Thelargenumberofalloyingelements typicallypresentinsteelreducesthisprobabilityofsatisfyingthis prerequisitecondition.However,inthiscontributionweevaluate theabilityofLIBStoreliablymeasurestrontiumandcesiumcon- taminationinAISIType304stainlesssteel,acommonconstruction materialusedthroughoutnuclearreprocessingfacilitiesowingto itsexcellentcorrosionresistanceproperties[18].Theselectionof thesetwoelementsreflectsthedominantcontributionofthefis- sionproducts90Sr(t1/2=28.8yrs)and137Cs(t1/2=30.2yrs)tothe totalinitial(<200years)activityofspentfuel(andresultingspent fuelreprocessingliquorsandstoragepondwaters)afterremoval fromreactors[19].Inaddition,wealsoassessthepotentialofLIBS tobeusedasadepthprofilingtechniqueforSrandCspenetration intosteelsusingLIBSmulti-pulseanalysisoverasingleposition,as previouslydocumentedforothermaterials[20,21].
2. Experimental 2.1. Steelsamples
ThechemicalcompositionoftheAISIType304stainlesssteel usedinexperiments isprovided inTable1.Steelcoupons with dimensionsof10×10×13mm(l×w×h)weremechanicallypol- ishedto4000gritusingSiCpaperonasingle10×10mmface.This
Contaminationexperimentsweremaintainedat60◦Cfor30days;
thereafter thecoupons wereremoved andwashed briefly with deionised water and then isopropyl alcohol (IPA) prior toLIBS analysis.Solutionanalysis(InductivelyCoupledPlasmaMassSpec- trometry)revealedthatthetotalamountofstrontiumdeposited ontothesteelsurfacesafteracidicandalkalinecontaminationwas 0.48and1.10mgcm−2,respectively.Incomparison,cesiumaccu- mulationwas0.51and0.56mgcm−2,respectively.
2.2. LIBSanalysis
AQ-switchedNd:YAGpulsedlaser(10HzEKSPLANanosecond E/OlasermodelNL301G-10;fundamentalwavelengthof1064nm) wasusedinthisstudy.Aschemeoftheinstrumentisprovidedinthe SupportingInformation.Thelaserpulseenergywassetto100mJ,a startingenergythatwasproventobeuseful,andwaslateradjusted asrequired.Thetemporallaserbeamwidthwas3–6ns,andthe repetitionratebetweensuccessivelaserpulseswasfixedat10Hz.
Thelightemittedfromthecoolingplasmawascollectedbyaplano- convexquartzlenswithfocallengthof75mmintotheentranceslit ofa0.5mfocallengthFiberOpticSpectrometer(AvaSpec-2048- USB2-APL27),equippedwithindexablegratingsof1200,2400,and 3600groovesmm−1,respectively.Thedispersedplasmalightwas detectedusingachargecoupleddevice(CCD)detector(2048pixel) withcontrolstoenablesettingthedelayandintegrationtime.An emissionspectrumwithintheregion185–904nmwasgenerated fromasinglelasershot,whichpermittedidentificationofthenec- essaryelementsthroughtheiruniquespectralfeatures.Fullwidth athalfmaximumforthespectrometerwasaminimumof0.03nm acrossthespectralrange.Thegate-widthwas1.1msandthedelay timeforanalysiswasfixedattheminimumvalueof1.27s,astan- dardtimedelay.Detectorandplasmaparametersarepresentedin theSupportingInformation.
2.3. Glowdischargeopticalemissionspectroscopydepthanalysis
ElementaldepthprofileanalysiswasconductedwithGD-OES (Glow Discharge Optical Emission Spectroscopy), using a GD- Profiler2 (HoribaJobinYvon)atanappliedpowerof 35Wand an Ar gas pressure of 635Pa. A 4mm diameter Cu anode was used.Theelementalemissionlinesusedwere371.999nmforFe, 425.439nmforCr,341.482nmforNi,130.223nmforO,460.739nm forSr,455.329nmforCs,and396.157nmforAl.Theselineswere detectedusingapolychromatorfocallengthof500mmandwith 30opticalwindows.Thephotondetectorwascalibratedusinga 1mm Alsheet.Inorder tomaintainthevacuumsealattheO- ring-sampleinterface,larger,analoguesteelcouponsofdimensions 20mm×20mm×13mmthicknesswereusedforGD-OESanalysis.
Fig.1. LIBSemissionspectrafor‘asreceived’AISIType304stainlesssteelunderair(A)andargon(B)atmospheres.
Fig.2.ComparisonofLIBSemissionspectraintheregionsofinterestfor(A)Sr,and(B)Csdetection.ThearrowshowsthepositionoftheSrII407.77nmline.Measurement wasperformedunderopenairconditionsatalaseroutputenergyof100mJ.Thespectrain(A)werenormalisedto1foreaseofcomparison.
2.4. ElectronmicroscopyandenergydispersiveX-rayelemental analysis
Todeterminethemorphologyofthesteelsurfaceaftertheabla- tionprocess,SEManalysiswasundertakenontheresultingcraters thatformeduponlaserimpact.Electronmicroscopyimageswere obtainedusingaFEIXL30200(E)SEM-FEGmicroscopeatanaccel- eratingvoltageof15keVunderhighvacuum,utilisingeitheran Everhart-Thornleydetector(ETD)oraCircularBackscatterdetector (CBS).AnEDAXGemeniEDSSpectrometerwasusedtodetermine thedistributionofmajorelements(Fe,Cr,Ni,O)onthesteelsurface ataspotsizeofapproximately1m.
2.5. Laserconfocalmicroscopyanalysis
ThemorphologyanddepthofcratersthatresultedfromLIBS analysisofsteelsurfaceswereanalysedwithaKeyenceVK-X200K 3Dlaserconfocalmicroscopeat200xmagnificationusinga0.5m stepsizeandsuperfineresolution(2048×1536pixels).Thearea analysedbythelaserconfocalmicroscopyoftheLIBScraterwas approximately2.0mmx1.5mm.
2.6. Timeofflightsecondaryionmassspectrometry
ElementaldistributionoftheLIBScrater(includingSrandCs) wasanalysedusingaTOF-SIMSinstrument(IONTOFGmbH,Mün- ster,Germany)ofthereflectron-type.Thesystemwasequipped witha 30kV Bi/Mnliquid metaliongun(LMIG)astheprimary ionsourceandwasoperatedatanemissioncurrentof0.8A.The pulsewidthofthebunchedprimaryionpulsewassetto30nsat a100scycletime,whichyieldedamassresolutionofsecondary ions>8000amu.Priortoanalysis,insitusputtercleaningofthe surfacewasundertakenwithanargongasclusterionbeam(GCIB) inordertoremovesurfaceboundhydrocarboncontamination.The scanningareaoftheLIBScraterwassetto500m×500m,the maximumareasizeforhighresolutionimaging.Themajorisotopes oftheelementsofinterestwereselectedforanalysis.
3. Resultsanddiscussion 3.1. Strontiumidentification
Initially,strontiumandcesiumemissionlineswereidentified within theLIBS spectrum of contaminated coupons. A fraction of theunderlyingstainless steel supportmatrix is also ablated in tandemwith thetarget elements, and therefore linescorre-
Fig.3.ComparisonofLIBSdepthprofilesmeasuredinanargonenvironmentforacidicandalkalinecontaminatedstainlesssteelat100mJand50mJlaserenergy.TheSr signalin(C)wasscaledbyafactorof0.1forclarity.
spondingto elementsinthe substratewill alsobeobserved in theemissionspectrum.Forthisreasonthepossibilityofspectral interferencemustbeconsideredwhenselectinganalytelinesfor positiveelementalidentification.IncontrasttomanypreviousLIBS investigationsofnuclearmaterials,the304stainlesssteelmatrix comprisesalargenumberofalloyingelementsandthusityields acomplexLIBSspectrum,irrespectiveofthesurroundingatmo- sphere,asshowninFig.1.Thenumberofstrontiumandcesium spectralfeaturesthatarefreeofinterferencecanbeexpectedtobe lowwithinthespectrumofstainlesssteelduetoitsmulti-element composition.Accordingly,thechoiceofelementallineforpositive detectionmustbecarefullyconsideredasthoseemployedforother materialsmaynotbesuitableonthesteelmatrix.
Previous research has used 407.77nm for Sr measurement (407.77nmrepresentsSrII;2S1/2→2P3/2)inLIBSexperimentation [2,8,23,24],andinaccordancethisemissionlinewasfirsttestedin thiswork.AsshowninFig.2A,a407.77nmspectrallineobservable inthecontaminatedmaterialisabsentinthereferenceuncontam- inatedspecimen,suggestingthatthisisthestrontiumlinethatis usefulformeasurements.Owingtothelargenumberofalloying elementspresentinthematrix,theLIBSspectraofstainlesssteel materialsareinherentlycomplex,subsequentlypeakassignment canoftenbea challenging task. Inconsideration of this,a LIBS spectrumofastandardstrontiumsalt(Sr(NO3)2)wasalsorecorded tosupportourassignmentasSrII407.77nm.Thenitratesaltwas deemedasanappropriatereferencestrontiummaterialasitwas
consideredthatnitrogenandoxygendonotcontributetothesignal ofinterest.Thiswasbasedontheomissionofthe407.77nmspec- tralfeatureintheopenairspectrumoftheuncontaminatedsteel material,despiteanabundanceofN2 andO2 inthesurrounding atmosphere.Hence,theobservationofthislineinthesaltspectrum validatesourdesignationasSrII407.77nmandstronglysuggests thatthesteelalloyingelementsalsodonotcontributetothesignal.
Therefore,theSrII407.77nmlinecanbesatisfactorilyutilisedfor thepositiveidentificationofstrontiumwithinthesteelmatrix.
A comparison ofthe normalisedintensitiesof theSrlinein thecontaminatedLIBSspectrarevealsuptaketobesignificantly enhancedinthealkalinecontaminationmatrix,consistentwiththe ICP-MSsolutiondata.Thisisalsoinaccordancewithpreviouscon- taminationstudiesthatreportedanincreasingaffinityformetal ionsorptionontosteelsurfacesinalkalinemedia[25,26],although noconsensusofthemechanismsinvolvedwasprovided.Inorder toquantifytheobservedpHdependenceonstrontiumdeposition, constructionofcalibrationcurvesusingstandardsampleswould benecessarytoconvertsignalintensityintoconcentration[23,27], whichisbeyondthescopeofthiswork.
3.2. Cesiumidentification
ThelackofLIBSinvestigationsintheliteraturewithCsasthe target element haspreviously beenattributed toits poorlimit ofdetection[10],and asa resultstudieshave largelybeenlim-
Fig.4. GD-OESdepthprofilesof304stainlesssteelcontaminatedin3MHNO3and1mMNaOH.ForclaritytheSrsignalshavebeenscaledbyafactorof100fortheacidic andalkalinesystems.
itedtotheanalysisofsampleswhereCs isa majorconstituent ofthesubstrate[27,28].Here,theprominentCsI852.11nmline wasalmostexclusivelyselectedforquantificationbutinthisstudy interferencewiththeatmospheric(air)ArIlineat852.14nmwas observed[29](SupportingInformationFig.S3),necessitatingthe selectionofanothercandidateline.TheprominentCsI455.53nm and459.32nmlines[30]werediscardedforidentificationpurposes onsimilargroundsasspectralinterferencewiththeCrI455.48[31]
andFeI459.52nm[32]matrixlineswereobserved(Fig.1).TheCs I894.35nmline(2S1/2→2P1/2)wasconsideredasanappropriate alternativeinthisworkduetoitsapplicabilityforanumberofsup- portingmatricesthathavepreviouslybeentested[30].Asseenin Fig.2BtheLIBSemissionspectrumofcontaminated304includesa linecentredat894.33nmthatisnotpresentintheuncontaminated spectrum.Toconfirmthis,wecompletedcomplimentaryanalysis ofCsNO3,andfoundthattheCsI894.35nmlinewasobserved.This line,however,onlybecamevisiblewhenthesolutionconcentra- tionswereincreasedbyonetotwoordersofmagnitudeinboththe alkalineandacidicsystems,respectively.Nodetectablequantities ofcesiumcouldbefoundonthesteelsurfaceusingcontamination solutionconditionsrepresentativeofconditionsfoundatnuclear reprocessingfacilities.Hence,thesensitivityofLIBSwithregards toCsdetectionwasnotsatisfactoryinourstudy[10].
3.3. Chemicaldepthprofiling
Afundamentalchallengeassociatedwithdepthprofilingtech- niquesis thecorrelationof analyteremoval ratewithdepthof analysis. LIBS is no exception to this, and currently no estab- lishedmethodfordepthquantificationexists.Arelativelysimple calibrationprocedureemployedinpreviousstudiesinvolvedcal- culationoftheaverageablationratebydividingthethicknessofa standardmaterialbythecorrespondingnumberofshotsrequired toablatefromtoptobottom[21,33].Subsequentmeasurement ofcrater depthbyelectron microscopyand stylusprofilometry demonstratedthisapproachtobedubious,bytheobservationof anobviousnon-linearablationrate[34].Adecreasingablationrate atgreater depths hassubsequently beenattributedto stronger absorptionofthelaserpulsebytheplasmaasitpenetratesfurther intothematerial,reducingitscapacitytoremovematerialupon contactwiththecraterbottom[35].Addingtothesedifficulties isapparentirregularcratermorphologywherethecraterbottom
typicallydoesnothavea flatsurface,thusmakingitdifficultto accuratelydeterminedepthbydirectmeasurement[36].
TheprocedureusedtoobtainLIBSchemicaldepthinformation inthisworkisdescribedasfollows.Thespontaneousformationof apassivesurfacechromiumoxidelayeronstainlesssteelmateri- alsisexploitedinananalogousapproachtotheaforementioned studiesofcoatedsystems,usingoxygenasthereferenceelement confinedtothesurface.Usingthisapproach,thedecreasingoxygen signalintheinitialpulsescorrespondstotheablationofthepas- sivelayerandthuscanbemonitoredtoevaluatecontamination relativetothissurfacefilm.ToavoidcontributionoftheLIBSsignal byatmosphericoxygen,analysismustbeperformedunderaninert atmosphere.Manyoftheplasmapropertiesincludingsize,temper- atureandexpansionratearestronglyinfluencedbyitssurrounding atmosphereandthereforethechoiceofatmospheremustbecare- fullyselectedtobecomplimentary withthenatureofanalysis.
Argonisreportedtoyieldintensespectrallineswhilstsimultane- ouslypossessingalowsurfaceablationrate[37],attributedtoits lowthermalconductivitythatsubsequentlyheatstheplasmamore effectivelybyinverseBremsstrahlung.Theresultingplasmaalso actsasaprotectivebarrier,opticallyshieldingthesurfaceandthus reducingthevolumeofablation.Thecombinationofintensespec- trallinescorrespondingtosmallervolumesofablatedmaterialfor eachindividualshotaffordsimproveddepthresolutionforchemi- calanalysisandforthisreasonargonwaschosenastheatmosphere fordepthprofilinganalysisinthisstudy.
LIBSdepthprofilesof Srand Cscontaminated steelcoupons areshowninFig.3.ThecapabilityofLIBStoreliablyperformCs depthcharacterisationwasnotconsideredowingtothechallenges involvingpositive detection under conditions representative of nuclearreprocessingfacilities.Toovercomepooraccuracy,nor- malizationofanalytesignalstotheFeI404.56nmmatrixlinewas performed;acommonpracticeinelementalanalysis[1,2].Inorder todemonstrate thevalidity ofthis approach, theCr Iemission lineat425.39nmwasalsomonitored,where therelativelysta- blesignalobservedsuggeststhattheassumptionofauniformFe distributionwithinthematrixisreasonable.Operatingatalaser energyof100mJshot−1revealsamaximumstrontiumconcentra- tionwithintheinitialpulses.However,strongfluctuationinthe strontiumsignalforthealkalinesystem,wherecontaminationis muchmorepronounced,makesitunfeasibletoaccuratelyevalu- atestrontiumcontaminationdepth.Thisobservationismuchless pronouncedforalaserenergyof50mJshot−1,suggestingittobe
Fig.5. SEMimageandSEM-EDXelementalmapsofLIBScratersformedonthehighpHcontaminatedsteelsurfaceafteradifferentnumberof100mJshotsunderargon.
aconsequenceofmatrixeffectsintrinsictotheLIBSexperimental configuration,ratherthanacorrectreflectionofstrontiumcontam- inationbehaviour.Inaddition,inconsistentoxygendepthprofiles werealsoobservedbetweentheacidicandalkalinesystemsathigh laserfluence,wherethesignalwassignificantlydiminishedforthe highpHspecimen.Thiseffectwasnotobservedatlowerlaserflu- ence.Apossibleexplanationfortheapparentsignificanceofthe laserparametersisaddressedlater.Measuringatareducedlaser outputitalsobecomesapparentthatoxygenpersistsforatleastan equalnumberofshotsasthecorrespondingstrontiumsignal.This observationnowbecomespossibleonlybythesmootherdecline ofthestrontiumsignalintheassociateddepthprofiles.Fromthis resultit wouldseem thatstrontium ismaintainedwithinclose proximitytothesurface,anddoesnotdiffuseintothebulkmaterial.
Tocheckthevalidityofourproposedmethod,complimentary depth-resolvedanalysis ofthecontaminated steelcouponswas undertakenusingacommercialGD-OES.Theseparationofsample
removalandexcitationprocessesinglowdischargetechniquesis akeydistinctionfromLIBSthat,intandemwithastabledischarge, isresponsibleforthereductionofmatrixeffects.Hence,suitable algorithmsareavailableforthequantificationofglowdischarge emissionyields[38]andfortheconversionofsputteringrateto depthinformation[39].Asaresult,GD-OEStodateremainsamore universaltechniqueforelementaldepthanalysis,asdemonstrated bythewidespreadinvestigationsreportedinvolvingsteelmaterials [40–43].
TheGD-OESdepthprofilesofthesteelcouponsaftercontam- inationareshowninFig.4.Bymonitoringtheoxygensignal,the interfacepositionwaslocatedafterasputteringtimeofapprox- imately 0.20and 0.30sfor thelow andhighpHcontaminating solutions, respectively.In both instances,strontium enrichment wasobservedwithintheCr2O3 passivelayer,asindicatedbythe alignmentofthestrontium,chromium,andoxygensignalmaxima.
Thisfinding is consistentwiththeLIBS result in thatcontami-
Fig.6.(A)SEMmorphologyand(B)confocalmicroscopyanalysisofacraterformedonanalkalinecontaminatedsteelsamplefromasingle100mJlasershotunderanargon atmosphere.TOF-SIMSelementalanalysisofthehighlightedregionin(B)fortheelements(C)Fe,(D)Cr,(E)Ni,(F)O,(G)Sr,and(H)Cs.
nationdiffusioniseffectively inhibitedbythepassivelayer.An importantdistinguishingfeaturehoweveristheimproveddepth resolutionaffordedbytheGD-OEStechnique,whichiscapableof discriminatingbetween surfacecomplexationand passivelayer embedment. This is a clear advantage over LIBS which in this study,isonlysufficienttoclassifycontaminationasasurfacephe- nomenon. Furthermore, noCs signalcould be detected during GD-OEScharacterisation,irrespectiveofcontaminationconditions.
Thisresultfurtheremphasisesthatopticalemissionbasedspec- troscopic techniquesarecurrently notsensitiveenough forthe analysisofcesium[10].
3.4. Crateranalysis
Theanalysisofmajorelementdistribution(Fe,Cr,O,Ni)around LIBScraters,asafunctionoftotalnumberoflasershotsdelivered,
isshowninFig.5.Inallcases,thecratersproducedwereapproxi- mately1mmindiameter.ThestudywascarriedoutusinghighpH contaminatedsteelsamplesusing100mJshotsinanargonmedium becausefluctuationofthestrontiumsignalwasmostevidentunder theseconditions.Itcanclearlybeseenthatwithanincreasingnum- berofshots,adistinctcratergeometrydevelopsinwhichdiscrete concentricringsofnon-uniformelemental compositionform.A prominentfeatureistheincreasingoxygenconcentrationatthe craterrimwithshotnumber,whichmaybeattributedtomate- rialbeingexpelledfromthecraterandaccumulatingaroundthe perimeterasasolidifiedoxidemelt.
Theabsenceofthemajorsteelalloyingelementsinthemelt howeversuggeststhatthenatureofthislaser-matterinteractionis notdominatedbytherelativeconcentrationsofthematrixele- mentswithin theplasma plume. Taking intoconsideration the irregularSrandO depthprofiles recordedathighlaserfluence
excludesthelikelihoodofstrontiumbeingdepositedasSrO,where theexactchemicalcompositionofthedebrisissubsequentlynot clear.Regardless,thepresence ofsettledstrontium and oxygen materialonthespecimensurfacemaysubsequentlybere-analysed intandemwithablationofdeeperregions,producingaresidualtail- ingoftheanalytesignalsthatinturnleadstoacompromiseddepth resolution.Thispossibilityhasbeenconsideredpreviouslyinstud- iesthatreportedanalogouscratermorphologiesonmetallicalloys [44]whereinoneinstancedirectobservationwaspossiblebyelec- tronprobemicroanalysis[45].Furthermore,intheaforementioned studiestheseverityofthetailingeffectwasoftenexacerbatedby alterationstoexperimentalparametersthatincreaselaserfluence.
Thisincludestighteningthebeamfocusandreducingworkingdis- tance,whichinturnyieldedmoreirregularcraterprofiles.
ItisnotthepurposeofthispapertoexplainLIBScratermorphol- ogyoritsformationindetail,whichhasbeendescribedelsewhere inpublishedliterature[46].Ofnote,cesiumcouldalsobedetected withTOF-SIMS(Fig.6)andherethestrontiumandcesiumspa- tialdistributionprofileswerealsoinconsistent,indicatingthatthe redistributionbehaviourisuniquetoeachindividualelementand thereforecannotreliablybeinferredfromthesurfaceresponseof anothermaterial.Amorecomprehensivestudyofcratermorphol- ogyandevolution isclearly necessarytobetterunderstandthe surface-laserinteractionthatisfundamentaltoLIBSasacharac- terisationtechnique.Thisundertakingisparticularlyprudentfor applicationsinnucleardecommissioning,sincethisanalysistech- niquecouldpotentiallyreintroduceradioactivematerialontothe materialsurface.
Inaddition,ithaspreviouslybeensuggestedthatlaserirradi- ancetreatmentsmayalsodeterioratethecorrosionresistanceof austeniticstainlesssteels[47].Thisbehaviourhasbeenattributed toalocaliseddiscontinuityofthepassivelayerundertheheating effectofthelaserathighfluence[48].Ifsimilarcorrosionphenom- enaareinitiatedduringtheLIBSmeasurementthiswillpresenta seriousproblemforinsitucharacterisationasareducedcorrosion performancewillincreasethelikelihoodofmaterialfailure.Fur- therinvestigationisthereforerequiredtoevaluatetheeffectofthe ablationprocessonthecorrosionresistanceofstainlesssteels.
4. Conclusions
WehaveshownthatLIBScanbeusedtoidentifySrandCscon- taminationon304L austeniticstainless steel,a materialwidely usedin thenuclearwaste storageand reprocessingoperations.
Here,theSrII(407.77nm)andCsI(894.35nm)emissionlinescan beseparatedfromtheoverallsteelemissionspectrum.However, whilstLIBScanreliablymeasureSrcontaminationonsteelatlevels representativeofthosefoundinthenuclearindustry(>0.5gcm−2),
This work was supported by the Sellafield Ltd. Decontami- nation and Effluents Centre of Expertise and the RCUK grants ST/N002474/1andNE/M014088/1.SmithisfundedbyaRoyalSoci- etyIndustryFellowship,andBanford,Horsfall,SmithandTrivedi acknowledgesupportfromNNL’sWasteManagementandDecom- missioningIR&DProgramme.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttps://doi.org/10.1016/j.jhazmat.2017.10.
064.
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