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JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725https://doi.
org/10.
1007/s10967-018-6316-0NitridefuelforGenIVnuclearpowersystemsChristianEkberg1·DiogoRibeiroCosta2,3·MarcusHedberg1·MikaelJolkkonen2Received:20October2018/Publishedonline:10November2018TheAuthor(s)2018AbstractNuclearenergyhasbeenapartoftheenergymixinmanycountriesfordecades.
Todayinprincipleallpowerproducingreactorsusethesametechniqe.
EitherPWRorBWRfuelledwithoxidefuels.
Thischoiceoffuelisnotselfevidentandtodaytherearesuggestionstochangetofuelswhichmaybesaferandmoreeconomicalandalsousedine.
g.
GenIVnuclearpowersystems.
Onesuchfueltypeisthenitrides.
Thenitrideshaveabetterthermalconductivitythantheoxidesandasimilarmeltingpointandarethushavelargersafetymarginstomeltingduringoperation.
Inadditiontheyarebetween30and40%moredensewithrespecttofissilematerial.
Drawbacksincludeinstabilitywithrespecttowaterandasometimescomplicatedfabricationroute.
TheformerisnotreallyanissuewithGenIVsystemsbutforuseinthepresentfleet.
Inthispaperwediscussbothproductionandrecyclingpotentialofnitridefuels.
KeywordsNuclearfuel·Nitridenuclearfuels·GenIV·Productionofnitrides·Nuclearfuelrecycling·DissolutionofnitridesIntroductionNuclearpoweristodayadisputedtechniquealthoughitisinprincipleCO2freeandhighlyenergetic.
However,thecur-rentnuclearsystemsareratherinefficient,onlyabout1%oftheinherentenergyisused,andthewasteneedtobestoredforabout100,000yearsbeforeitsradiotoxicityissimilartotheoriginallymineduranium.
InsomecountriessuchasFrancethereisareprocessingsystemusedwheretheplu-toniumisusedintherefabricatingofmixedoxidefuelthatisoncemoreusedinapowerproducingreactor.
Thisis,however,onlydoneonceincreasingtheenergyutilisationtoabout1.
2%butsavingabout17%offreshlyminedura-nium[1]Inthelast2decadesacompletecircularuseofthefuelmaterial,thesocalledGenIVnuclearpowersystemshaveemergedasapotentialsolutiontotheaforementionedissues.
ThefollowinggoalsforGenIVweredefinedprevi-ouslyintheGenerationIVInternationalForum(GIF)[2,3]:sustainability,safetyandreliability,economiccompetitive-ness,andproliferationresistanceandphysicalprotection.
Suchasystemcomprisefastreactors,separationfacilitiesforrecoveryofthestillusefulactinidesandafuelfabricationplanttocompletetherecycling.
Naturally,severalquestionsarisewhendiscussinganewpowerproductionsystemssuchaswhichcombinationofcoolant,claddingandfueltouseinthereactoraswellasaselectionofseparationsystemandfueltypeandfabricationroute.
Overtheyearstherehasbeendifferentinvestigationstofindtheoptimalnuclearfuel.
Initially,inthe1940s,uraniummetalwaschosenasapotentialcandidatemainlybecauseitshighthermalconductivityandfissiledensity.
However,itslowmeltingpoint,associatedwithitsphasetransforma-tionduringheating,chemicalinstability,andhighfissiongasswelling,becameadrawbacktoitsindustrialapplication.
Toworkaroundtheseproblems,themostpromisingalter-nativewastheceramicoxidefuel,specificallytheuraniumdioxide(UO2),whichhashighmeltingpoint,goodchemi-calstabilityandcompatibilitywiththefissionproducts,andreasonablethermalstabilityduringburn-up.
Themaindownside,otherwise,wasitsverylowthermalconductivity.
Paralleltouraniumdioxidedevelopment,manyresearchprojectshavebeencarriedoutinordertoconsiderotherfueloptions,suchasnitrides,carbidesandsilicides[4].
Among*ChristianEkbergche@chalmers.
se1NuclearChemistry/IndustrialMaterialsRecycling,Chalmers,Gteborg,Sweden2DivisionofNuclearEngineering,KungligaTekniskaHgskolan,Stockholm,Sweden3WestinghouseElectricSwedenAB,Vsters,Sweden1714JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725them,nitrideshasshownmanysuperiorqualitiesoverthestandardoxidefuelsuchas:Higherfissiledensity(40%moreuraniuminUNthaninUO2)[4]:leadingtohigherconversionratios,andpoten-tiallyhigherburn-ups.
Higherthermalconductivityreductionofthefuelcentre-linetemperature,decreaseintheenergystoredperunitmasswhileincreasingthemarginforfuelmelting[5,6],anddelaythemigrationoffissionproductsandactinides,whichpositivelyaffectsthefuelswelling[7].
Reprocessinggooddissolutioninnitricacid(HNO3),makingthisfuelcompatiblewiththePUREXprocess,whichusesHNO3fordissolutionofspentfuel[8].
Stabilitygoodchemicalcompatibilitywithmostpotentialcladdingmaterials,aswellasirradiationstability[4].
LongerfuelcycletimeowingtoneutronicbehaviourofUNinthecore[9],thefuelcyclecouldincreasefromthecommonlyapplied18months(standardUO2)toabout25months,basedonaburnupof50GWd/tU.
Thisincreaseleadstofewershutdownsforreloading,thusbeinganeconomicalbenefitfornitridefuelimplementa-tion[4].
Ontheotherhand,therearesomedrawbacksregardingthenitridefuels:Theproductionofminoractinide(orevenplutonium)containingnitridefuelisnotstraightforwardandrequiresomedifficultproductionsteps.
Oxidationresistance:thenitridepelletsreadilyoxi-dizesinsuperheatedsteam,withcompletedegradationobtainedwithin1hin0.
50barsteamat500°CforUN[10].
Theas-manufacturednitridepowderispyrophoric,whichmeansthatitreactsimmediatelywithoxygenfromair,forinstance.
Thus,theatmosphereduringhandlingtheUNpowdermustbeoxygen-free,whichrequiresadditionalimplementationforindustrialscaleproduc-tion[11].
Fuelcost:mainlyduetothefactthatthenitrogencom-ponenthastobehighlyenrichedin15Ntoincreasetheneutroneconomyandavoidthe(n,p)formationof14Cfrom14N[12].
Thisreactionalsoenhancestheamountofradioactivematerialinthespentfuel,whichisunde-sirable.
However,theadditionalcostsrelatedtonitro-genenrichmentareoffsetbyloweruraniumenrichmentrequirements,aswellasthereducednumberoffuelassembliesineachfuelreload,resultinginanestimatedUS$5millionperyearsavings[4].
Sincetheearlieststudies,backinthe1900s,manymethodsofobtainingthenitrideshavebeendeveloped,suchas:directnitriding,hydriding–dehydriding–nitriding,carbothermicnitriding,oxidativeammonolysis,andothers[13,14].
Anoverviewofuraniumnitridepaperspublishedthroughtheyears,from1948to2018isshowninFig.
1[15].
Therearetworegionsinwhichthenumberofpapersincreasedconsiderably.
Thefirst,between1964and1970,mightberelatedtothemostcomprehensiveuraniumnitridecharacterizationaswellasthenewpreparationroutesfromuraniumtetrachlorideandtetrafluoride,partofthe(herementioned)"goldenage"regardinguraniumnitrideresearchanddevelopment[16–18].
Thesecond,mostclearlydefinedafter2011,isresponsibleforaround42%ofthepublishedpapers,asseenabove.
Thiscorrespondstosomethingabout9papersperyear,againstapproximately1paperperyearfrom1946to2010.
ThatbehaviourismainlyimpactedbytheresponsefromthescientificcommunityaftertheFuku-shimaDaiichiaccidenton11March2011,causedbytheearthquakeandthefollowingtsunami.
Thisscenarioexceedstheseverityofthedesignbasisaccidents(DBA)[19].
After-wards,theinternationalgoalwaschangedfromjustimprov-ingtheexistingUO2-zirconiumfuelsystem,toathoughtfulandrigorousattempttoreplacethecurrentsystemsoastomakeitmuchmorerobustandsafetyunderDBA.
Asoftoday,nitridefuelshasbeensuccessfullytestedinthesodiumcooledBR-10reactorinRussia[20]aswellasintheFFTFandEBR-IItestreactors[1].
ThustherearegoodgroundsforconsideringnitridesasastrongcandidateforGenIVreactorsduetotheirmanyadvantagespreviouslydescribedanditsresultsunderoperation.
Inthispaperwewilldiscussdifferentproductionroutesaswellasfuelrecy-clingoptionsfornitridefuels.
ProductionTherearemanydifferentmethodsofproducingnitrideswithdifferentprosandconsaswillbeoutlinedbelow.
Itis,how-ever,clearthattheuseofdifferentfissilenitrideswillaffecttheproductionrouteconsiderably.
Ifminoractinidesaretobeincludedtheirhighspecificradioactivitywillmoreorlessrequireadirectcouplingtotheseparationsolutiontoavoidunnecessaryhandlingoffine,radioactivepowder.
However,ifnitridesaretobebasedonmainlyuraniumandplutoniumthechallengeswithrespecttomanufacturearesimilarasfornormalmixedoxidefuelashasbeenshowninRussiawhereindustrialnitridefuelmanufacturingisongoing[21].
ProductionofnitridesfrommetalThemainadvantageofusingmetallicuraniumasstartingmaterialisthatneitheroxygennorcarbonneedtobeintro-ducedatanystageoftheprocess,hencevirtuallyeliminat-ingtheissuesoftheseelementsappearingasimpuritiesintheproduct,asisfrequentlythecasewiththecarbothermic1715JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725nitridingmethod.
Thedifferentvariantsofthemetalroutearealsocarriedoutatlowertemperatures,whichreducesvolatilisationinthemanufactureofminoractinide-con-tainingfuels.
Additionally,andunlikeinthecarbothermicnitridingmethodwherenitrogenisusedinadoublecapac-ityasreactantandascarriergastoremoveformedcarbonmonoxide,quantitativeincorporationof15Ncanbeachieved,whichwasfirstdemonstratedwithnaturalN2[22]andlaterrepeatedwithactual15N2[23].
Nitrideproductionfrommetalhasbeendemonstratedtobefeasibleinlargescale[24,25].
Themaindrawbackisthathigh-purityuraniummetalisrequiredasfeedstock.
Metallicfuelshavebecomeuncommoninmostcontexts,thereisalmostnocivilianpro-ductionofenrichedmetallicuranium,andnewindustrialinfrastructurewouldberequiredforlarge-scalenitridefuelproductionbythisroute.
Itshouldalsobenotedthatallthepowdersinvolved,includingthenitrideitself,aremoreorlesspyrophoricandcannotbehandledintheopen.
Denselysinterednitridepellets,ontheotherhand,arestableinairorwateratambienttemperatures.
Whilethereareafewtechnicallydifferentapproaches,theyaretypicallybasedonthesequentialorcombinedreactionofuraniummetalwithhydrogenandnitrogen.
Theimmediateproductis,inthecaseofuranium,amoreorlesshyperstoichiometricsesquinitridewhichnecessitatesaconcludingstoichiometryadjustmentstepconsistingofdenitridinginvacuumorunderinertgasat1100–1300°C.
Denitridingisnotneededinthecaseofplutonium,whichdoesnotformastablesesquinitride,butgivesplutoniummononitridePuNastheimmediateproductfromnitriding.
Inprinciple,ifnotinpractice,thereactioncouldbecar-riedoutjustbyexposingbulkuraniummetaltonitrogengasaccordingtothereactions:Inactualproduction,severalmethodsareemployedtoincreasethereactionrateandyield,asoutlinedbelow.
NitridingofmetalwithnitrogengasSoliduraniummetalreactssluggishlyandoftenincom-pletelywithelementalnitrogen.
Evenifthemetalsurfaceisthoroughlycleanedfromoxidelayersbeforethesynthesis,initialsurfacenitridingofthemetalproducesabarrierwhichinterfereswithcontinuedreaction[26].
Onewayofaddress-ingthisproblemistoreducethemetaltoaveryfineUH3powderbyreactionwithhydrogengas[27]at200–250°C,andthenreducingitbacktometalatatemperatureexceeding(1)2U+3+x2N2→U2N3+x(2)U2N3+x→2UN+1+x2N2Fig.
1Numberofuraniumnitridepaperspublishedthoughtheyears.
ThesearchwasdoneusingScopuswebsite,with"uraniumnitride"aspartofthepapertitle;othersnitrides-typewerenotcounted[15]1716JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725thedissociationtemperatureofthehydridebutnotcausingmeltingorsinteringofthemetalpowder[28].
Sincethedissociationtemperatureisafunctionoftheambientpartialpressureofhydrogen,vacuumorflowingargonisemployedtokeepthispressurelow.
Theobtainedmetalpowder,havingaverylargespecificsurface,willreadilycombinewithN2.
Thereactionsequencethusbecomesfollowedbyreactions(1)and(2).
Onedrawback,apartfromtheintroductionoftwoaddi-tionalsteps,isthattheuraniumpowderisinanexception-allyreactivestate,willbeveryeasilyoxidisedandhasatendencytoself-ignitioninair.
Hencethenitridingstepshouldbeperformedinimmediatesequenceandwithoutanytransferofthepowder.
Itisalsonoted[28]thattheheatreleasedintheexothermicnitridingreactioncancausecak-ingormeltingofthemetalpowder.
Afluidised-bedsetupissuitableinthisaswellasinseveralofthefollowingvariantsofsynthesis,asitbothpromotesfastanduniformreactionduetoenhancedgas–solidinteractionandreducestheriskforcaking.
Analternativemethodistomelturaniummetalundernitrogenatmosphere,optionallyatapressureofseveralatmospheres.
Whilesmallamountsofuraniumnitridehavebeenproducedbyarcmelting[29],thatmethodisnotlikelytobeconvenientlyappliedinindustrialscale.
Inthecaseofconventionalmelting,suchasinacrucible,surfacenitridingstillstronglyinterfereswiththereactionbetweentheliquiduraniumandnitrogengas[30].
NitridingofhydridewithnitrogengasInsteadofreducinguraniumhydridetoformareactivemetalpowder,thehydridecanbereacteddirectlywithN2.
Inaddi-tiontoeliminatingoneprocessstep,theendothermiccharac-terofthehydridedecompositionreducestheoverallreactionenthalpyandhencetheriskforemergenceofliquiduranium.
Inthismethod,hydridingisperformedasaboveandtheformedhydrideisthennitridedwithN2at300–500°C[31].
Theprocesswillthenstartwithreaction(3),followedbyanddenitridingaccordingto(2).
Afterthestoichiometryadjustmentstep,theobtainedpowderisdirectlysuitableforpressingandsintering.
ThecharacteristicUNpowdermorphology,producedbyhydriding–nitridingroutewithaverageparticlessizeabout5m,isshowedinFig.
2[32].
(3)2U+3H2→2UH3(4)2UH3→2U+3H2(5)2UH3+3+x2N2→U2N3+x+3H2Theas-synthesisedUNpowder,whensinteringbysparkplasmasintering(SPS)at1650°Cand3min,formsaveryhighdensity(~99.
8%TD)fuelwithverylowporosity,aspresentedinFig.
3[31].
NitridingofmetalwithammoniaAnattractivefeatureofammoniaasnitridingagentisthatthereactioncanbecarriedoutinasinglestep(exceptforthefinalstoichiometryadjustment).
Thereactiontemperature,200–300°C,isevenlowerthanforthenitridingofhydride,whichwouldfurtherloweranyevaporationintheproduc-tionofminoractinide-richfuelsfortransmutationpurposes.
Fig.
2UNpowdermorphologyimageobtainedbyscanningelectronmicroscopy[32]Fig.
3ScanningelectronmicrographyofUNpelletsinteredbySPSat1650°Cand3min[31]1717JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725Thenitridingwithammoniainvolvestwoalternativepaths[33].
ThenitrogeninammoniahashigherchemicalpotentialthanthatinN2,andhencedirectnitridingofmetalproceedsmorereadilyandatlowertemperature[34].
Inaparallelreaction,uraniummetalishydrided,whichmaycon-tributetospallingandhenceexposureoffreshmetalsur-face,unlikeinnitridingofmetalwithN2.
Intheserespects,themethodcanbeseenasacombinationofreactions(1),(3)and(5)inparallel.
However,theenhancedpotentialfordirectnitridingofmetalisrelatedtothetendencyofammo-niatopartiallydecomposeintoN2andH2atthereactiontemperature,andwhilethisreactionwillintheabsenceofcatalysingsurfacesberatherslow,nitridingworksbestiffreshammoniaiscontinuouslysupplied[33],andtheN2formedbyNH3decompositionwillnotbeefficientlyincor-poratedinthenitrideatthelowtemperatureused.
Hence,ifisotopicallyenriched15Nisused,theunconsumedfractionofNH3andN2willeitherrepresentacostlylossorhavetoberecycled.
TosummarisethedifferenturaniumnitrideroutesfromUmetal,thefollowingpicturewasmadetoillustratealltheabovementionedprocessestogether(Fig.
4).
ProductionofnitridesfromfluoridesEventhoughsynthesisbynitridingofuraniummetalisinseveralwaysbothattractiveandconvenient,theround-aboutpathoverthemetallicformisanundesirablecomplicationfromthepointofindustrialproduction.
Anobviousoptionwouldbetouseuraniumhexafluoride,UF6,oruraniumtetrafluoride,UF4,whichareindustry-standardintermedi-ariesintheisotopicenrichmentprocessandintheproduc-tionofuraniumdioxide,therebycuttingoutanypreliminaryconversionstepswhichnotonlyincreasecostandcomplex-ity,butalsotendtointroduceimpurities.
Intheammonoly-sisofuraniumfluorides[35,36],justasinthesynthesisfrommetal,nocarbonoroxygencompoundsfeatureinthereaction,whichallowsaveryhighdegreeofpurityoftheproduct.
WhetherthestartingmaterialbeUF6orUF4isnotcriticalfortheprocess,sincethefirststepofreactionofammoniawithUF6isreductionoftheuraniumtotetravalentshape,forminganadductofUF4andammoniumhydrogenfluorideNH4F[37]:Thereactionisspontaneousat100–200°C,andtheprod-uctcanifsodesiredbedecomposedtopureUF4andNH4Fbyfurtherheatingto500°C[37],althoughthisisunneces-saryforthecontinuednitridesynthesis.
Fromthatpointon,thereactionsofUF6orUF4areessen-tiallyidentical.
Hencethechoicebetweenthefluorideswillmostlydependonpracticaldecisionsconcerningthepro-cessandequipment.
Theexclusiveuseofgaseousreactants(UF6andNH3)toformasolidproductcanbeanattractivefeaturewhendevelopingacontinuousindustrialprocess;ontheotherhand,UF4hastheadvantageofbeingmoreeasilyhandled.
Also,sincepartofany15NH3usedwillformNH4F,andthisamountissmallerwithalessfluorinatedrawmate-rial,theuseofUF4reducestheneedofammoniarecycling.
Theintermediatespeciesformedinthenitridingreactiondependonthetemperatureandthecompositionofthegasphase,andinvolvetetravalentammoniumuraniumfluoridecomplexesofthetype(NH4)x4UFxnNH3,whichformulacanberewrittenas(UF4)(NH4F)xnNH3.
Onesuggestedreactionformula[35]correspondstowhereassociatedammoniamoleculesareomittedfromthenotation,andtheproduct"UN2"ismorecorrectlyanitro-gen-richformofuraniumsesquinitride,whichneedstobeconvertedtomononitrideinafinaldenitridingstepaccord-ingto(2).
ProductionofnitridesfromsolutionInsteadofworkingwithfinepowdersduringfuelproductionorsimplyconnecttheproductiondirectlytotheseparation/(6)3UF6+8NH3→3NH4UF5+3NH4F+N2(7)NH44UF8+6NH3→UN2+8NH4F+H2Fig.
4OverviewofUNproduc-tionfromUmetal:thesolidline(red)representsthedirectnitridingroute;thedashedline(green)characterisesthehydrid-ing-nitridingroute;andthedottedline(blue)illustratesthehydriding-dehydriding-nitridingroute.
(Colorfigureonline)1718JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725recyclingprocessitispossibletosubstitutethepowderswithsmallactinidecontainingkernelscommonlyreferredtoasmicrospheres.
Theterm"micro"shouldhoweverbeinterpretedfairlyfreelysinceitispracticallypossibletoproducespheresofvarioussizesandfinalspherediameterstypicallyrangesfromaround50muptoabout1000m[38].
Forafullypowderfreeproductionroute,freeflowingmicrospherescanbepoureddirectlyintocladdingtubes.
Thisconceptisoftenreferredtoassphere-pacfuel[39].
Alternativelytheproducedspherescanbecompactedandsinteredintotraditionalfuelpellets.
Thisapproachmayatfirstglancelooklikeafullypowderfreeprocessaswellbutanyfinalphysicalhomogenizationofthesinteredpelletsbygrindingandpolishingwillintroduceastepwherealimitedfractionofpowderisgenerated.
Thereareseveraldifferentgelationtechniquesavailablefortheproductionofmicrospheres.
Commonforalltech-niquesisthatthestartingmaterialisanaqueousmetalsolu-tionthatissomehowdispersedasdropletsandsolidifiedintokernels.
Thesolidificationprocesscangenerallybedividedintotwosub-types.
Inthefirsttypeaprecipitationisinducedinthedropletsbyextractionofwaterand/oracidfromthedropletsintothedispersionmedia.
ThesecondtypeisbasedongelationbypHincreaseinthedropletscausingthemetalionstogel/precipitate.
Theresultspresentedherearebasedonthesecondtypeofgelationusingamethodcalledtheinternalgelationprocess.
TheinternalgelationprocessTheinternalgelationprocesswasoriginallynotdevelopedfortheproductionofnitridebasedmaterialsbutratherfortheproductionofuraniumoxidefuelkernels[40,41].
Throughcontinuesresearchanddevelopmentovertheyearstheprocesshasbeenadaptedtoproductionofoxides,nitridesandcarbidesandworkhasbeenperformedonbothsinglemetalcontainingmicrospheresaswellasmetalmixes.
MicrospherescontainingamongothersU,Zr,PuandmixedU–Pu,Zr–CeandZr–Puhavebeenproducedbyvariousinternalgelationprocesses[42–47].
TheinternalgelationprocessisbasedontemperatureinducedpHincreasecausingthemetalionsinthesoltogel/precipitate.
Generallythemethodstartswithametalnitratesolution.
Thesolutionisgenerallycooleddowntoinbetween0and4°C[38].
UreaandHexamethylenetetramine(HMTA)areaddedtothesol,eitherassolidstodissolveorasamixedsolution.
TheHMTAistheprincipalgelationagentinthesolwhileureaisaddedasacomplexationagentinordertopreventprematuregelationofthesol.
Iftheaimistoproducecarbideornitrideacarboncontainingsource,commonlyafinecarbonpowder,isalsodispersedinthesol.
Thesolisintroducedassmalldropletsintoanimmiscibleheatcarriercommonlyheatedtoinbetween50and100°C[42,48].
WhenthedropletsareheatedintheimmiscibleheatcarriertheHMTAdegrades,thiscausesanincreaseinpHinthedropletswhichresultsingelation/precipitationofthemetalionsinthedroplet.
Moreworkhasbeencarriedoutonproductionofuraniumbasedmicrospheresusingtheinternalgelationprocessascomparedtogelationoftrans-uraniumactinidesorgelationofinertmatrixmaterialssuchasZr.
Theprincipalreactionstepsofthegelationprocessarethereforepresentedusinggelationofuraniumasthemodelsystem.
Theprincipalstepspftheinternalgelationofuraniumbasedsolshavebeendeterminedto[49].
Decomplexationofurea:Hydrolysisoftheuranylions:ProtonationofHMTA:HMTAdecomposition:Thecontinuoushydrolysisoftheuranylionsthatleadtotheformationofasolidmaterialisthereforedrivenfor-wardbyremovalofH+fromthesystembyreactionwiththeHMTA.
Aftergelationofthesphereshasbeencompletedtheyarewashedinordertoremovesiliconeoilandexcessgela-tionchemicals.
Firstthesiliconeoiliswashedofusingforexamplepetroleumetherorcarbontetrachloride.
Afterthesiliconeoilhasbeenwashedofthespheresaregenerallywashed/agedinaqueousammonia.
Thereasonforwashing/ageinginaqueousammoniasolutionistwofold.
Theageingpartispurposedtosupplyanabundanceofhydroxylionsforthemicrospherestomakesurethatalltheuranylionshavebeenproperlygelled/precipitated.
Thewashingofthemicrospheresinaqueoussolutionisperformedinordertoremoveions,suchasNH4+andNO3forexample,aswellasresidualgelationchemicalsfromtheformedmicrospheres.
ResidualgelationchemicalsandNH4NO3inthedrysphereswillincreasethedegreeofmechanicaldamageinthespheresduringheattreatments,suchasthecarbothermicreduction,whentheresidualchemicalsaredecomposedtogasesandleavesthemicrospheres.
Themicrospheresproducedbytheinternalgelationtechniquearegenerallysmoothandoffairlygoodspherical(8)UO2CONH222+22CONH22+UO2+2(9)UO2+2+2H2OUO2(OH)2+2H+(10)CH26N4+H+CH26N4H+(11)CH26N4H++3H++4NO3+6H2O4NH+4+4NO3+6CH2O1719JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725shape.
ExamplesofwashedanddriedmicrospherespriortoanyheattreatmentarepresentedinFig.
5[50].
Choosingasuitablecarbonsourceisdetrimentaltothesuccessinmakingthefinalnitridematerial.
Sincethecon-versionofmetaloxidemicrospherestometalnitrideinvolvesthesolidstatereactionbetweencarbonandmetaloxide,andnomillingofthemicrospherescanbeperformedifthegoalistohavealowdustprocess,itisimportantthatthecarboniswelldispersedinthemicrospheresalreadyatthegela-tionstagetoprovidebeneficialconditionsforthenitrideformation.
Theeffectofgelationperformedusingacarbonsourcewithlowdispersionandshortsettlingtimeduringthegela-tionprocessisshowedinFig.
6[50].
FromFigs.
5and6thecomparisonbecomesskewedsinceFig.
5istakenatmuchlowermagnificationbutalsowhencomparingtolargermagnificationimages(Fig.
7)itcanbeseenthatthemicrospheresproducedinthebatchrepresentedinFig.
5hasmuchmorehomogeneouscarbondistribution[50].
BothmicrospheresinFigs.
5and6havebeenpreparedusinggraphitepowderascarbonsourcebutthemaindiffer-enceisthattheparticlesizeofthecarbonusedinFig.
5isabitbelow0.
5mwhilethecarbonparticlesinFig.
6havealargesizefractionranginguptoabout10–20msize.
InadditiontothedifferenceincarbondistributionwithinthemicrospheresitisalsoobservableinFigs.
5and6thatthemicrospheresintheimagesareofverydifferentsize.
Whenproducingmaterialsusingtheinternalgela-tionprocessitispossibletocontrolthesizeofthemicro-spheresbycontrollingthegelationparametersusedduringproduction.
Thesizeofthedropletsthatareintroducedintotheheatcarryingsiliconeoilcanbecontrolledbythesizeofthenozzlethatthesolisdrippedthroughandbyphysicallybreakingupthesolintosmalldropletsatthenozzletipbyapplicationofoscillationofthenozzle.
Itisalsopossibletoaffectthesizeofthedriedmicrospheresbychangingthemetalconcentrationinthesol.
Asthemetalconcentrationinthesoldecreasesthemicrospherewillshrinkmoreduringdryingandthefinalsizeofthedriedmicrosphereswilldecrease.
Fig.
5SEMimageshowinganexampleofacarboncontainingura-niummicrosphereafterwashinganddrying.
Themicrosphereintheimagecontainacarbontometalmolarrationof2.
15[50]Fig.
6SEMimageillustratingawashedanddriedzirconiumbasedmicrosphereusingacoursegraphitepowderascarbonsource.
Themolarcarbontometalratiointhemicrosphereis2.
3[50]Fig.
7LargermagnificationimageofthesurfaceofthemicrosphereinFig.
5.
Ascanbeseenalsoatlargemagnificationthereisnoclearaggregationofcarbonparticlesobservableonthesurfaceofthemicrosphere[50]1720JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725CarbothermicreductionCarbothermicreductionisanitrideproductiontechniquethatutilizesmetaloxidesasstartingmaterial.
Thenamederivesfromthatcarbonismixedintothemetaloxidematerialasareductionagentinordertofacilitatetheremovalofoxy-genfromthesystemduringthereaction.
Ifthedioxideofthemetaltobenitridedisusedasinputmaterialinthesynthesis,suchasUO2orPuO2forexample,aminimumcarbontometalmolarratioof2isrequiredtotheoreticallyeliminatealloxygenfromthesampleandproduceanitridematerialwithoutoxy-genimpurities.
However,inrealitythereactionsystemsexpe-rienceslimitationsduetotheinabilitytoproduceinfinitelyfinemixturesofcarbonandmetaloxide.
Inordertoproducenitridematerialslowinoxygenimpuritiesusingreasonablereactiontimes,carboniscommonlyaddedinexcesstothereactionsystemcomparedtothestoichiometricrequirements.
Thismeansthatinpractice,carbontometalratiosexceed-ingthetheoreticalmolarratio2areusedinordertosuppressresidualoxygenimpuritiesinthefinalnitridematerials.
Car-bothermicreductioncanbeutilizedfornitrideproductionfromalloftheactinideoxides,rangingfromuraniumtocurium,whicharetheelementsusuallyofinterestwhendiscussingproduction,implementationandrecyclingofnuclearfuelsinaGenerationIVfuelcycle.
Itishowevernotthecasethatasin-glespecificsetofreactionparametersistheperfectoptimumregardlessofwhichoneoftheactinideoxidesonewishestosynthesize.
Onthecontrary,thereactionconditionsandopti-malcarbontometalratioforcarbothermicreductionvariesdependingonifitisU,Np,Pu,AmorCmoxidethatisbeingconvertedintonitride[51–54].
Productionofactinidenitridesbycarbothermicreductionhasbeenperformedonalloftheabovementionedelements,eitherinpureformorinactinidemixes,acrosstheworld.
Themajorityofsynthesisexperiencehoweverderivefromproduc-tionofuraniumnitride,whichiswhyuraniumwillbeusedasmodelelementwhendescribingthedifferentreactionsinvolvedinthesynthesis.
Whenproducinguraniumnitridethefirststepistoremoveexcessoxygenpresentintheuraniumoxideeitherasuraniumtrioxide,UO3,orashyperstoichiometricuraniumdioxide,UO2+x.
Thiscanbeachievedbyreductionusinghydrogenininertcarriergasasreducingagent[55]Ifcarbonisalreadypresentinthesystem,asinthecaseofwhengelationderivedmicrospheresarebeingnitrideforexample,somecarboninthesystemcanbelostascarbonmonoxideduetooxidationbysteam[56]Analternativetousinghydrogenistousecarbonasreductionagent[47,57](12)UO2+x+xH2UO2.
0+xH2O(13)C+H2O(g)CO+H2Notethatreactions(12)and(13)aremodelreactionsillustratingonecasewhenH2isusedasreducingagentandonecasewhencarbonisusedasreducingagent.
Onecouldequallywellhavebalancedreaction(12)withUO3ontheleftreactionsideandreaction(14)withUO2+xontheleftreactionside.
Whentheactinideoxidehasbeenbroughttothedesiredoxidationstatethecarbothermicreductionreactioncanbecarriedout.
Thenitrogenrequiredforthenitrideformationduringthecarbothermicreductionreactionissuppliedviathereactionatmosphere.
DifferentreactionatmospheressuchaspureN2,mixedN2+H2orNH3canbeused[57–60].
WhenthecarbothermicreductionisperformedinN2asreactionatmosphereandthetemperatureiskeptbelow1723Kithasbeenreportedtoproceedby[57]:Above1723Kacarbonitrideisreportedtoforminsteadaccordingto[57]:Theconformationofcarbideinthesystemduringcarbo-thermicreductioncantheoreticallyberemovedbyprolongedheattreatmentbyEliminationofcarbidefromthenitridematerialisthusdependingontheabilitytoremoveelementalcarbonfromthereactionmixture.
Aslongasthereisafreecarbonphasetoequilibrateagainstinthesystemthefinalnitridematerialwillcontaincarbideimpurities.
IthasbeenestimatedthataslongasanequilibratingcarbonphaseexistsinthesystemtheUNmaterialproducedwillcontainabout15mol%car-bide[61].
OnepossibleremovalmechanismisthereactionbetweenexcesscarbonandunreactedUO2accordingtoreac-tion(15).
However,sincecarbontometalratioshigherthanthetheoretical2needstobeappliedinordertoefficientlyproducenitrideswithlowoxygenimpuritylevelstherewillobviouslynotbeenoughUO2presentinthesystemtoremoveallelementalcarbonasCO.
Thereforesomeaddi-tionalmechanismforcarbonremovalneedstobepresentinordertodrivereaction(17)forward.
OnepossibilitytoremovecarbonisbyadditionofH2tothereactionatmosphere.
ByintroducingH2intotheN2atmospheretheelementalcarboncanbeconvertedintohydrogencyanideaccordingtothereaction[62]:Limitationsduetochemicalequilibriuminreaction(18)canbeovercomebyusingflowingreactiongas,thuspurgingthesystemofformedHCNandallowingcarbontobeelim-inatedfromthesolidsample.
Thermodynamicmodelling(14)UO3+0.
5CUO2+0.
5CO2(15)UO2+2C+0.
5N2UN+2CO(16)UO2+(2+x)C+(1x)∕2N2UN1xCx+2CO(17)UN1xCx+(x∕2)N2UN+xC(18)H2+N2+2C2HCN1721JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725madeonnon-uraniumcontainingnitridefuelhasalsoidenti-fiedHCNasthelikelyreactionproductduringdecarburiza-tionofthenitridematerialduringsynthesis[63].
Anadvantageofusingthecarbothermicreductionpro-cesscomparedtootherprocessesfornitrideproductionisthatithasbeenidentifiedasaprocessthatmaybemorewellsuitedforscalinguptolargescaleproductioncomparedtoothernitrideproductionprocesses[64].
Theresultingmicrospheres,aftercarbothermicreductionhasbeenperformed,canpossessawiderangeofmicrostruc-tures.
Dependingonhowmuchoftheresidualchemicalsthatwaswashedoutofthemicrospherespostgelationandwhatheatingrampsandreactiontemperaturesthatarebeingusedduringthecarbothermicreductionthefinalstructureofthemicrospherescanbestronglyaffected.
ExamplesofpossiblenitridemicrospherestructuresarepresentedinFigs.
8and9[50].
Ineverymicrospherebatchproducedtherewillofcoursebeinternalvariancewithsomemicrospheresshowinghigherorlowerdegreeofphysicaldamagebutonaveragethestruc-tureofthefinalmicrospherescanbecontrolledbycontrol-lingtheheattreatmentofthematerials.
Themaindiffer-encebetweenFigs.
8and9,apartfromthedifferentmetalinthemicrospheres,istheheatingrate.
TheZrNmicrosphereexampleinFig.
8washeatedat20°C/minuptoamaximumtemperatureof1700°C.
TheUNmicrosphereinFig.
9washeatedatarateof3°C/minto350°Cfollowedby10°C/minto800°Ctoremoveresidualchemicalsandmoisturebeforebeingcooleddown.
Theoxide+carbonspheresformedwassubsequentlyheated15°C/mintoamaximumtemperatureof1650°Cduringthecarbothermicreductionitself.
Thisisanillustrationofhowthemicrospheremicrostructurecanbecontrolledinordertopotentiallyproducematerialswithtailoredsuitabilityforuseeitherpelletpressingordirectlyassphere-pacfuel.
Reachinghighnitridepurityusingthecarbothermicreductionprocessisnotstraightforwardandisdependentonmetaloxidetobenitrided,theheattreatmentappliedduringcarbothermicreduction,onthecarbontometalratiousedandonthedegreeofhomogenousmixingofthecarbonwiththemetaloxide.
Measurementofimpuritiescanbeper-formedeitherbydirectmeasurementoftheoxygen,carbonandnitrogenlevelsinthefinalnitridebutalsobyindirectmeasurementssuchasX-raydiffraction(XRD).
X-raydif-fractionofthematerialscan,apartfromdetectingundesiredphasesinthematerialsuchasresidualoxides,alsoestimatesolidsolutionimpuritiesbycomparingthelatticeparameteroftheproducednitridetoreferenceliteraturedata.
Shiftsinlatticeparametercanbeusedtoestimateforexamplecarbideimpuritiesinnitridebymakinganinterpolationbetweenthelatticeparametersofmetalnitrideandcarbideaccord-ingtoVegardslaw.
ExamplesofnitridepuritiesthatcanbeachievedusingcarbothermicreductionarepresentedinTable1.
ThedatainTable1isaselectedcompilationofdatafrom[45],PuN-data,[65],twofirstZrN-datapointsand[66],mixedZrN-PuNdataandtheremainderbeinghith-ertounpublisheddata.
ThetablepresentspuritiesmeasuredeitherbydirectmeasurementofN,O,andCcontentandCcontentestimatedbyVegardslawfromXRDdata.
NoneofthehighactivitysamplesinTable1wasmeas-uredfordirectdeterminationofN,OorCcontent,therea-sonbeingradiationsafetybasedatthefacilitywheretheworkwasperformed.
HoweverinallthesamplesinTable1Fig.
8SEMimageofaZrNmicrosphereaftercarbothermicreduc-tion.
Thespherehasfracturedseverelyduringtheheattreatmentstepfollowedbypartialsinteringduringthelatestagesofthecarbother-micreduction.
ThisresultsinadamagedfinalmicrospherecontainingzonesofpartlysinteredZrN[50]Fig.
9ExampleofaSEMimageofaUNmicrosphereaftercarboth-ermicreduction.
Thesphereiscoveredinsmallporesbutessentiallythespherehassurvivedtheheattreatmentstepswithoutsufferinganyphysicaldamage[50]1722JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725whereC-contentisestimatedonlyfromXRDdatanoresid-ualoxidephasecouldbedetectedandtheimpurityestimateisthereforemadeonsolidsolutioncarbonitridematerials.
Theimpuritiespresentedinthosematerialsarethuscarbonimpuritiesinotherwisefairlypurenitridematerials.
PelletfabricationfromnitridemicrospheresNitridesusedfornuclearfuelapplicationsaregenerallydiffi-cultmaterialstosintercomparedtowhenmakingoxidefuel.
Duetohighmeltingtemperaturesnitridefuelsneedtobesinteredathightemperatures[67].
Thetendencyofthefuelnitrides,suchasUNorPuNforexample,todissociateintometalannitrogenathightemperaturesandlowN2partialpressuresaddstothechallengeofsuccessfullysinterthesematerials[68].
Pressingpelletsfromnitridemicrospheresaddsanadditionalchallengetothetask.
Sincethenitridesarehardmaterialsmechanicalcompactionofthemicro-spheresintogoodqualitygreenpelletsbecomesincreasinglyharderastheporosityofthemicrospheresdecreases.
Thelevelofresidualporosityanddegreeoffracturinginthemicrospheresisinturndependentonthethermaltreatmentappliedwhenconvertingthegelledspheresintonitrides.
Inordertoproduceamicrospherebasedmaterialsuitableformechanicalcompactionitisthusimportanttofactorinthefinalmechanicalpropertiesofthesphereswhendesigningtheheattreatmentstepsandnotonlythedesigncriteriawithrespecttofinalnitridepurity.
AnexampleofthebehavioroffinalZrNpelletsinwhichoneofthepelletswasmadefrommicrospheresthatwerelargelydisintegratedduringcompactioncomparedtowhenthemicrospheresretainedtheirindividualshapeareshowninFig.
10.
Bothpelletsarepressedusingthesamecompac-tionpressureandsinteredat2000°CinflowingN2gas[50].
Analternativetoconventionalpressurelesssinteringofthenitridemicrosphereswouldbetoapplysomeversionoffieldassistedsinteringinstead.
Onepotentialtechniquecouldbesparkplasmasintering(SPS).
SPShasbeensuccessfullyTable1ExamplesofnitridepuritiesreachablebythecarbothermicreductiontechniquewhenproducingmetalnitridesofZr,UandPuThe(–)notationinthetableindicatesnotmeasuredDesiredmaterialNitrogencontent(wt%)Oxygencontent(wt%)Carboncontent(wt%)Carboncontent(wt%)accordingtoXRDUN5.
430.
290.
0110.
08UN5.
310.
100.
0050.
13UN5.
260.
080.
2120.
36UN4.
030.
242.
271.
84PuN–––0.
17PuN–––0.
14PuN–––0.
31PuN–––0.
06(Zr0.
6Pu0.
4)N–––0.
95ZrN––6.
115.
53ZrN––2.
062.
98ZrN10.
500.
362.
962.
12Fig.
10ComparisonbetweentwoZrNpelletssinteredat2000°C.
Thereisadistinctdifferenceobservablebyocularinspectionbetweenthepelletsbasedonifthemicrospherespressedweredestroyedorretainedtheirindividualshapeduringcompaction[50]1723JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725appliedtosinterUNpowderstopelletsreachingdensitiesclosetothetheoreticaldensities[69].
ApplicationofSPStosinternitridemicrosphereshasbeenperformedtosinterZrN[68].
DensitiesoffinalZrNpelletscouldbeincreasedusingSPScomparedtopressurelesssintering.
InthatstudyZrNpelletamaximumdensityofabout87%oftheoreticaldensitywasachievedwhensinteringZrNmicrospheresat1700°Cfor30mininargonusingapressureof99MPadur-ingsintering.
NostudyofsinteringofUNmicrospheresbySPStechniqueexisttothisdatetotheauthors'knowledge.
DissolutionandseparationInordertoclosethenuclearfuelloopitisvitalthatafterusetheusedfuelcanbedissolvedandthentheusefulcompo-nentscanbeseparatedoutforre-manufacture.
Inthecaseofnitridefuelsthemainchallengeisthedissolutionstep.
Uraniumnitridecaninitselfbedissolvedinnitricacid(HNO3)muchinthesamewayasconventionalUO2fuels[70].
Thishasbeenfurtherdemonstratedwithpureaswellaswithcarbon-andoxygen-richUNpellets[71].
Hencerepro-cessingbythePUREXmethodappearstobefeasible.
Thepracticalexperiencewithirradiatednitridefuelsishoweverratherlimited.
Alargequantityofspent(~80GWd/ton)(U,Pu)Nfuelwassuccessfullyreprocessedbyasomewhatmodifiedaciddissolutionandliquidextractionprocess,inwhichdissolutionwasassistedbyasmalladditionoffluo-ride[72].
Inparticular,actinidenitridepelletswithalargeadmixtureofzirconiumnitridehavebeenshowntobeexcep-tionallydifficulttodissolvewithoutadditionoffluorideionsorhydrofluoricacidHF(whichtoobecomeequivalentinthenitricacidenvironment)[71].
ConsideringthatZrisoneofthemostabundantfissionproducts,itcannotbeexcludedthatitcaninterferewiththenitricaciddissolutionofhigh-burnup,pureUNfuelsunlessHFisaddedtothesolution.
Ifthefuelisenrichedin15N,thenitrogenmustforeco-nomicalreasonsberecycled.
DissolutioninHNO3producesnitrogen-containinggasesbyseveralreactions,withpartofthisnitrogencomingfromtheacidandhencebeingofnaturalisotopiccomposition[73].
Assomeofthesegase-ousspecieswillformfromboththenitrideandtheacid,theresultisanisotopicdilutionwhichwouldnecessitatecostlyre-enrichmentof15N.
Itwillthereforebenecessarytorecoverthe15Nbeforethedissolutioninnitricacid,mostlikelybyaprecedingstepofvoloxidation.
Thiscouldbedonebycombustionofthefuelinoxygen,formingNO2,tobefurtherconvertedtoasuitablereagentforre-use,oroxidisingitinsuperheatedsteam[74],formingNH3whichisdirectlysuitableformanufactureofnewfuel.
Ithashoweverbeennotedthatasignificantfractionofnitrogenremainsintheoxidicproductevenafterratherextensiveoxidation[11],andcompleteoxidationtoUO3,whichcannotbeachievedwithsteam,islikelyneededtoliberateallthe15N.
Ifontheotherhandnaturalnitrogen(>99%14N)isusedinthefuelmanufacture,theadditionalissue(besidestheneutroneconomypenalty)of14Cproductionmustbedealtwith.
Thefeasibilityof14Ccapturehasbeendemonstrated[72],and14Cisalreadyseparatedandstoredinindustrial-scalereprocessing[75].
Yetthisresultsinanadditionallong-livedwastefractionthatcanbeavoidedif15Nisused.
Oncethenitridefuelhaseitherbeendirectlydis-solved(ignoringthe15Nissue)orfollowinganoxidativedissolutiontheresultingsolutionscanbeuseddirectlyinoneofthenewlydevelopedGroupedActinideExtractionSystems(GANEX)[76–78].
Theseprocessesweredevel-opedtohavenopurestreamofe.
g.
plutoniumsoallacti-nidesareextractedtogether(excepturanium)thusmakingtheprocessesconsiderablymoreproliferationsafeandatthesametimepotentiallyopeningthepossibilitytoadirectconnectiontothefuelfabricationlineusingawetprocess.
Electrometallurgicaldissolutionandrefining(oftencalledpyroprocessing)isanoftensuggestedalternativetoaqueousreprocessingmethods,andhastheadvantagethatthe15Ncomponentcanberecoveredinundilutedform.
Suchmeth-odsofelectrolyticaldissolutionintoamoltensaltmediumappeartobeparticularlysuitablefornitridesbecauseoftheirappreciableelectricconductivity.
Thisapproachhasbeenshowntoworkforunirradiated(U,Pu)Nfuel[79].
Anover-viewofdifferenttechnicalsolutionscanbefoundin[80]ConclusionsThedevelopmentofnitridefuelshasbeengoingonandoffinthelast5decades.
Fromthebeginningtheywerepromis-ingasnuclearfuelsduetotheirhighfissiledensityandther-malconductivity.
However,unfavourablereactionswithhotwateraswellasamorecomplicatedproductionmadethemobsoletecomparedoxidesinthe60ies.
Todaythenitridesgotanewspring.
Bothforuseasaccidenttolerantfuelsaswellasfuelinfast,metalcooledreactorsforGenIVsys-tems.
Researchisongoingonhowtoeasetheproductionandtoovercometheinteractionswithwater.
Industrialscaleproductionhasbegunine.
g.
Russia.
OpenAccessThisarticleisdistributedunderthetermsoftheCrea-tiveCommonsAttribution4.
0InternationalLicense(http://creativecommons.
org/licenses/by/4.
0/),whichpermitsunrestricteduse,distribu-tion,andreproductioninanymedium,providedyougiveappropriatecredittotheoriginalauthor(s)andthesource,providealinktotheCreativeCommonslicense,andindicateifchangesweremade.
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org/10.
1007/s10967-018-6316-0NitridefuelforGenIVnuclearpowersystemsChristianEkberg1·DiogoRibeiroCosta2,3·MarcusHedberg1·MikaelJolkkonen2Received:20October2018/Publishedonline:10November2018TheAuthor(s)2018AbstractNuclearenergyhasbeenapartoftheenergymixinmanycountriesfordecades.
Todayinprincipleallpowerproducingreactorsusethesametechniqe.
EitherPWRorBWRfuelledwithoxidefuels.
Thischoiceoffuelisnotselfevidentandtodaytherearesuggestionstochangetofuelswhichmaybesaferandmoreeconomicalandalsousedine.
g.
GenIVnuclearpowersystems.
Onesuchfueltypeisthenitrides.
Thenitrideshaveabetterthermalconductivitythantheoxidesandasimilarmeltingpointandarethushavelargersafetymarginstomeltingduringoperation.
Inadditiontheyarebetween30and40%moredensewithrespecttofissilematerial.
Drawbacksincludeinstabilitywithrespecttowaterandasometimescomplicatedfabricationroute.
TheformerisnotreallyanissuewithGenIVsystemsbutforuseinthepresentfleet.
Inthispaperwediscussbothproductionandrecyclingpotentialofnitridefuels.
KeywordsNuclearfuel·Nitridenuclearfuels·GenIV·Productionofnitrides·Nuclearfuelrecycling·DissolutionofnitridesIntroductionNuclearpoweristodayadisputedtechniquealthoughitisinprincipleCO2freeandhighlyenergetic.
However,thecur-rentnuclearsystemsareratherinefficient,onlyabout1%oftheinherentenergyisused,andthewasteneedtobestoredforabout100,000yearsbeforeitsradiotoxicityissimilartotheoriginallymineduranium.
InsomecountriessuchasFrancethereisareprocessingsystemusedwheretheplu-toniumisusedintherefabricatingofmixedoxidefuelthatisoncemoreusedinapowerproducingreactor.
Thisis,however,onlydoneonceincreasingtheenergyutilisationtoabout1.
2%butsavingabout17%offreshlyminedura-nium[1]Inthelast2decadesacompletecircularuseofthefuelmaterial,thesocalledGenIVnuclearpowersystemshaveemergedasapotentialsolutiontotheaforementionedissues.
ThefollowinggoalsforGenIVweredefinedprevi-ouslyintheGenerationIVInternationalForum(GIF)[2,3]:sustainability,safetyandreliability,economiccompetitive-ness,andproliferationresistanceandphysicalprotection.
Suchasystemcomprisefastreactors,separationfacilitiesforrecoveryofthestillusefulactinidesandafuelfabricationplanttocompletetherecycling.
Naturally,severalquestionsarisewhendiscussinganewpowerproductionsystemssuchaswhichcombinationofcoolant,claddingandfueltouseinthereactoraswellasaselectionofseparationsystemandfueltypeandfabricationroute.
Overtheyearstherehasbeendifferentinvestigationstofindtheoptimalnuclearfuel.
Initially,inthe1940s,uraniummetalwaschosenasapotentialcandidatemainlybecauseitshighthermalconductivityandfissiledensity.
However,itslowmeltingpoint,associatedwithitsphasetransforma-tionduringheating,chemicalinstability,andhighfissiongasswelling,becameadrawbacktoitsindustrialapplication.
Toworkaroundtheseproblems,themostpromisingalter-nativewastheceramicoxidefuel,specificallytheuraniumdioxide(UO2),whichhashighmeltingpoint,goodchemi-calstabilityandcompatibilitywiththefissionproducts,andreasonablethermalstabilityduringburn-up.
Themaindownside,otherwise,wasitsverylowthermalconductivity.
Paralleltouraniumdioxidedevelopment,manyresearchprojectshavebeencarriedoutinordertoconsiderotherfueloptions,suchasnitrides,carbidesandsilicides[4].
Among*ChristianEkbergche@chalmers.
se1NuclearChemistry/IndustrialMaterialsRecycling,Chalmers,Gteborg,Sweden2DivisionofNuclearEngineering,KungligaTekniskaHgskolan,Stockholm,Sweden3WestinghouseElectricSwedenAB,Vsters,Sweden1714JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725them,nitrideshasshownmanysuperiorqualitiesoverthestandardoxidefuelsuchas:Higherfissiledensity(40%moreuraniuminUNthaninUO2)[4]:leadingtohigherconversionratios,andpoten-tiallyhigherburn-ups.
Higherthermalconductivityreductionofthefuelcentre-linetemperature,decreaseintheenergystoredperunitmasswhileincreasingthemarginforfuelmelting[5,6],anddelaythemigrationoffissionproductsandactinides,whichpositivelyaffectsthefuelswelling[7].
Reprocessinggooddissolutioninnitricacid(HNO3),makingthisfuelcompatiblewiththePUREXprocess,whichusesHNO3fordissolutionofspentfuel[8].
Stabilitygoodchemicalcompatibilitywithmostpotentialcladdingmaterials,aswellasirradiationstability[4].
LongerfuelcycletimeowingtoneutronicbehaviourofUNinthecore[9],thefuelcyclecouldincreasefromthecommonlyapplied18months(standardUO2)toabout25months,basedonaburnupof50GWd/tU.
Thisincreaseleadstofewershutdownsforreloading,thusbeinganeconomicalbenefitfornitridefuelimplementa-tion[4].
Ontheotherhand,therearesomedrawbacksregardingthenitridefuels:Theproductionofminoractinide(orevenplutonium)containingnitridefuelisnotstraightforwardandrequiresomedifficultproductionsteps.
Oxidationresistance:thenitridepelletsreadilyoxi-dizesinsuperheatedsteam,withcompletedegradationobtainedwithin1hin0.
50barsteamat500°CforUN[10].
Theas-manufacturednitridepowderispyrophoric,whichmeansthatitreactsimmediatelywithoxygenfromair,forinstance.
Thus,theatmosphereduringhandlingtheUNpowdermustbeoxygen-free,whichrequiresadditionalimplementationforindustrialscaleproduc-tion[11].
Fuelcost:mainlyduetothefactthatthenitrogencom-ponenthastobehighlyenrichedin15Ntoincreasetheneutroneconomyandavoidthe(n,p)formationof14Cfrom14N[12].
Thisreactionalsoenhancestheamountofradioactivematerialinthespentfuel,whichisunde-sirable.
However,theadditionalcostsrelatedtonitro-genenrichmentareoffsetbyloweruraniumenrichmentrequirements,aswellasthereducednumberoffuelassembliesineachfuelreload,resultinginanestimatedUS$5millionperyearsavings[4].
Sincetheearlieststudies,backinthe1900s,manymethodsofobtainingthenitrideshavebeendeveloped,suchas:directnitriding,hydriding–dehydriding–nitriding,carbothermicnitriding,oxidativeammonolysis,andothers[13,14].
Anoverviewofuraniumnitridepaperspublishedthroughtheyears,from1948to2018isshowninFig.
1[15].
Therearetworegionsinwhichthenumberofpapersincreasedconsiderably.
Thefirst,between1964and1970,mightberelatedtothemostcomprehensiveuraniumnitridecharacterizationaswellasthenewpreparationroutesfromuraniumtetrachlorideandtetrafluoride,partofthe(herementioned)"goldenage"regardinguraniumnitrideresearchanddevelopment[16–18].
Thesecond,mostclearlydefinedafter2011,isresponsibleforaround42%ofthepublishedpapers,asseenabove.
Thiscorrespondstosomethingabout9papersperyear,againstapproximately1paperperyearfrom1946to2010.
ThatbehaviourismainlyimpactedbytheresponsefromthescientificcommunityaftertheFuku-shimaDaiichiaccidenton11March2011,causedbytheearthquakeandthefollowingtsunami.
Thisscenarioexceedstheseverityofthedesignbasisaccidents(DBA)[19].
After-wards,theinternationalgoalwaschangedfromjustimprov-ingtheexistingUO2-zirconiumfuelsystem,toathoughtfulandrigorousattempttoreplacethecurrentsystemsoastomakeitmuchmorerobustandsafetyunderDBA.
Asoftoday,nitridefuelshasbeensuccessfullytestedinthesodiumcooledBR-10reactorinRussia[20]aswellasintheFFTFandEBR-IItestreactors[1].
ThustherearegoodgroundsforconsideringnitridesasastrongcandidateforGenIVreactorsduetotheirmanyadvantagespreviouslydescribedanditsresultsunderoperation.
Inthispaperwewilldiscussdifferentproductionroutesaswellasfuelrecy-clingoptionsfornitridefuels.
ProductionTherearemanydifferentmethodsofproducingnitrideswithdifferentprosandconsaswillbeoutlinedbelow.
Itis,how-ever,clearthattheuseofdifferentfissilenitrideswillaffecttheproductionrouteconsiderably.
Ifminoractinidesaretobeincludedtheirhighspecificradioactivitywillmoreorlessrequireadirectcouplingtotheseparationsolutiontoavoidunnecessaryhandlingoffine,radioactivepowder.
However,ifnitridesaretobebasedonmainlyuraniumandplutoniumthechallengeswithrespecttomanufacturearesimilarasfornormalmixedoxidefuelashasbeenshowninRussiawhereindustrialnitridefuelmanufacturingisongoing[21].
ProductionofnitridesfrommetalThemainadvantageofusingmetallicuraniumasstartingmaterialisthatneitheroxygennorcarbonneedtobeintro-ducedatanystageoftheprocess,hencevirtuallyeliminat-ingtheissuesoftheseelementsappearingasimpuritiesintheproduct,asisfrequentlythecasewiththecarbothermic1715JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725nitridingmethod.
Thedifferentvariantsofthemetalroutearealsocarriedoutatlowertemperatures,whichreducesvolatilisationinthemanufactureofminoractinide-con-tainingfuels.
Additionally,andunlikeinthecarbothermicnitridingmethodwherenitrogenisusedinadoublecapac-ityasreactantandascarriergastoremoveformedcarbonmonoxide,quantitativeincorporationof15Ncanbeachieved,whichwasfirstdemonstratedwithnaturalN2[22]andlaterrepeatedwithactual15N2[23].
Nitrideproductionfrommetalhasbeendemonstratedtobefeasibleinlargescale[24,25].
Themaindrawbackisthathigh-purityuraniummetalisrequiredasfeedstock.
Metallicfuelshavebecomeuncommoninmostcontexts,thereisalmostnocivilianpro-ductionofenrichedmetallicuranium,andnewindustrialinfrastructurewouldberequiredforlarge-scalenitridefuelproductionbythisroute.
Itshouldalsobenotedthatallthepowdersinvolved,includingthenitrideitself,aremoreorlesspyrophoricandcannotbehandledintheopen.
Denselysinterednitridepellets,ontheotherhand,arestableinairorwateratambienttemperatures.
Whilethereareafewtechnicallydifferentapproaches,theyaretypicallybasedonthesequentialorcombinedreactionofuraniummetalwithhydrogenandnitrogen.
Theimmediateproductis,inthecaseofuranium,amoreorlesshyperstoichiometricsesquinitridewhichnecessitatesaconcludingstoichiometryadjustmentstepconsistingofdenitridinginvacuumorunderinertgasat1100–1300°C.
Denitridingisnotneededinthecaseofplutonium,whichdoesnotformastablesesquinitride,butgivesplutoniummononitridePuNastheimmediateproductfromnitriding.
Inprinciple,ifnotinpractice,thereactioncouldbecar-riedoutjustbyexposingbulkuraniummetaltonitrogengasaccordingtothereactions:Inactualproduction,severalmethodsareemployedtoincreasethereactionrateandyield,asoutlinedbelow.
NitridingofmetalwithnitrogengasSoliduraniummetalreactssluggishlyandoftenincom-pletelywithelementalnitrogen.
Evenifthemetalsurfaceisthoroughlycleanedfromoxidelayersbeforethesynthesis,initialsurfacenitridingofthemetalproducesabarrierwhichinterfereswithcontinuedreaction[26].
Onewayofaddress-ingthisproblemistoreducethemetaltoaveryfineUH3powderbyreactionwithhydrogengas[27]at200–250°C,andthenreducingitbacktometalatatemperatureexceeding(1)2U+3+x2N2→U2N3+x(2)U2N3+x→2UN+1+x2N2Fig.
1Numberofuraniumnitridepaperspublishedthoughtheyears.
ThesearchwasdoneusingScopuswebsite,with"uraniumnitride"aspartofthepapertitle;othersnitrides-typewerenotcounted[15]1716JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725thedissociationtemperatureofthehydridebutnotcausingmeltingorsinteringofthemetalpowder[28].
Sincethedissociationtemperatureisafunctionoftheambientpartialpressureofhydrogen,vacuumorflowingargonisemployedtokeepthispressurelow.
Theobtainedmetalpowder,havingaverylargespecificsurface,willreadilycombinewithN2.
Thereactionsequencethusbecomesfollowedbyreactions(1)and(2).
Onedrawback,apartfromtheintroductionoftwoaddi-tionalsteps,isthattheuraniumpowderisinanexception-allyreactivestate,willbeveryeasilyoxidisedandhasatendencytoself-ignitioninair.
Hencethenitridingstepshouldbeperformedinimmediatesequenceandwithoutanytransferofthepowder.
Itisalsonoted[28]thattheheatreleasedintheexothermicnitridingreactioncancausecak-ingormeltingofthemetalpowder.
Afluidised-bedsetupissuitableinthisaswellasinseveralofthefollowingvariantsofsynthesis,asitbothpromotesfastanduniformreactionduetoenhancedgas–solidinteractionandreducestheriskforcaking.
Analternativemethodistomelturaniummetalundernitrogenatmosphere,optionallyatapressureofseveralatmospheres.
Whilesmallamountsofuraniumnitridehavebeenproducedbyarcmelting[29],thatmethodisnotlikelytobeconvenientlyappliedinindustrialscale.
Inthecaseofconventionalmelting,suchasinacrucible,surfacenitridingstillstronglyinterfereswiththereactionbetweentheliquiduraniumandnitrogengas[30].
NitridingofhydridewithnitrogengasInsteadofreducinguraniumhydridetoformareactivemetalpowder,thehydridecanbereacteddirectlywithN2.
Inaddi-tiontoeliminatingoneprocessstep,theendothermiccharac-terofthehydridedecompositionreducestheoverallreactionenthalpyandhencetheriskforemergenceofliquiduranium.
Inthismethod,hydridingisperformedasaboveandtheformedhydrideisthennitridedwithN2at300–500°C[31].
Theprocesswillthenstartwithreaction(3),followedbyanddenitridingaccordingto(2).
Afterthestoichiometryadjustmentstep,theobtainedpowderisdirectlysuitableforpressingandsintering.
ThecharacteristicUNpowdermorphology,producedbyhydriding–nitridingroutewithaverageparticlessizeabout5m,isshowedinFig.
2[32].
(3)2U+3H2→2UH3(4)2UH3→2U+3H2(5)2UH3+3+x2N2→U2N3+x+3H2Theas-synthesisedUNpowder,whensinteringbysparkplasmasintering(SPS)at1650°Cand3min,formsaveryhighdensity(~99.
8%TD)fuelwithverylowporosity,aspresentedinFig.
3[31].
NitridingofmetalwithammoniaAnattractivefeatureofammoniaasnitridingagentisthatthereactioncanbecarriedoutinasinglestep(exceptforthefinalstoichiometryadjustment).
Thereactiontemperature,200–300°C,isevenlowerthanforthenitridingofhydride,whichwouldfurtherloweranyevaporationintheproduc-tionofminoractinide-richfuelsfortransmutationpurposes.
Fig.
2UNpowdermorphologyimageobtainedbyscanningelectronmicroscopy[32]Fig.
3ScanningelectronmicrographyofUNpelletsinteredbySPSat1650°Cand3min[31]1717JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725Thenitridingwithammoniainvolvestwoalternativepaths[33].
ThenitrogeninammoniahashigherchemicalpotentialthanthatinN2,andhencedirectnitridingofmetalproceedsmorereadilyandatlowertemperature[34].
Inaparallelreaction,uraniummetalishydrided,whichmaycon-tributetospallingandhenceexposureoffreshmetalsur-face,unlikeinnitridingofmetalwithN2.
Intheserespects,themethodcanbeseenasacombinationofreactions(1),(3)and(5)inparallel.
However,theenhancedpotentialfordirectnitridingofmetalisrelatedtothetendencyofammo-niatopartiallydecomposeintoN2andH2atthereactiontemperature,andwhilethisreactionwillintheabsenceofcatalysingsurfacesberatherslow,nitridingworksbestiffreshammoniaiscontinuouslysupplied[33],andtheN2formedbyNH3decompositionwillnotbeefficientlyincor-poratedinthenitrideatthelowtemperatureused.
Hence,ifisotopicallyenriched15Nisused,theunconsumedfractionofNH3andN2willeitherrepresentacostlylossorhavetoberecycled.
TosummarisethedifferenturaniumnitrideroutesfromUmetal,thefollowingpicturewasmadetoillustratealltheabovementionedprocessestogether(Fig.
4).
ProductionofnitridesfromfluoridesEventhoughsynthesisbynitridingofuraniummetalisinseveralwaysbothattractiveandconvenient,theround-aboutpathoverthemetallicformisanundesirablecomplicationfromthepointofindustrialproduction.
Anobviousoptionwouldbetouseuraniumhexafluoride,UF6,oruraniumtetrafluoride,UF4,whichareindustry-standardintermedi-ariesintheisotopicenrichmentprocessandintheproduc-tionofuraniumdioxide,therebycuttingoutanypreliminaryconversionstepswhichnotonlyincreasecostandcomplex-ity,butalsotendtointroduceimpurities.
Intheammonoly-sisofuraniumfluorides[35,36],justasinthesynthesisfrommetal,nocarbonoroxygencompoundsfeatureinthereaction,whichallowsaveryhighdegreeofpurityoftheproduct.
WhetherthestartingmaterialbeUF6orUF4isnotcriticalfortheprocess,sincethefirststepofreactionofammoniawithUF6isreductionoftheuraniumtotetravalentshape,forminganadductofUF4andammoniumhydrogenfluorideNH4F[37]:Thereactionisspontaneousat100–200°C,andtheprod-uctcanifsodesiredbedecomposedtopureUF4andNH4Fbyfurtherheatingto500°C[37],althoughthisisunneces-saryforthecontinuednitridesynthesis.
Fromthatpointon,thereactionsofUF6orUF4areessen-tiallyidentical.
Hencethechoicebetweenthefluorideswillmostlydependonpracticaldecisionsconcerningthepro-cessandequipment.
Theexclusiveuseofgaseousreactants(UF6andNH3)toformasolidproductcanbeanattractivefeaturewhendevelopingacontinuousindustrialprocess;ontheotherhand,UF4hastheadvantageofbeingmoreeasilyhandled.
Also,sincepartofany15NH3usedwillformNH4F,andthisamountissmallerwithalessfluorinatedrawmate-rial,theuseofUF4reducestheneedofammoniarecycling.
Theintermediatespeciesformedinthenitridingreactiondependonthetemperatureandthecompositionofthegasphase,andinvolvetetravalentammoniumuraniumfluoridecomplexesofthetype(NH4)x4UFxnNH3,whichformulacanberewrittenas(UF4)(NH4F)xnNH3.
Onesuggestedreactionformula[35]correspondstowhereassociatedammoniamoleculesareomittedfromthenotation,andtheproduct"UN2"ismorecorrectlyanitro-gen-richformofuraniumsesquinitride,whichneedstobeconvertedtomononitrideinafinaldenitridingstepaccord-ingto(2).
ProductionofnitridesfromsolutionInsteadofworkingwithfinepowdersduringfuelproductionorsimplyconnecttheproductiondirectlytotheseparation/(6)3UF6+8NH3→3NH4UF5+3NH4F+N2(7)NH44UF8+6NH3→UN2+8NH4F+H2Fig.
4OverviewofUNproduc-tionfromUmetal:thesolidline(red)representsthedirectnitridingroute;thedashedline(green)characterisesthehydrid-ing-nitridingroute;andthedottedline(blue)illustratesthehydriding-dehydriding-nitridingroute.
(Colorfigureonline)1718JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725recyclingprocessitispossibletosubstitutethepowderswithsmallactinidecontainingkernelscommonlyreferredtoasmicrospheres.
Theterm"micro"shouldhoweverbeinterpretedfairlyfreelysinceitispracticallypossibletoproducespheresofvarioussizesandfinalspherediameterstypicallyrangesfromaround50muptoabout1000m[38].
Forafullypowderfreeproductionroute,freeflowingmicrospherescanbepoureddirectlyintocladdingtubes.
Thisconceptisoftenreferredtoassphere-pacfuel[39].
Alternativelytheproducedspherescanbecompactedandsinteredintotraditionalfuelpellets.
Thisapproachmayatfirstglancelooklikeafullypowderfreeprocessaswellbutanyfinalphysicalhomogenizationofthesinteredpelletsbygrindingandpolishingwillintroduceastepwherealimitedfractionofpowderisgenerated.
Thereareseveraldifferentgelationtechniquesavailablefortheproductionofmicrospheres.
Commonforalltech-niquesisthatthestartingmaterialisanaqueousmetalsolu-tionthatissomehowdispersedasdropletsandsolidifiedintokernels.
Thesolidificationprocesscangenerallybedividedintotwosub-types.
Inthefirsttypeaprecipitationisinducedinthedropletsbyextractionofwaterand/oracidfromthedropletsintothedispersionmedia.
ThesecondtypeisbasedongelationbypHincreaseinthedropletscausingthemetalionstogel/precipitate.
Theresultspresentedherearebasedonthesecondtypeofgelationusingamethodcalledtheinternalgelationprocess.
TheinternalgelationprocessTheinternalgelationprocesswasoriginallynotdevelopedfortheproductionofnitridebasedmaterialsbutratherfortheproductionofuraniumoxidefuelkernels[40,41].
Throughcontinuesresearchanddevelopmentovertheyearstheprocesshasbeenadaptedtoproductionofoxides,nitridesandcarbidesandworkhasbeenperformedonbothsinglemetalcontainingmicrospheresaswellasmetalmixes.
MicrospherescontainingamongothersU,Zr,PuandmixedU–Pu,Zr–CeandZr–Puhavebeenproducedbyvariousinternalgelationprocesses[42–47].
TheinternalgelationprocessisbasedontemperatureinducedpHincreasecausingthemetalionsinthesoltogel/precipitate.
Generallythemethodstartswithametalnitratesolution.
Thesolutionisgenerallycooleddowntoinbetween0and4°C[38].
UreaandHexamethylenetetramine(HMTA)areaddedtothesol,eitherassolidstodissolveorasamixedsolution.
TheHMTAistheprincipalgelationagentinthesolwhileureaisaddedasacomplexationagentinordertopreventprematuregelationofthesol.
Iftheaimistoproducecarbideornitrideacarboncontainingsource,commonlyafinecarbonpowder,isalsodispersedinthesol.
Thesolisintroducedassmalldropletsintoanimmiscibleheatcarriercommonlyheatedtoinbetween50and100°C[42,48].
WhenthedropletsareheatedintheimmiscibleheatcarriertheHMTAdegrades,thiscausesanincreaseinpHinthedropletswhichresultsingelation/precipitationofthemetalionsinthedroplet.
Moreworkhasbeencarriedoutonproductionofuraniumbasedmicrospheresusingtheinternalgelationprocessascomparedtogelationoftrans-uraniumactinidesorgelationofinertmatrixmaterialssuchasZr.
Theprincipalreactionstepsofthegelationprocessarethereforepresentedusinggelationofuraniumasthemodelsystem.
Theprincipalstepspftheinternalgelationofuraniumbasedsolshavebeendeterminedto[49].
Decomplexationofurea:Hydrolysisoftheuranylions:ProtonationofHMTA:HMTAdecomposition:Thecontinuoushydrolysisoftheuranylionsthatleadtotheformationofasolidmaterialisthereforedrivenfor-wardbyremovalofH+fromthesystembyreactionwiththeHMTA.
Aftergelationofthesphereshasbeencompletedtheyarewashedinordertoremovesiliconeoilandexcessgela-tionchemicals.
Firstthesiliconeoiliswashedofusingforexamplepetroleumetherorcarbontetrachloride.
Afterthesiliconeoilhasbeenwashedofthespheresaregenerallywashed/agedinaqueousammonia.
Thereasonforwashing/ageinginaqueousammoniasolutionistwofold.
Theageingpartispurposedtosupplyanabundanceofhydroxylionsforthemicrospherestomakesurethatalltheuranylionshavebeenproperlygelled/precipitated.
Thewashingofthemicrospheresinaqueoussolutionisperformedinordertoremoveions,suchasNH4+andNO3forexample,aswellasresidualgelationchemicalsfromtheformedmicrospheres.
ResidualgelationchemicalsandNH4NO3inthedrysphereswillincreasethedegreeofmechanicaldamageinthespheresduringheattreatments,suchasthecarbothermicreduction,whentheresidualchemicalsaredecomposedtogasesandleavesthemicrospheres.
Themicrospheresproducedbytheinternalgelationtechniquearegenerallysmoothandoffairlygoodspherical(8)UO2CONH222+22CONH22+UO2+2(9)UO2+2+2H2OUO2(OH)2+2H+(10)CH26N4+H+CH26N4H+(11)CH26N4H++3H++4NO3+6H2O4NH+4+4NO3+6CH2O1719JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725shape.
ExamplesofwashedanddriedmicrospherespriortoanyheattreatmentarepresentedinFig.
5[50].
Choosingasuitablecarbonsourceisdetrimentaltothesuccessinmakingthefinalnitridematerial.
Sincethecon-versionofmetaloxidemicrospherestometalnitrideinvolvesthesolidstatereactionbetweencarbonandmetaloxide,andnomillingofthemicrospherescanbeperformedifthegoalistohavealowdustprocess,itisimportantthatthecarboniswelldispersedinthemicrospheresalreadyatthegela-tionstagetoprovidebeneficialconditionsforthenitrideformation.
Theeffectofgelationperformedusingacarbonsourcewithlowdispersionandshortsettlingtimeduringthegela-tionprocessisshowedinFig.
6[50].
FromFigs.
5and6thecomparisonbecomesskewedsinceFig.
5istakenatmuchlowermagnificationbutalsowhencomparingtolargermagnificationimages(Fig.
7)itcanbeseenthatthemicrospheresproducedinthebatchrepresentedinFig.
5hasmuchmorehomogeneouscarbondistribution[50].
BothmicrospheresinFigs.
5and6havebeenpreparedusinggraphitepowderascarbonsourcebutthemaindiffer-enceisthattheparticlesizeofthecarbonusedinFig.
5isabitbelow0.
5mwhilethecarbonparticlesinFig.
6havealargesizefractionranginguptoabout10–20msize.
InadditiontothedifferenceincarbondistributionwithinthemicrospheresitisalsoobservableinFigs.
5and6thatthemicrospheresintheimagesareofverydifferentsize.
Whenproducingmaterialsusingtheinternalgela-tionprocessitispossibletocontrolthesizeofthemicro-spheresbycontrollingthegelationparametersusedduringproduction.
Thesizeofthedropletsthatareintroducedintotheheatcarryingsiliconeoilcanbecontrolledbythesizeofthenozzlethatthesolisdrippedthroughandbyphysicallybreakingupthesolintosmalldropletsatthenozzletipbyapplicationofoscillationofthenozzle.
Itisalsopossibletoaffectthesizeofthedriedmicrospheresbychangingthemetalconcentrationinthesol.
Asthemetalconcentrationinthesoldecreasesthemicrospherewillshrinkmoreduringdryingandthefinalsizeofthedriedmicrosphereswilldecrease.
Fig.
5SEMimageshowinganexampleofacarboncontainingura-niummicrosphereafterwashinganddrying.
Themicrosphereintheimagecontainacarbontometalmolarrationof2.
15[50]Fig.
6SEMimageillustratingawashedanddriedzirconiumbasedmicrosphereusingacoursegraphitepowderascarbonsource.
Themolarcarbontometalratiointhemicrosphereis2.
3[50]Fig.
7LargermagnificationimageofthesurfaceofthemicrosphereinFig.
5.
Ascanbeseenalsoatlargemagnificationthereisnoclearaggregationofcarbonparticlesobservableonthesurfaceofthemicrosphere[50]1720JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725CarbothermicreductionCarbothermicreductionisanitrideproductiontechniquethatutilizesmetaloxidesasstartingmaterial.
Thenamederivesfromthatcarbonismixedintothemetaloxidematerialasareductionagentinordertofacilitatetheremovalofoxy-genfromthesystemduringthereaction.
Ifthedioxideofthemetaltobenitridedisusedasinputmaterialinthesynthesis,suchasUO2orPuO2forexample,aminimumcarbontometalmolarratioof2isrequiredtotheoreticallyeliminatealloxygenfromthesampleandproduceanitridematerialwithoutoxy-genimpurities.
However,inrealitythereactionsystemsexpe-rienceslimitationsduetotheinabilitytoproduceinfinitelyfinemixturesofcarbonandmetaloxide.
Inordertoproducenitridematerialslowinoxygenimpuritiesusingreasonablereactiontimes,carboniscommonlyaddedinexcesstothereactionsystemcomparedtothestoichiometricrequirements.
Thismeansthatinpractice,carbontometalratiosexceed-ingthetheoreticalmolarratio2areusedinordertosuppressresidualoxygenimpuritiesinthefinalnitridematerials.
Car-bothermicreductioncanbeutilizedfornitrideproductionfromalloftheactinideoxides,rangingfromuraniumtocurium,whicharetheelementsusuallyofinterestwhendiscussingproduction,implementationandrecyclingofnuclearfuelsinaGenerationIVfuelcycle.
Itishowevernotthecasethatasin-glespecificsetofreactionparametersistheperfectoptimumregardlessofwhichoneoftheactinideoxidesonewishestosynthesize.
Onthecontrary,thereactionconditionsandopti-malcarbontometalratioforcarbothermicreductionvariesdependingonifitisU,Np,Pu,AmorCmoxidethatisbeingconvertedintonitride[51–54].
Productionofactinidenitridesbycarbothermicreductionhasbeenperformedonalloftheabovementionedelements,eitherinpureformorinactinidemixes,acrosstheworld.
Themajorityofsynthesisexperiencehoweverderivefromproduc-tionofuraniumnitride,whichiswhyuraniumwillbeusedasmodelelementwhendescribingthedifferentreactionsinvolvedinthesynthesis.
Whenproducinguraniumnitridethefirststepistoremoveexcessoxygenpresentintheuraniumoxideeitherasuraniumtrioxide,UO3,orashyperstoichiometricuraniumdioxide,UO2+x.
Thiscanbeachievedbyreductionusinghydrogenininertcarriergasasreducingagent[55]Ifcarbonisalreadypresentinthesystem,asinthecaseofwhengelationderivedmicrospheresarebeingnitrideforexample,somecarboninthesystemcanbelostascarbonmonoxideduetooxidationbysteam[56]Analternativetousinghydrogenistousecarbonasreductionagent[47,57](12)UO2+x+xH2UO2.
0+xH2O(13)C+H2O(g)CO+H2Notethatreactions(12)and(13)aremodelreactionsillustratingonecasewhenH2isusedasreducingagentandonecasewhencarbonisusedasreducingagent.
Onecouldequallywellhavebalancedreaction(12)withUO3ontheleftreactionsideandreaction(14)withUO2+xontheleftreactionside.
Whentheactinideoxidehasbeenbroughttothedesiredoxidationstatethecarbothermicreductionreactioncanbecarriedout.
Thenitrogenrequiredforthenitrideformationduringthecarbothermicreductionreactionissuppliedviathereactionatmosphere.
DifferentreactionatmospheressuchaspureN2,mixedN2+H2orNH3canbeused[57–60].
WhenthecarbothermicreductionisperformedinN2asreactionatmosphereandthetemperatureiskeptbelow1723Kithasbeenreportedtoproceedby[57]:Above1723Kacarbonitrideisreportedtoforminsteadaccordingto[57]:Theconformationofcarbideinthesystemduringcarbo-thermicreductioncantheoreticallyberemovedbyprolongedheattreatmentbyEliminationofcarbidefromthenitridematerialisthusdependingontheabilitytoremoveelementalcarbonfromthereactionmixture.
Aslongasthereisafreecarbonphasetoequilibrateagainstinthesystemthefinalnitridematerialwillcontaincarbideimpurities.
IthasbeenestimatedthataslongasanequilibratingcarbonphaseexistsinthesystemtheUNmaterialproducedwillcontainabout15mol%car-bide[61].
OnepossibleremovalmechanismisthereactionbetweenexcesscarbonandunreactedUO2accordingtoreac-tion(15).
However,sincecarbontometalratioshigherthanthetheoretical2needstobeappliedinordertoefficientlyproducenitrideswithlowoxygenimpuritylevelstherewillobviouslynotbeenoughUO2presentinthesystemtoremoveallelementalcarbonasCO.
Thereforesomeaddi-tionalmechanismforcarbonremovalneedstobepresentinordertodrivereaction(17)forward.
OnepossibilitytoremovecarbonisbyadditionofH2tothereactionatmosphere.
ByintroducingH2intotheN2atmospheretheelementalcarboncanbeconvertedintohydrogencyanideaccordingtothereaction[62]:Limitationsduetochemicalequilibriuminreaction(18)canbeovercomebyusingflowingreactiongas,thuspurgingthesystemofformedHCNandallowingcarbontobeelim-inatedfromthesolidsample.
Thermodynamicmodelling(14)UO3+0.
5CUO2+0.
5CO2(15)UO2+2C+0.
5N2UN+2CO(16)UO2+(2+x)C+(1x)∕2N2UN1xCx+2CO(17)UN1xCx+(x∕2)N2UN+xC(18)H2+N2+2C2HCN1721JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725madeonnon-uraniumcontainingnitridefuelhasalsoidenti-fiedHCNasthelikelyreactionproductduringdecarburiza-tionofthenitridematerialduringsynthesis[63].
Anadvantageofusingthecarbothermicreductionpro-cesscomparedtootherprocessesfornitrideproductionisthatithasbeenidentifiedasaprocessthatmaybemorewellsuitedforscalinguptolargescaleproductioncomparedtoothernitrideproductionprocesses[64].
Theresultingmicrospheres,aftercarbothermicreductionhasbeenperformed,canpossessawiderangeofmicrostruc-tures.
Dependingonhowmuchoftheresidualchemicalsthatwaswashedoutofthemicrospherespostgelationandwhatheatingrampsandreactiontemperaturesthatarebeingusedduringthecarbothermicreductionthefinalstructureofthemicrospherescanbestronglyaffected.
ExamplesofpossiblenitridemicrospherestructuresarepresentedinFigs.
8and9[50].
Ineverymicrospherebatchproducedtherewillofcoursebeinternalvariancewithsomemicrospheresshowinghigherorlowerdegreeofphysicaldamagebutonaveragethestruc-tureofthefinalmicrospherescanbecontrolledbycontrol-lingtheheattreatmentofthematerials.
Themaindiffer-encebetweenFigs.
8and9,apartfromthedifferentmetalinthemicrospheres,istheheatingrate.
TheZrNmicrosphereexampleinFig.
8washeatedat20°C/minuptoamaximumtemperatureof1700°C.
TheUNmicrosphereinFig.
9washeatedatarateof3°C/minto350°Cfollowedby10°C/minto800°Ctoremoveresidualchemicalsandmoisturebeforebeingcooleddown.
Theoxide+carbonspheresformedwassubsequentlyheated15°C/mintoamaximumtemperatureof1650°Cduringthecarbothermicreductionitself.
Thisisanillustrationofhowthemicrospheremicrostructurecanbecontrolledinordertopotentiallyproducematerialswithtailoredsuitabilityforuseeitherpelletpressingordirectlyassphere-pacfuel.
Reachinghighnitridepurityusingthecarbothermicreductionprocessisnotstraightforwardandisdependentonmetaloxidetobenitrided,theheattreatmentappliedduringcarbothermicreduction,onthecarbontometalratiousedandonthedegreeofhomogenousmixingofthecarbonwiththemetaloxide.
Measurementofimpuritiescanbeper-formedeitherbydirectmeasurementoftheoxygen,carbonandnitrogenlevelsinthefinalnitridebutalsobyindirectmeasurementssuchasX-raydiffraction(XRD).
X-raydif-fractionofthematerialscan,apartfromdetectingundesiredphasesinthematerialsuchasresidualoxides,alsoestimatesolidsolutionimpuritiesbycomparingthelatticeparameteroftheproducednitridetoreferenceliteraturedata.
Shiftsinlatticeparametercanbeusedtoestimateforexamplecarbideimpuritiesinnitridebymakinganinterpolationbetweenthelatticeparametersofmetalnitrideandcarbideaccord-ingtoVegardslaw.
ExamplesofnitridepuritiesthatcanbeachievedusingcarbothermicreductionarepresentedinTable1.
ThedatainTable1isaselectedcompilationofdatafrom[45],PuN-data,[65],twofirstZrN-datapointsand[66],mixedZrN-PuNdataandtheremainderbeinghith-ertounpublisheddata.
ThetablepresentspuritiesmeasuredeitherbydirectmeasurementofN,O,andCcontentandCcontentestimatedbyVegardslawfromXRDdata.
NoneofthehighactivitysamplesinTable1wasmeas-uredfordirectdeterminationofN,OorCcontent,therea-sonbeingradiationsafetybasedatthefacilitywheretheworkwasperformed.
HoweverinallthesamplesinTable1Fig.
8SEMimageofaZrNmicrosphereaftercarbothermicreduc-tion.
Thespherehasfracturedseverelyduringtheheattreatmentstepfollowedbypartialsinteringduringthelatestagesofthecarbother-micreduction.
ThisresultsinadamagedfinalmicrospherecontainingzonesofpartlysinteredZrN[50]Fig.
9ExampleofaSEMimageofaUNmicrosphereaftercarboth-ermicreduction.
Thesphereiscoveredinsmallporesbutessentiallythespherehassurvivedtheheattreatmentstepswithoutsufferinganyphysicaldamage[50]1722JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725whereC-contentisestimatedonlyfromXRDdatanoresid-ualoxidephasecouldbedetectedandtheimpurityestimateisthereforemadeonsolidsolutioncarbonitridematerials.
Theimpuritiespresentedinthosematerialsarethuscarbonimpuritiesinotherwisefairlypurenitridematerials.
PelletfabricationfromnitridemicrospheresNitridesusedfornuclearfuelapplicationsaregenerallydiffi-cultmaterialstosintercomparedtowhenmakingoxidefuel.
Duetohighmeltingtemperaturesnitridefuelsneedtobesinteredathightemperatures[67].
Thetendencyofthefuelnitrides,suchasUNorPuNforexample,todissociateintometalannitrogenathightemperaturesandlowN2partialpressuresaddstothechallengeofsuccessfullysinterthesematerials[68].
Pressingpelletsfromnitridemicrospheresaddsanadditionalchallengetothetask.
Sincethenitridesarehardmaterialsmechanicalcompactionofthemicro-spheresintogoodqualitygreenpelletsbecomesincreasinglyharderastheporosityofthemicrospheresdecreases.
Thelevelofresidualporosityanddegreeoffracturinginthemicrospheresisinturndependentonthethermaltreatmentappliedwhenconvertingthegelledspheresintonitrides.
Inordertoproduceamicrospherebasedmaterialsuitableformechanicalcompactionitisthusimportanttofactorinthefinalmechanicalpropertiesofthesphereswhendesigningtheheattreatmentstepsandnotonlythedesigncriteriawithrespecttofinalnitridepurity.
AnexampleofthebehavioroffinalZrNpelletsinwhichoneofthepelletswasmadefrommicrospheresthatwerelargelydisintegratedduringcompactioncomparedtowhenthemicrospheresretainedtheirindividualshapeareshowninFig.
10.
Bothpelletsarepressedusingthesamecompac-tionpressureandsinteredat2000°CinflowingN2gas[50].
Analternativetoconventionalpressurelesssinteringofthenitridemicrosphereswouldbetoapplysomeversionoffieldassistedsinteringinstead.
Onepotentialtechniquecouldbesparkplasmasintering(SPS).
SPShasbeensuccessfullyTable1ExamplesofnitridepuritiesreachablebythecarbothermicreductiontechniquewhenproducingmetalnitridesofZr,UandPuThe(–)notationinthetableindicatesnotmeasuredDesiredmaterialNitrogencontent(wt%)Oxygencontent(wt%)Carboncontent(wt%)Carboncontent(wt%)accordingtoXRDUN5.
430.
290.
0110.
08UN5.
310.
100.
0050.
13UN5.
260.
080.
2120.
36UN4.
030.
242.
271.
84PuN–––0.
17PuN–––0.
14PuN–––0.
31PuN–––0.
06(Zr0.
6Pu0.
4)N–––0.
95ZrN––6.
115.
53ZrN––2.
062.
98ZrN10.
500.
362.
962.
12Fig.
10ComparisonbetweentwoZrNpelletssinteredat2000°C.
Thereisadistinctdifferenceobservablebyocularinspectionbetweenthepelletsbasedonifthemicrospherespressedweredestroyedorretainedtheirindividualshapeduringcompaction[50]1723JournalofRadioanalyticalandNuclearChemistry(2018)318:1713–1725appliedtosinterUNpowderstopelletsreachingdensitiesclosetothetheoreticaldensities[69].
ApplicationofSPStosinternitridemicrosphereshasbeenperformedtosinterZrN[68].
DensitiesoffinalZrNpelletscouldbeincreasedusingSPScomparedtopressurelesssintering.
InthatstudyZrNpelletamaximumdensityofabout87%oftheoreticaldensitywasachievedwhensinteringZrNmicrospheresat1700°Cfor30mininargonusingapressureof99MPadur-ingsintering.
NostudyofsinteringofUNmicrospheresbySPStechniqueexisttothisdatetotheauthors'knowledge.
DissolutionandseparationInordertoclosethenuclearfuelloopitisvitalthatafterusetheusedfuelcanbedissolvedandthentheusefulcompo-nentscanbeseparatedoutforre-manufacture.
Inthecaseofnitridefuelsthemainchallengeisthedissolutionstep.
Uraniumnitridecaninitselfbedissolvedinnitricacid(HNO3)muchinthesamewayasconventionalUO2fuels[70].
Thishasbeenfurtherdemonstratedwithpureaswellaswithcarbon-andoxygen-richUNpellets[71].
Hencerepro-cessingbythePUREXmethodappearstobefeasible.
Thepracticalexperiencewithirradiatednitridefuelsishoweverratherlimited.
Alargequantityofspent(~80GWd/ton)(U,Pu)Nfuelwassuccessfullyreprocessedbyasomewhatmodifiedaciddissolutionandliquidextractionprocess,inwhichdissolutionwasassistedbyasmalladditionoffluo-ride[72].
Inparticular,actinidenitridepelletswithalargeadmixtureofzirconiumnitridehavebeenshowntobeexcep-tionallydifficulttodissolvewithoutadditionoffluorideionsorhydrofluoricacidHF(whichtoobecomeequivalentinthenitricacidenvironment)[71].
ConsideringthatZrisoneofthemostabundantfissionproducts,itcannotbeexcludedthatitcaninterferewiththenitricaciddissolutionofhigh-burnup,pureUNfuelsunlessHFisaddedtothesolution.
Ifthefuelisenrichedin15N,thenitrogenmustforeco-nomicalreasonsberecycled.
DissolutioninHNO3producesnitrogen-containinggasesbyseveralreactions,withpartofthisnitrogencomingfromtheacidandhencebeingofnaturalisotopiccomposition[73].
Assomeofthesegase-ousspecieswillformfromboththenitrideandtheacid,theresultisanisotopicdilutionwhichwouldnecessitatecostlyre-enrichmentof15N.
Itwillthereforebenecessarytorecoverthe15Nbeforethedissolutioninnitricacid,mostlikelybyaprecedingstepofvoloxidation.
Thiscouldbedonebycombustionofthefuelinoxygen,formingNO2,tobefurtherconvertedtoasuitablereagentforre-use,oroxidisingitinsuperheatedsteam[74],formingNH3whichisdirectlysuitableformanufactureofnewfuel.
Ithashoweverbeennotedthatasignificantfractionofnitrogenremainsintheoxidicproductevenafterratherextensiveoxidation[11],andcompleteoxidationtoUO3,whichcannotbeachievedwithsteam,islikelyneededtoliberateallthe15N.
Ifontheotherhandnaturalnitrogen(>99%14N)isusedinthefuelmanufacture,theadditionalissue(besidestheneutroneconomypenalty)of14Cproductionmustbedealtwith.
Thefeasibilityof14Ccapturehasbeendemonstrated[72],and14Cisalreadyseparatedandstoredinindustrial-scalereprocessing[75].
Yetthisresultsinanadditionallong-livedwastefractionthatcanbeavoidedif15Nisused.
Oncethenitridefuelhaseitherbeendirectlydis-solved(ignoringthe15Nissue)orfollowinganoxidativedissolutiontheresultingsolutionscanbeuseddirectlyinoneofthenewlydevelopedGroupedActinideExtractionSystems(GANEX)[76–78].
Theseprocessesweredevel-opedtohavenopurestreamofe.
g.
plutoniumsoallacti-nidesareextractedtogether(excepturanium)thusmakingtheprocessesconsiderablymoreproliferationsafeandatthesametimepotentiallyopeningthepossibilitytoadirectconnectiontothefuelfabricationlineusingawetprocess.
Electrometallurgicaldissolutionandrefining(oftencalledpyroprocessing)isanoftensuggestedalternativetoaqueousreprocessingmethods,andhastheadvantagethatthe15Ncomponentcanberecoveredinundilutedform.
Suchmeth-odsofelectrolyticaldissolutionintoamoltensaltmediumappeartobeparticularlysuitablefornitridesbecauseoftheirappreciableelectricconductivity.
Thisapproachhasbeenshowntoworkforunirradiated(U,Pu)Nfuel[79].
Anover-viewofdifferenttechnicalsolutionscanbefoundin[80]ConclusionsThedevelopmentofnitridefuelshasbeengoingonandoffinthelast5decades.
Fromthebeginningtheywerepromis-ingasnuclearfuelsduetotheirhighfissiledensityandther-malconductivity.
However,unfavourablereactionswithhotwateraswellasamorecomplicatedproductionmadethemobsoletecomparedoxidesinthe60ies.
Todaythenitridesgotanewspring.
Bothforuseasaccidenttolerantfuelsaswellasfuelinfast,metalcooledreactorsforGenIVsys-tems.
Researchisongoingonhowtoeasetheproductionandtoovercometheinteractionswithwater.
Industrialscaleproductionhasbegunine.
g.
Russia.
OpenAccessThisarticleisdistributedunderthetermsoftheCrea-tiveCommonsAttribution4.
0InternationalLicense(http://creativecommons.
org/licenses/by/4.
0/),whichpermitsunrestricteduse,distribu-tion,andreproductioninanymedium,providedyougiveappropriatecredittotheoriginalauthor(s)andthesource,providealinktotheCreativeCommonslicense,andindicateifchangesweremade.
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