Oxidation Mechanisms in Zirconium Alloys for Nuclear Applications

205 Pages·2017·6.7 MB·English

DPhil Thesis
Oxidation Mechanisms in Zirconium Alloys for
Nuclear Applications
Supervised by: Professor
Author: Sean S. Yardley
C.R.M. Grovenor and
Professor S. Lozano-Perez
January2017
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Preface
This thesis describes works carriedout bythe authorintheDepartment of Materials,
UniversityofOxford,from January2011toJanuary2017underthesupervisionofProfessor
C.R.M. GrovenorandProfessorS. Lozano-Perez. Nopart ofthis thesis has beenpreviously
submittedforadegreeat this oranyotheruniversity.Theworkofotherauthors has been
drawnuponandhas been referencedand acknowledgedinthetext.Alist ofreferences is
given at theendof each chapter.
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Abstract
OxidationMechanisms inZirconium Alloys for Nuclear Applications
Athesis submittedforthedegreeofDoctorofPhilosophy
S.S.Yardley
LinacreCollege
Michaelmas Term 2016
Theoxidationofzirconium alloys under aqueous conditions has beenstudiedformorethan
50 years inrelationtoits roleas afuel claddingelement inwater-coolednuclear reactors.
Themost well-knownandinterestingphenomenonassociatedwiththecorrosionof
zirconium alloys is the cyclicoxidationrate.Oxidationinitiallyproceeds rapidly,formingan
oxidefilm.As this film thickens, therateofoxidationslows beforetransitioning,andbegins
oxidisingat thehigherrateagain.This effect has beenwell studied,althoughthe causeofrate
transitionis not understood.
Usingsamples oxidisedinconditions verycloselymatchingthoseofanuclearreactor,
althoughwithout radiation,several techniques wereusedtocharacterisethe oxidebeforeand
afterthetransitioninoxidationrate.Comparison oftheseresults allows someinsight intothe
mechanism operating.
Dopingsamples withisotopictracers duringdifferent points intheoxidationcycleand
subsequent analysis withNanoSIMS gaveinsight intotheporosityandprotectiveness ofthe
oxidefilm beforeandaftertransition.Beforetransition,theentireoxidefilm acts as abarrier,
ratherthan asmall sublayer.Aftertransition,the wholelayerofpreviouslyformedoxide
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becameporous andtheoxidisingmediawas admitteddirectlytothemetal/oxideinterface.
Examiningsamples ofdifferingoxidationtimes showedeachsubsequent transitionwas a
closerepeat ofthe first.Oxidethickness,ratherthantimeundercorrosion conditions, was
identifiedas thebest predictorofwhetherasamplewouldundergotransition.Oxideporosity
(andthus ratetransition)was demonstratedtobe a local process onscales as small <1µm.
Residual stress intheoxideandunderlyingmetal substratehavebeenstudiedforzirconium
alloys for some years. Here,anewtechniqueis appliedthat largelyconfirms previous
measurements madebyXRD.Testingpre- andpost-transitionoxides revealedalargedropin
thecompressivestress aftertransition,showingthepost-transitionoxideisbothporous and
contains muchlower compressivestresses.
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Acknowledgements
This thesis, suchas it is, wouldnot havebeenpossiblewithout thehelpofthefollowing
people,towhom Iam incrediblygrateful
ProfessorChristopherGrovenor,andProfessorSergio LozanoPerez,bothofwhom were
extremelypatient,knowledgeable andhelpful throughout.
DrKatieMoore,whodedicatedmuchofhertime tohelpmyexperiments anddataanalysis.
Thestaff at theDepartment ofMaterials, Oxford University,who facilitatedthewhole
endeavour.
KatiePlummer,whosupportedmethroughall stages ofstudy.
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Contents
OxidationMechanisms inZirconium Alloys for Nuclear Applications....................................1
LiteratureReview.......................................................................................................................8
Introduction............................................................................................................................8
Applications ofzirconium intheNuclear Industry..............................................................10
Zirconium Alloys.................................................................................................................13
CorrosionofZirconium Alloys............................................................................................14
Types ofCorrosion...............................................................................................................17
TheEffect ofTemperatureonCorrosion.............................................................................20
KineticTransitions inOxidationRate..................................................................................21
StructureofOxidelayers......................................................................................................25
Effect ofSecondaryPhaseParticles.....................................................................................34
Effect ofSolidSolutionAlloyingElements.........................................................................36
Effect ofstress onKineticTransitions.................................................................................37
Theeffect of radiationon in-reactorcorrosion....................................................................40
HydrogenPick-Up................................................................................................................41
Zirconium Hydrides.............................................................................................................42
TheEffect ofHydrides onOxidation...................................................................................47
DelayedHydrideCracking...................................................................................................48
InvestigatingOxideTransitions usingIsotopicallyDopedSamples andNanoSIMS Analysis
..................................................................................................................................................52
Introduction..........................................................................................................................52
SamplePreparation..............................................................................................................53
NanoSIMS Instrumentation..................................................................................................61
DataCollection.....................................................................................................................64
DataAnalysis....................................................................................................................66
Results..................................................................................................................................70
16Oand18OAnalysis............................................................................................................71
Calculating18Oenrichment..................................................................................................83
Analysis of Hydrides............................................................................................................89
Boronanalysis......................................................................................................................91
Analysis ofSPPs..................................................................................................................94
Penetrationprofiledifferences between alloys..................................................................100
Thebarrierlayer.................................................................................................................112
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Residual Stress Analysis byRingCoreMillingandDigital ImageCorrelation...................114
Experimental Measurements ofResidual Stress intheOxide Layer: Previous Work.......116
MeasuringResidual Stress: Digital ImageCorrelation......................................................121
CalculatingStress...............................................................................................................121
Experimental procedure.....................................................................................................122
Samplepreparation.........................................................................................................122
FIB Procedure.................................................................................................................125
DataAnalysis.....................................................................................................................128
Noisereduction..................................................................................................................131
Results............................................................................................................................146
Stress evaluation.................................................................................................................164
Errorestimations................................................................................................................174
Discussion..........................................................................................................................175
Further work.......................................................................................................................177
PorosityAnalysis byCathodicPolarisation...........................................................................179
Conclusions............................................................................................................................193
FutureWork...........................................................................................................................196
References..............................................................................................................................201
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Literature Review
Introduction
Zirconium is agroup4transitionmetal usedextensivelyinthenuclearindustryforthe
claddingandsupport structures ofnuclear fuel elements insidewater-cooledreactors. The
operatingenvironment forthezirconium claddingis highlychallenging,demandingthat the
material retains its structural integrityunderconditions ofhightemperature,highneutron flux
andcorrosivesurroundings. Zirconium alloys are theleadingmaterial used inthis application
primarilybecauseofits lowneutroncapturecross section,whichpermits exchangeof
neutrons betweenfuel elements sustainingthenuclear chainreaction. Inordertocreate a
zirconium-basedalloywiththerequiredproperties,careful considerationmust begiventoits
composition,microstructureandmanufacture. In ordertoachievethis, all productionsteps
must beconsidered,andtheeffect theyhaveonthefinal product throughout its servicelife
[1].
Zirconium has fivecommonisotopes, threeofwhichoccurnaturally, givingit anaverage
atomicmass of91.244andatomicnumber40[2].Inits pureform,zirconium is soft and
ductile,solidat room temperature, alloyingelements cansignificantlychangethese
properties [3,4] .It also has ahighmeltingpoint of2128.15K[5],allowingnuclearplants to
operate at highertemperatures andthus moreefficiently.
Zirconium is typicallyextractedfrom Ilmeniteand Rutileas aby-product of titanium mining.
Global zirconium productionreached1440tonnes in2011[6].Refinedzirconium metal is
usuallyreferredtoas “commercial”purityinits naturallyextractedstate as it is mixedwith
betweenoneandfourpercent hafnium[7].
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Extractionofzirconiumoccurs mainlyas aby-product oftitanium refinement[8] viatheKroll
process [9].TheKroll process involves theapplicationofchlorine gas totheoreof
zirconium,which yields metal chlorides. Thesechlorides are extractedviafractional
distillation,andcanthenbereducedtothepuremetal.Forsome applications, theHunter
process [10] maybeused tomakeultra-purezirconium. IntheHunterProcess,impuremetal
is heatedinthepresence ofiodine,whichvolatises as ZrI ,Thevolatised gas is decomposed
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onahightemperaturefilament,depositingthepuremetal. TheKroll Process is thedominant
extractionmethodofzirconium today.
Fornuclear applications, the1-4wt%hafnium present in‘commerciallypure’zirconium
must beeliminated,as althoughthe chemistryofthetwoelements is verysimilar,theyhave
stronglydifferingneutroncross sections [11].“Reactor grade”zirconium is moreexpensive
but necessary, becauseevenasmall amount ofhafnium inthemetal will significantly
increasethetotal neutron capture cross sectionofthefuel rod cladding.Hafnium has a
neutroncross section600times greaterthanthat ofzirconium.
Separationofzirconium from hafnium is difficult duetotheirsimilarchemistry; most
processes exploit theslight disparityinthesolubilityofthechloridecompounds, zirconium
compounds tendingtobe slightlyless soluble. Inordertoachievethehighest possiblepurity,
iterativepurificationsteps maybe required.
Zirconium metal consists mainlyoftheHCP alpha phase,thoughtheBCC betaphaseis
observedat temperatures above1155K,or at lowertemperatures withtheadditionofcertain
alloyingelements tostabiliseit [12].Alphaandbetazirconium haveextremelydifferent
properties withregards tothediffusionandstorageofhydrogen,so understandingthe
occurrenceofthesephases is ofutmost importancewhendesigningfuel claddingelements.
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Zirconium possesses properties otherthanits neutroncross sectionthat makeit suitedtouse
innuclearreactors; most importantly,its corrosion resistance andits mechanical strength.
Whileneitheroftheseproperties areexceptional, theyaresufficient toallowthefuel
claddingelements toisolatethefuel pellets from theprimarycoolingwater [11].
Zirconium has adensityof6.52g/cm3,andYoung’s modulus of88GPa[13],althoughas the
tubes usedforfuel claddingtendtobeverythin,theweight ofzirconium is not ofprime
importance.
Theneutroncapture cross sectionofzirconium is 0.18barns, whichis some13times lower
thanthat ofsteel.This has manybenefits foruseinanuclearreactor, as the freeneutrons
producedfrom fissionreactions caneasilyaccess thefissionablematerial intheotherfuel
rods, meaningfewerneutrons are “wasted”.More efficient transmissionbetweenfuel rods
means moreneutrons are abletoinitiatefurtherfissionreactions, sustainingthechain
reaction.This allows reactors withzirconium basedcladdingtouseless enrichedfuel
comparedtootherreactortypes, whichis desirableforeconomicandnuclearproliferation
reasons.
Applications ofzirconiumintheNuclearIndustry
Zirconium was usedveryearlyinthehistoryofnuclear reactors, actingas a spacerinthe
EBR-1reactorin Idahosince1957[14,15],althoughit was not until later that it was usedas
afuel cladding. It hadbeenreviewedforuseinthereactors ofproposedsubmarines as early
as 1949. Inmanyearlyreactors, Magnox (magnesium non-oxidising)[16]was usedtoprotect
thefuel elements. Thedesign goal ofthe claddingwas toconcentratethe radioactivityinone
placetostopthecoolant from acquiringtoomuch activity.[14]
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