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Electrochemical Sensors for Controlling Oxygen Content and Corrosion Processes in Lead-Bismuth Eutectic Coolant—State of the Art

Authors:
Sergey N. Orlov
Sergey N. Orlov
Sergey N. Orlov
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Nikita A. Bogachev
Nikita A. Bogachev
Nikita A. Bogachev
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Andrey S. Mereshchenko
Andrey S. Mereshchenko
Andrey S. Mereshchenko
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Alexandr A. Zmitrodan
Alexandr A. Zmitrodan
Alexandr A. Zmitrodan
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Abstract and Figures

Controlling oxygen content in the primary circuit of nuclear reactors is one of the key tasks needed to ensure the safe operation of nuclear power plants where lead-bismuth eutectic alloy (LBE) is used as a coolant. If the oxygen concentration is low, active corrosion of structural materials takes place; upon increase in oxygen content, slag accumulates due to the formation of lead oxide. The generally accepted method of measuring the oxygen content in LBE is currently potentiometry. The sensors for measuring oxygen activity (electrochemical oxygen sensors) are galvanic cells with two electrodes (lead-bismuth coolant serves as working electrode) separated by a solid electrolyte. Control of corrosion and slag accumulation processes in circuits exploring LBE as a coolant is also based on data obtained by electrochemical oxygen sensors. The disadvantages of this approach are the low efficiency and low sensitivity of control. The alternative, Impedance Spectroscopy (EIS) Sensors, are proposed for Real-Time Corrosion Monitoring in LBE system. Currently their applicability in static LBE at temperatures up to 600 °C is shown.
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Sensors2023,23,812.https://doi.org/10.3390/s23020812www.mdpi.com/journal/sensors
Review
ElectrochemicalSensorsforControllingOxygenContentand
CorrosionProcessesinLead‐BismuthEutecticCoolant—State
oftheArt
SergeyN.Orlov
1,2,3
,NikitaA.Bogachev
1
,AndreyS.Mereshchenko
1
,AlexandrA.Zmitrodan
2

andMikhailYu.Skripkin
1,
*
1
InstituteofChemistry,Saint‐PetersburgStateUniversity,7/9UniversitetskayaEmb.,
199034St.Petersburg,Russia
2
FederalStateUnitaryEnterprise“AlexandrovResearchInstituteofTechnology”,72,KoporskoeShosse,
188540SosnovyBor,Russia
3
InstituteofNuclearIndustry,PetertheGreatSt.PetersburgPolytechnicUniversity(SPbSU),29,
PolytechnicheskayaStreet,195251St.Petersburg,Russia
*Correspondence:m.skripkin@spbu.ru
Abstract:Controllingoxygencontentintheprimarycircuitofnuclearreactorsisoneofthekeytasks
neededtoensurethesafeoperationofnuclearpowerplantswherelead‐bismutheutecticalloy(LBE)
isusedasacoolant.Iftheoxygenconcentrationislow,activecorrosionofstructuralmaterialstakes
place;uponincreaseinoxygencontent,slagaccumulatesduetotheformationofleadoxide.The
generallyacceptedmethodofmeasuringtheoxygencontentinLBEiscurrentlypotentiometry.The
sensorsformeasuringoxygenactivity(electrochemicaloxygensensors)aregalvaniccellswithtwo
electrodes(lead‐bismuthcoolantservesasworkingelectrode)separatedbyasolidelectrolyte.Con‐
trolofcorrosionandslagaccumulationprocessesincircuitsexploringLBEasacoolantisalsobased
ondataobtainedbyelectrochemicaloxygensensors.Thedisadvantagesofthisapproacharethe
lowefficiencyandlowsensitivityofcontrol.Thealternative,ImpedanceSpectroscopy(EIS)Sensors,
areproposedforReal‐TimeCorrosionMonitoringinLBEsystem.Currentlytheirapplicabilityin
staticLBEattemperaturesupto600°Cisshown.
Keywords:lead‐bismutheutecticcoolant;structuralmaterials;protectivefilm;corrosion;oxygen
activitysensors;impedancespectroscopy
1.Introduction
Fastneutronreactorswithleadorlead‐bismutheutecticcoolantareoneofthesix
promisingreactortechnologiesselectedwithintheframeworkoftheGIF(GenerationIV
InternationalForum)internationalprojectasthebasisfornuclearpowerofthefuture[1].
Theadvantagesoflead‐bismutheutecticcoolantarearelativelylowmeltingpoint(125
°C),ahighboilingpoint(above1700°C),lowchemicalactivitycomparedtosodiumand
sodium‐potassiumcoolant.Allthesefactorsdetermineafairlywiderangeofnuclear
powerplantswithlead‐bismutheutecticcoolantcurrentlybeingdevelopedinRussia[2],
USA[3],China[4,5]andJapan[6].Atthesametime,thelead‐bismutheutecticcoolantis
highlycorrosion‐aggressiveinrelationtothestructuralmaterialsofthefirstcircuit[7]
becauseofthesignificantsolubilityofstructuralmaterialscomponentsinlead.
Thestructuralmaterialsoftheprimarycircuitareprotectedfromthedestructionby
oxidefilmsformedontheirsurface[8].Thestructureandprotectivepropertiesofthese
filmsdependonthechemicalcompositionofstructuralmaterials,flowrateandtemper‐
atureoftheLBEand,primarily,ontheoxygencontentinthecoolant[9].Anotherproblem
thataccompaniestheexplorationofreactorswithlead‐bismutheutecticcoolantisslag
Citation:Orlov,S.N.;
Bogachev,N.A.;
Mereshchenko,A.S.;
Zmitrodan,A.A.;Skripkin,M.Y.
ElectrochemicalSensorsfor
ControllingOxygenContentand
CorrosionProcessesin
Lead‐BismuthEutectic
Coolant—StateoftheArt.Sensors
2023,23,812.https://doi.org/10.3390/
s23020812
AcademicEditors:MatjažFinšgar
andTinkaraMastnak
Received:24November2022
Revised:7January2023
Accepted:9January2023
Published:10January2023
Copyright:©2023bytheauthors.Li‐
censeeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon‐
ditionsoftheCreativeCommonsAt‐
tribution(CCBY)license(https://cre‐
ativecommons.org/licenses/by/4.0/).
Sensors2023,23,8122of14
accumulationduetooxidationofthecoolantifoxygencontentinthecircuitistoohigh.
Insufficientattentionthatwaspaidtothisissueattheearlystageofoperationledtoa
seriousradiationaccidentwiththemeltingofthecoreontheK‐27nuclearsubmarineof
Project645[10].
Thus,anadequatechoiceofoperatingrangesandeffectivecontroloftheoxygencon‐
tentintheprimarycircuitisthebasisfortrouble‐freeoperationofnuclearreactorswhere
lead‐bismutheutecticcoolantisexplored.Inaddition,animportantissuerelatedtothe
aggressivenessofLBEinrelationtostructuralmaterialsistheorganizationofcorrosion
monitoring.
Theaimofthisreviewistoanalyzeexistingapproachestothemeasurementofoxy‐
gencontentandcontrolofcorrosionandslagaccumulationprocessesinLBE.Specialat‐
tentionispaidtotheuseofoxygenactivitysensorsandalternativeelectrochemicalmeth‐
odsformonitoringcorrosionprocessesandslagaccumulationincircuitswithLBE.An
additionaltaskofthereviewistofamiliarizetheinternationalscientificcommunitywith
theresultsoftheworkofRussianscientists,currentlypublishedmainlyinRussian.
2.Discussion
2.1.TheOperatingRangeofOxygenConcentrations
Aswasmentionedabove,theprotectivepropertiesoffilmsformeduponoxygenin‐
hibitionofcorrosionaredeterminedbytheconditionsoftheirformation—thetempera‐
tureandflowrateoftheLBE,thetemperaturegradientinthecircuitandtheconcentration
ofoxygeninthecoolant.Dependingontheoxygencontentinthecoolant,theconditions
inthecircuitcanbedividedintothreegroups[9]:
(1) Highoxygencontent:from1×10−6%to1×10−3%byweight;
(2) Averageoxygencontent:from1×10−6%–1×10−7%byweight;
(3) Lowoxygencontent:≤1×10−7%.
Iftheoxygencontentinthecircuitislow,thestructuralmaterialsareactivelydis‐
solved.Asimilarsituationisalsoobservedattemperaturessohighthattheresultingox‐
idelayercannotpreventthedissolutionofstructuralmaterials[11].Atanaverageoxygen
concentration,anoxidefilmformsonthesurfaceofstructuralmaterials.Atthesametime,
oxygenreactswiththedissolvedcomponentsofsteel.Theresultinginsolubleoxidesof
corrosionproductsaredepositedonthesurfaceofthesteel,asaresultofwhichasingle‐
layeroxidefilmofferro‐chromiumspinel(Fe2‐xCrx)2O4isformedonthesurface[11].
Atahighoxygenconcentration,theoxidationofironatomsdiffusedoutwardfrom
structuralmaterialtakesplaceontheoxidefilm/LBEinterface,resultingintheformation
ofanexternaloxidelayerofmagnetite.Asnotalloxygenisboundinthisreaction,the
remainingmoleculesmigratethroughtheoxidelayerandreactwithbothironandchro‐
miumunderthefilm,increasingthethicknessoftheferro‐chromiumspinellayer.Asa
result,atwo‐layeroxidefilmisformedonthesurfaceofthestructuralmaterial,inwhich
onlytheinnerlayerhasprotectivefunctions[12,13].
Theprocessesofcorrosionandoxidationofstructuralmaterialsintheflowofliquid
metalcoolantdifferfromthecaseofstaticLBE.Klok[14]hasshownthatthecorrosion
rateisproportionaltov0.6−0.8,wherevisthelinearvelocityofLBE.Theincreaseinthe
corrosionratewithanincreaseintheflowrateofthecoolantisexplained,firstly,bydam‐
ageoftheoxidelayerduetoerosionand,secondly,byacceleratingthediffusionofcorro‐
sionproductsfromthesurfaceofthestructuralmaterialintothevolumeofthecoolant.
Theeffectoftheflowvelocityonthestructureoftheoxidefilmisdemonstratedbythe
resultsofWiesenburgandhiscoauthors[15].Thecorrosionresistanceofmartensiticsteel
T91wasanalyzed.Itwasshownthat,atanoxygencontentof10−8–10−10%,acoolanttem‐
peratureof450°C,aflowrateof1m/sfor2000hoftheexperimentandthepenetration
ofLBEintothesteeltoadepthof10micronsoccurred.Atanoxygencontentof10−8%,a
coolanttemperatureof300°Candaflowrateof1m/sfor10,000hoftheexperiment,
neithertheformationofanoxidefilmnorthedissolutionofsteelwasobserved.Whenthe
Sensors2023,23,8123of14
temperatureofthecoolantroseto450°Cundersimilarconditions,athinlayerofoxide
filmwasformedonT91steel,preventingitscorrosion.Atanoxygencontentof10−6%,a
coolanttemperatureof480°C,aflowvelocityof1.3m/sandtwo‐layer(magnetiteand
ferro‐chromiumspinel)filmwithathicknessof28micronswasformedonsteelduring
6600hoftheexperiment.Anincreaseintemperatureupto550°Cundersimilarcondi‐
tionsledtoanincreaseinthefilmthicknessupto45microns.Withanincreaseintheflow
rateupto2m/s(oxygencontent10−6%,coolanttemperature550°C)for15,000hofthe
experiment,asingle‐layerspinelfilmwithathicknessof45micronswasformedonT91
steel.Additionally,oneoftheexperimentsconductedintheLBEstream(1m/s),atan
oxygenconcentrationof10−8%ontheLINCEloop(Madrid),shouldbenoted[16]—when
thesamplewasheldfor5000h,thedissolutionofT91steel(upto300microns)wasob‐
served,althoughneitherforsampleswithanexposureof2000h,norforsampleswithan
exposureof10,000,suchapatternwasnotobserved.Theincreasedcorrosiondamagein
theconsideredexperimentwasexplainedbytheturbulentflowofLBE,whichconfirmed
theinfluenceofthecharacteristicsofthecoolantflowonthestateofstructuralmaterials.
Theareasofleadoxidesandprotectivefilmsformationdependingonthetempera‐
tureandoxygenconcentrationintheLBE[17,18]areshowninFigure1.
Figure1.ConditionsofleadoxideandprotectivefilmsformationaccordingtoMartynov[17]and
Müller[18].Theredlineindicatesthemaximumcoretemperatureforcurrentlydesigned
reactorswithLBE.
Sensors2023,23,8124of14
AsonecanseefromthedatapresentedinFigure1,ifthetemperatureoflead‐bismuth
eutecticcoolantwasabove350°Candoxygenconcentrationwasabove(7–8)×10−6wt%,
leadmonooxidewasformedinthecircuit.Theoptimaloxygenconcentrationvalues,
whichensuredtheformationofstrongprotectivefilmsandatthesametimeprevented
theformationofslag,areinanarrowrangewith(1–4)×10−6wt%width[19].Currently,
attemptsarebeingmadetoexpandthisrangebymodifyingthecompositionofstructural
materialsandapplyingthincoatingsenrichedwithelementswithahighaffinityforoxy‐
gentotheirsurface.ItwasshownbyDingetal.[20]thattheenergiesoftheelement‐
oxygenbondintheLBEmediumforthepairsFe–O,Cr–O,Ni–O,Al–OandSi–Ohadthe
followingvalues:−0.93;−1.26;−0.56;−1.45and−1.75eV,respectively.Thus,silicon,chro‐
miumandaluminumformedmorestableprotectiveoxidefilmscomparedtoirononthe
surfaceofstructuralmaterials.Aluminumiscurrentlythemostwidelyusedmaterial
whenapplyingprotectivecoatings.
Alltheprincipalindustrialmethodsofsurfacefilmsformationwereusedforcoating
structuralmaterials.Amongthemwerechemicalandphysicaldepositionfromthegas
phase(batchcementing,pulsedlaserspraying,vacuumarctechnology,thermalspray‐
ing),mechanicalmillingandsoon[21–28].Themainproblemoftheapplicationofthese
coatingstoinhibitcorrosionofstructuralmaterialsintheLBEenvironmentwasensuring
theirsufficientmechanicalstrengthandadhesiononthesurfaceofthemainalloy[27].
Themethodofhigh‐currentpulsedelectronbeamsofmicrosecondrange[29–31]isa
promisingapproachtocreateprotectivecoatingsonthesurfaceofstructuralmaterialsin
nuclearreactorswithLBE.Whentheelectronbeamactedonthepreliminarylayerofalu‐
minum,itsmeltingwiththestructuralmaterialtookplace.Asaresult,anintermetallic
phaseoftheAlxFeycompositionwithadepthof20–30micronsformedonthesurface.The
mixingprocesshadavortexcharacter.Becauseoftheoxidationoftheintermetallicphase
inLBE,anextremelydenseultrathinAl2O3oxidefilmformedonthesurfaceofthemate‐
rial,whichpersistedduringthetestingprocess.Thisfilmcompletelypreventedliquid
metalcorrosionatanoxygencontentof10−8–10−7wt%andallowedtheexpansionofthe
oxygenrangeofnormaloperationofreactorswithheavyliquidmetalcoolants[32].
Thus,maintainingtheoxygencontentatagivenlevelintheLBEensuredthecorro‐
sionresistanceofstructuralmaterialsthatdeterminestheactualityofthemonitoringof
theoxygenconcentrationinthecoolant.
2.2.MeasurementofOxygeninaLead‐BismuthEutecticCoolant
Oneofthemostcommonwaystomeasureoxygenconcentrationinliquidsisthe
electrochemicalone[33].Theconventionalelectrochemicalsensorstomeasureoxygenac‐
tivityinalead‐bismuthcoolantareagalvaniccellwithtwoelectrodesseparatedbyasolid
electrolyte.InsensorsutilizedatnuclearplantsinRussia,thereferenceelectrodeisasat‐
uratedoxygensolution(withaconstantoxygenactivityequalto1)inmetalswithalow
meltingpoint.Usually[34–39],Bi‐Bi2O3system(meltingpoint544K)isusedfortherefer‐
enceelectrode.Theoperatingtemperaturerangeoftheoxygensensorcanbeexpanded
bysubstitutionofthissystemonIn‐In2O3[40,41](meltingpoint433K)eitherSn‐SnO2
[42,43](meltingpoint505K)systems.
Alternativeoptionsforreferenceelectrodesare:
Air/metal(Pt)[43,44]orperovskiteoxidee.g.,LanthanumStrontiumManganite
(LSM)andLanthanumStrontiumManganite‐GadoliniumDopedCeria(LSM‐GDC)
[45,46];
Solidmetal/metaloxidemixturese.g.,Fe/Fe3O4[47–50],Cu/Cu2O[51].
ThemaindisadvantageofthePt‐airreferenceelectrodeisthehighminimumtem‐
peratureofthereadings—about400°C.Atlowertemperatures,especiallyintheareaof
lowoxygenconcentrations,asignificantmeasurementerrorispossible[52,53].
Sensors2023,23,8125of14
Todate,thepossibilityofusingperovskitesforhigh‐temperaturesensorshasbeen
proven[54].Atthesametime,asignificantdisadvantageofallairelectrodesisthepossi‐
bilityofcontactofthecoolantwiththeexternalatmospherewhentheinsulatinglayerof
theelectrodeisdamaged[44].
Liquidmetal/metaloxidereferenceelectrodesmightcausethemechanicalfailureof
thesolidelectrolyteoncethemeltingpointofthemetalphaseiscrossede.g.,indiummelts
at157°CandLBEmeltsat125°C[44].Atthesametime,animportantadvantageofthe
liquidbismuth/bismuthoxidereferenceelectrodeistherefinementofitstechnologyand
extensiveexperienceinapplicationofthesensorsbasedonitbothinreactorswithstatic
andwithcirculatedcoolantatnumerousnuclearindustryenterprisesinRussia(JSC“NI‐
KIET”,Moscow.CRISM“Prometey”,Saint‐Petersburg,OKB“Giropress”JSC,Podolsk
etc.)andabroad(e.g.,ENEAcenteratBrazimone,Italy)[49].
Asaworkingelectrode,alead‐bismutheutecticcoolantacts,theoxygenactivityin
whichislessthanorequalto1.Solidelectrolytesseparatingtheworkingandreference
electrodeinthesensorsaresolidsolutionsofthecompositionMeOx*Me’Oy.Thesolutions
arebasedonrefractoryoxidesZrO2,HfO2,ThO2,stabilizedbytheadditionofCaO,Y2O3
andSc2O3[50,55].Theseoxideadditivesprovideionicconductivityandmechanical
strengthofelectrolytesduetostabilizationofthecubiccrystallatticesimilartofluorite.
Thehighestionicconductivityofelectrolytesisachievedatconcentrationsofstabilizing
additivesof7–15mol%[50].
Theseoxidesdemonstraten‐typeconductivity,whichispreservedinsolidsolutions.
Asaresult,solidelectrolytesinoxygensensorshavemixedion‐electronconductivity.To
accountforthisfact,theaveragetransfernumberswereused—iontiandelectronte[50].
Thesenumbersareequaltothefractionsofionicandelectronicconductivity,respectively.
𝑡𝑡 =1(1)
Theionictransfernumbersoftheoxygensensorelectrolyteweredeterminedexper‐
imentally.Itisconsideredtobeaconditionedsensoriftiisintherange0.95–1.
Whenthesensorwasoperating,theionizationreactionofoxygenatomsproceeded
onthereferenceelectrode,followedbythetransferofanionsthroughtheelectrolyteto
theworkingelectrode,wheretheanionsweredischarged[56].Itwasassumedthatoxy‐
gendissolvedinaliquidmetalandinareference,electrodewasinatomicform[57]and
wassubjecttotheSievertslawthatallowsthepredictionofitssolubilityinmetals.
OxygenactivityinLBEcanbedeterminedfromtheNernstequation:
𝐸 𝑅𝑇
2𝐹ln 𝑎
𝑎(2)
whereETisthecalculatedEMFofthegalvaniccell,V;Ristheuniversalgasconstantequal
to8.31J/(mol×K);FistheFaradaynumberequalto96,500C/mol;Tisthetemperatureof
thecoolantinthesensorarea,K;aref—thermodynamicactivityofoxygeninthereference
electrode;aLBE—thermodynamicactivityofoxygeninLBE.
ThecalculatedEMFofagalvaniccellcorrelateswiththemeasuredoneasfollows:
Eo=ti∙ET(3)
ForasensorwithabismuthreferenceelectrodeinLBE,therelationshipbetweenthe
valueofthemeasuredEMFandactivityofoxygencanbeexpressedintheform[58]:
Eo=ti∙(0.088−9.91∙10−5T(0.18+lgaLBE)),(4)
wherethermodynamicactivityofoxygeninLBEcanberelatedtoitsconcentrationbythe
simpleequation:
aLBE=CLBE/Cs(5)
whereCLBEistheoxygenconcentrationinLBE,mass%;Csisthesolubilityofoxygenin
LBE,mass%.ThesolubilityofoxygeninLBEcanbecalculatedaccordingtotheequation:
Sensors2023,23,8126of14
Cs=A−B/T(6)
whereAandBareempiricalcoefficients.ThevaluesofparametersAandBobtainedby
variousauthorsweresummarizedbyAskhadullin[49].TheyarelistedbelowinTable1.
Table1.ParametersforcalculatingthelimitsolubilityofoxygeninLBE[49].
AuthorYearTemperatureRange,°CAB
Martynov1998400–7001.203400
Ghetta2004300–5003.274852
Muller2003200–6002.524803
Gouruouau2004350–5003.344962
Ganesan2006540–7402.424287
Pityk[56]hasmentionedthattheuseoftheabovementionedequationtocalculate
thelimitsolubilityofoxygeninLBEisnotmetrologicallyjustified,and,therefore,the
oxygenconcentrationvaluesobtainedusingthisequationcanonlybeusedasillustrative
andhavenomeaningasphysicalquantities.
AnotherproblemarisinguponuseofthermodynamicactivitysensorsinLBEwasthe
polarizationoftheelectrodes.Thefollowingpolarizationfactorsthatchangedtheelec‐
trodepotentialcanbeimplementedontheworkingelectrodeofthesensor[50]:
Oxidesdepositiononthesurfaceofthesolidelectrolyte;
adsorptionofmetallicimpuritiesontheelectrolytesurface;
formationofmicrocracks;
reductionoftheresistanceoftheinterelectrodeinsulator;
thepresenceofopenmicroporesonthesurfaceoftheelectrolyte.
Polarizingfactorsincreasedtheioniccurrentinthesensorcircuitandtheoxygen
activityontheelectrolytesurface,whichledtoanincreaseintheelectrodepotentialby
thepolarizationvalueη.Therefore,themeasuredEMFoftheoxygensensorwasreduced
bythesamevalue:
E=E0−η.(7)
ToaccountforthemagnitudeofpolarizationMusikhin[50]calculatedthecorre‐
spondingfunctionsthatallowedthemodelingandpredictingofthepolarizationpro‐
cessesontheworkingelectrode.
Oxygenactivitysensorsaredesignedtoworkinacorrosiveenvironmentwithaddi‐
tionalexposuretoradiationandneutronflux,whichcancauseadditionaldamagetosen‐
sormaterials[59].Okuboandco‐authors[60,61]havestudiedeffectoftheirradiationofa
solidelectrolyte—zirconiumdioxidestabilizedbytheadditionofyttriumoxide—by
gammaradiationandelectronswithanenergyof100keV.Theyshowedthatbend
strengthwasnotaffectedbytheirradiationevenathighdoseof10mGy.Atthesame
time,forpartiallystabilizedzirconiawithyttriacontentof3%byweight,apartialtransi‐
tionofthemonoclinicphasetothetetragonalphasewasobserved.
Despitetheharshoperatingconditions,thelifetime(servicelife)ofsensorsinLBEis
4000hormore[62].Tocontrolpossiblesensorfailures,itwasproposedtoinstallatleast
threeDACsinthecircuit[63].Inthiscase,beforemeasuringtheoxygenactivityinthe
coolant,itwaspossibletochecktheoperabilityofthesensors.Thistestconsistedinmeas‐
uringthepotentialdifferencebetweenthereferenceelectrodesofapairofsolid‐electrolyte
sensors.Ifthisdifferencewasconstantwithintheabsolutemeasurementerror,thesesen‐
sorswereoperational.Ifthisconditionwasnotmet,thenbysequentiallysortingthrough
pairsofsensors,aninoperablesensorwasdetectedandexcludedfromfurtherconsidera‐
tion.
Tosummarizetheresults,themeasurementofoxygencontentinthecoolantwas
currentlycarriedoutbythepotentiometricmethod;thehardwareaspectsofthemethod
Sensors2023,23,8127of14
werewelldeveloped.Atthesametime,questionswereraisedabouttherelationshipbe‐
tweenthevaluesobtainedbymeansofsensorsandtheactualoxygenconcentrationinthe
coolant,aswellasconfirmationoftheoperabilityofsensorsinstalledintheflowcircuits
withLBE.
2.3.ControlofCorrosionandSlagFormationProcessesinLBEUsingOxygenActivitySensors
Diagnosticsofthestateofstructuralmaterialsandprotectivecoatingsontheirsur‐
facesduringtheoperationofnuclearpowerplantswithLBEinRussiawascarriedout
duringtheejectionofgasmixturesinisothermalmodeat300–330°C[64].Twovariants
oftheintroducedgasmixtureswereprovided:He+O2andHe+H2O(g)+N2+H2.
Whenthefirstcompositionwasused,achangeinthethermodynamicactivityofox‐
ygenwasconsideredanindicator.Inthecaseofsignificantamountsofnon‐oxidizedcor‐
rosionproductsofstructuralmaterialspresentinthecoolant(liquidmetalcorrosion)or
violationofprotectivecoatingsonthesurfaceofthecircuittakesplace,therateofdeoxi‐
dationofthecoolantincreased.Onthecontrary,inthepresenceofsignificantamountsof
slags(oxidecompounds)inthecircuit,therateofdeoxidationdecreased.Ifareducinggas
mixturewasusedindiagnosticmode,therateofincreaseintheconcentrationofhydrogen
intheprotectivegaswasmonitored.Inthepresenceofoxideslagsinthecoolant,therate
ofhydrogenincomingdecreased,inthepresenceofnon‐oxidizedcorrosionproducts,it
increased.Infact,whenthisapproachwasused,thecontrolwasratherqualitativeand
didnotprovidearelationshipwiththequantitativevaluesofthecontentofcorrosion
productsandslaginthecoolant.
Diagnosticsofthestateofthesurfacesofstructuralmaterialsofpromisingnuclear
powerplantswithLBEwasproposedtobecarriedoutonthebasisofmonitoringthe
amountofdissolvedoxygensuppliedfromthemasstransferapparatus[65].Anincrease
inthisindicatorreflectedtheintensificationofcorrosionprocesses,adecreaseistheevi‐
denceoftheaccumulationofslagsortheappearanceofanadditionalsourceofoxygen.
Establishingtherelationshipofthesechangeswiththeratesofcorrosionprocessesand,
moreover,thelocalizationofcorrosionsites,thedeterminationofcorrodingequipment
waspracticallyimpossible.
Toincreasetheinformativenessofthecontrolofcorrosionandslagaccumulation
processesbasedonthedataoftheoxygenactivitysensors,IvanovandSalaevhaveana‐
lyzedtherelationshipbetweenthekindoftemperaturedependencesofoxygenactivity
andthecontentofchemicallyactiveimpuritiesinthecoolant[66,67].Thedeviationfrom
thelinearformofthedependenceofthereadingsoftheoxygenactivitysensoronthe
temperatureofthecoolanttakesplaceasaresultofachangeinthephysico‐chemicalstate
ofthecoolantassociatedwiththeappearanceofimpuritiesinit.Theresearcherssupposed
that,inthefuture,arelationshipwillbedevelopedthatquantitativelylinkstheoxygen
activitysensorreadingsobtainedinthemodesofthermalcyclingofthecoolantwiththe
contentofimpuritiesinthecoolant,butatthemomentthisworkhasnotbeencompleted.
Tosummarizetheresults,thedevelopmentofaquantitativerelationshipbetween
theindicationsofoxygenactivitysensorsandcorrosionprocessesinthecircuitwithLBE
isacomplexandnon‐trivialproblem,thesolutionofwhichhasnotyetbeenobtained.
2.4.ImpedanceSpectroscopyApplicationtoMonitorCorrosionProcessesinLBE
Atthemoment,theElectrochemicalImpedanceSpectroscopymethodiswidelyused
tomonitorthecorrosionprocessesinvariousenvironments[68–72].Thismethodisused
tostudythestructureofcorrosiondepositsandthemechanismoftheirformation[73],the
resistanceofcoatingsonthesurfaceofmaterials[74]andtheeffectivenessofcorrosion
inhibitors[75].TheadvantageoftheImpedanceSpectroscopymethodisitsabilitytotrack
changesinthestateofthemetal/liquidmetalinterface,whichisextremelyimportantin
thecaseofusingoxidefilmstoinhibitcorrosionprocessesinLBE.
Electrochemicalimpedancespectroscopyisbasedonmonitoringtheresponseofan
electrochemicalsystemtoasinusoidaldisturbanceofsmallamplitude(severalmV)ina
Sensors2023,23,8128of14
widefrequencyrange(10
7
to10
−4
Hz).Quantitativeanalysisofthefrequencydependence
oftheimpedanceonthebasisoftheselectedequivalentcircuitmakesitpossibletointer‐
pretitselementsinaccordancewiththephysicochemicalnatureoftheprocessesoccurring
ontheelectrodes[76,77].
Initially,theapplicationofthismethodtothestudyofoxidefilmsinLBEwaspro‐
posedbyLillardandStubbinsin2004–2008[78–80].Asmallsinusoidalvoltageexcitation
wasmeasuredbetweenthesampleandreferenceelectrodeinathree‐electrodeconfigura‐
tion,wherethecurrentfromtheworkingelectrodepassedthroughacounterelectrode
andtheimpedancewasmeasuredasafunctionoffrequency.Inthefirstsetoftheexper‐
iments,theworkingelectroderepresentedametalrodinaceramictube[81]andimped‐
ancespectroscopyprobewasformedbypushingastainless‐steelweldingroddownthe
centerofaceramictube,untilthetipoftherodpenetratedveryslightlyfromtheendof
thetube.AsteelreferenceelectrodewasalsoplacedintheLBE.Thedifferencebetween
theimpedancesmeasuredfromthesteelwithandwithouttheoxidelayersindicatedthe
existenceoftheoxidelayersinthefirstcase,andthemagnitudeand/orkindofdifference
yieldedinformationaboutprocessesoccurringontheelectrodes[77].Thisdevicepro‐
ducedreasonableresultsonlyattemperaturesupto200°Cor300°C,whichwasnot
enoughtoorganizereal‐timecorrosionmonitoringinthecircuitsofnuclearreactorswith
LBE.
Tosolvethisproblemthemodificationofworkingelectrodewassuggested[81,82].
Themodifiedelectrodeconsistedofa410stainless‐steeldisk,completelycoatedwithalu‐
minaexceptfortwosmallareasonthebackside,forelectricalconnections,andonesmall
areaonthefrontside,actingastheactiveareatobeexposedtotheLBE.Thisactivearea
wasacircleof0.25‐inchdiameter.Thisdiskwasthenpushedontothemostlyclosedbot‐
tomofastainless‐steelcylinder;thebottomhada0.375‐inch‐diameterholedrilled
throughittoallowtheactiveareaofthedisktocommunicatewiththeoutsideofthe
cylinder.
Thisimprovementofworkingelectrodemadeitpossibletoraisetheoperatingrange
oftheimpedancespectroscopymethodinSVTtoatemperatureof600°C.Kondo[83,84]
hasshownthattheapplicationofthismethodinaleadcoolantispossibleuptoatemper‐
atureof500°C.
AsimplifiedRandleselectricalequivalentcircuit(EEC),withouttheWarburgdiffu‐
sionelement[84],wasusedforequivalentcircuitfittinginliquidmetal,asshowninFig‐
ure2.Forequivalent‐circuitfitting,aparallelcombinationofaresistorandconstant‐phase
element(CPE)wasused.TheresistorandCPEinthiscircuithavebeenassignedtothe
resistanceandcapacitanceoftheoxidelayer.
Figure2.Electricalequivalentcircuitusedtofitimpedancedata.
CPEImpedanceZ
CPE
[78]isequalto:
𝑍
1
𝐴
󰇛𝑖𝑤󰇜(8)
wherew—frequency,Hz;A—frequencydependentparameter;n—mathematicalconstant
withoutobviousphysicalsense.
Theequivalentcapacitanceofthesurfaceoxidefilmcanbecalculatedas
Sensors2023,23,8129of14
𝐶𝑅


𝐴
(9)
whereRoxide—polarizationresistanceofthesurfaceoxidefilmformedonsample,Ohm.
AlltheparameterscouldbeacquiredfromthefittingoftheNyquistplot,basedon
theexperimentaldata.TheNyquistplotisacomplexohmicplane,impedancerealcom‐
ponentZ′wasdepositedalongthexaxis,andtheimaginarycomponentofresistanceZ′′
wasdepositedalongtheyaxis
𝑍𝑍
󰆒𝑖𝑍󰆒󰆒(10)
𝑍󰆒 𝑍cos 𝜑(11)
Z′′ 𝑍sin 𝜑(12)
whereZ0isvoluminousimpedanceofthesampleandφisphaseangle.
WhenconstructingtheimpedancespectruminNyquistcoordinates,toobtaineach
pointofthecurve,thevaluesZ′andZ″arecalculatedfromthedataarray(ω,Z0,φ)orthe
vectorZ0isplacedontheplaneatanangleφtothexaxisandthepositionofitsendpoint
isfixed(thecoordinategridmustbesquare).
TheexampleofNyquistplotfortheneatironinLBEandtheresultsoffittingtheEIS
datausingtheZviewsoftware[85]areshownatFigure3below.Measuredandfitted
impedanceagreeoneanotherfairlywell.
Figure3.NiquistplotforironinLBE:experimentaldataaremarkedwithsquares,solidlinerepre‐
sentsfittinone.Fittingparametersare:n=0.964,Q0=8.67∙10−10S∙sn/cm2,RLBE=3.02∙10−4Ω/cm2,Roxide
=2.71∙104Ω/cm2.
Fromequivalentcapacitancethethicknessofoxidefilmcanbecalculatedasfollows:
𝑑𝜀𝜀𝑠
𝐶(13)
wheredisoxidelayerthickness,μm;ε—absolutepermittivity,(F/m);ε0—vacuumpermit‐
tivity,(F/m);s—theelectrodearea(surfaceexposureareaofthespecimeninLBE),cm2.
Qiuetal.[86]consideredtheimpedancepropertiesofironoxidefilmsinLBE.They
showedthattheimpedancevaluewasproportionaltothethicknessofthefilm.Whenthe
thicknessoftheironoxidefilmwaslessthan50nm,theimpedancevaluewasinsignifi‐
cant.Withanincreaseinthethicknessoftheoxidefilmto200nm,theimpedancevalue
increasedto1kΩcm2.Disruptionoftheoxideviascratchingledtoanimmediatelossof
Sensors2023,23,81210of14
impedanceduetoshortcircuiting.Thisfacttestifiestothepossibilityofimpedancespec‐
troscopysensorsapplicationtodetectdamagetotheoxidefilmonstructuralmaterialsin
LBEinrealtime.
Thenextsteptowardstheintroductionofimpedancespectroscopyintothecorrosion
monitoringsystemofstructuralmaterialsofthefirstcircuitofthenuclearreactorswith
LBEcoolantisthetestingofthismethodinLBEstreamsatcirculationstandsthatallow
ustodrawthefinalconclusionabouttheapplicabilityofthiskindofsensors.
3.Conclusions
Theelementsthatmakeupausteniticandmartensiticsteelswereeffectivelydis‐
solvedinlead‐bismuthcoolant.Oxidefilmsprotectedthestructuralmaterialsofthe
primarycircuitfromdestruction.
Topreservetheprotectivefunctionsofoxidefilms,astableoxygenconcentrationwas
maintainedintheprimarycoolantinanarrowrangefrom1×10−6%to4×10−6%or
upto1×10−8–1×10−7%whenusingprotectivecoatingsmadeofaluminumoxide.
MeasurementoftheoxygencontentintheLBEwascarriedoutbythemethodof
potentiometry.Atthemoment,thismethodisfullydeveloped,butdoesnotexclude
anumberofmetrologicaldifficultiesininterpretationandensuringthereliabilityof
measurementresults.
Theuseofoxygenactivitysensorstomonitortheprocessesofcorrosionandslag
formationinLBEwashamperedbythelackofaquantitativerelationshipbetween
thesensorreadingsandthecontentofimpuritiesinthecoolant.
Toobtaindataonthestateofprotectiveoxidefilms(includingthicknessandme‐
chanicaldamage)onstructuralmaterialsinstaticLBEattemperaturesupto600°C,
impedancespectroscopysensorscanbeused.
Thenextstagefortheintroductionofimpedancespectroscopysensorsintothecor‐
rosionmonitoringsystemofthefirstcircuitofpromisingnuclearreactorswithLBE
istoconfirmtheiroperabilityinflowingLBEloops.
AuthorContributions:Conceptualization:S.N.O.andM.Y.S.;methodology:A.A.Z.;software:
N.A.B.;validation:A.S.M.;formalanalysis:A.S.M.;investigation:S.N.O.;resources:A.A.Z.;data
curation:M.Y.S.;writing—originaldraftpreparation:S.N.O.;writing—reviewandediting:M.Y.S.;
visualization:N.A.B.;supervision:A.S.M.;projectadministration:N.A.B.;findingacquisition:
M.Y.S.Allauthorshavereadandagreedtothepublishedversionofthemanuscript.
Funding:Thisresearchreceivednoexternalfunding.
InstitutionalReviewBoardStatement:Notapplicable.
InformedConsentStatement:Notapplicable.
DataAvailabilityStatement:Notapplicable.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterests.
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... If the oxygen concentration in LBE is more than 10 −6 wt%, the elemental O in liquid LBE will react chemically with the metal elements (Fe, Cr, etc.) in the steel material, thus causing oxidation corrosion on the surface of the structural steel [8,11,[13][14][15][16][52][53][54][55][56]. If a continuous and dense oxide film is produced on the surface of the structural steel, then oxidative corrosion will be effectively blocked. ...
... At high concentrations (C O > 10 −6 wt%) [56], the structural steel undergoes an oxidation reaction (Figure 1b). If multiple layers of poorly adherent oxide films are formed, these films can delaminate and oxidize the underlying surface. ...
... At low levels of oxygen in the LBE (C O < 5 × 10 −7 wt%), dissolution corrosion occurs mainly in structural steel. On the contrary, if the oxygen content in LBE is high (C O > 10 −6 wt%), oxidative corrosion will increase and Pb-O compounds will be formed, contaminating the liquid LBE and clogging the pipeline [56]. When the LBE's oxygen content is in this range, which can produce a stable Fe 3 O 4 that is not formed between Pb-O compounds, then a double-structured oxide layer will be formed. ...
A Review of Corrosion Behavior of Structural Steel in Liquid Lead–Bismuth Eutectic
Article
Full-text available
  • Jun 2023
Liquid lead–bismuth eutectic alloy is one of the candidate coolants for fourth-generation nuclear power systems because of its good physical and chemical properties, neutron economic performance, and safety. However, the compatibility between the coolant and structural steel is still the main factor restricting its large-scale industrial application in the nuclear energy field. Structural steel in a liquid lead–bismuth eutectic alloy for a long time would cause severe corrosion. The erosion of structural steel by high-flow-rate liquid lead–bismuth alloy will lead to a more complex corrosion process. This paper mainly reviews the corrosion characteristics of liquid lead–bismuth and the corrosion behavior of structural steel in liquid lead-bismuth eutectic. The main methods of inhibiting liquid lead–bismuth corrosion are summarized, and future research directions are suggested.
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... With respect to iron-based material calculations of the adsorption energy of liquid metal atoms and the escape energy of iron atoms on iron surfaces, bismuth exhibits a greater capacity for adsorption and dissolution corrosion compared to lead on Fe surfaces [8] According to the current literature, two reliable methods have been identified for mitigating the corrosion of steels in lead-bismuth Pb-Bi systems. The first method involves maintaining the oxygen concentration within a specified range [9], and the second method entails applying corrosion-resistant coatings to the surface of the materials [10]. ...
... After assembling, the experimental cells were tightly closed, evacuated, filled with high-purity argon (99.999%), and placed in vertical tube furnaces preheated to the desired temperature. (3); nickel heat reflecting screens (4); titanium getter (5); crucible (6); test samples (7); liquid metal (8); thermocouple (9). ...
Corrosion of Steels in Liquid Bismuth–Lithium Alloy
Article
Full-text available
  • Oct 2024
The corrosion resistance of several types of steel (AISI 410, 321, 316L, 904L) was determined in a liquid Bi-Li (5 mol.%) alloy (BLA) medium at 650 °C combining gravimetric analysis of steel samples and chemical analysis of corrosion products’ content accumulating in the BLA phase. Energy dispersive X-ray spectrometry, scanning electron microscopy, X-ray fluorescence and inductively coupled plasma–atomic emission spectrometry analysis were employed for characterizing steel structure and alloy composition. AISI 321, 316L and 904L nickel-containing corrosion-resistant steels underwent severe corrosion in BLA, and their corrosion rates depended on the nickel content in the material. AISI 410 steel exhibited the lowest corrosion rate of all the materials investigated, and this type of steel can be considered as a reasonable structural material for work in BLA environments. The corrosion rates of AISI 410, 321, 316L and 904L steels in BLA at 650 °C were 77, 244, 252 and 280 µm/year, respectively. It was also found that chromium was etched more intensively than iron from the surface of steel samples.
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Elaboration and Characterization of a New Heavy Metal Sensor Functionalized by Extracellular Polymeric Substances Isolated from a Tunisian Thermophilic Microalga Strain Graesiella sp
Article
Full-text available
  • Jan 2023
  • SENSORS-BASEL
The present study aimed to develop and characterize new heavy metal sensors functionalized by extracellular polymeric substances (EPSs) isolated from a Tunisian thermophilic microalga strain Graesiella sp. The elaborated sensor showed a highly homogeneous character and revealed a microstructural lamellar arrangement, high crystalline nature, and several functional groups. Electrochemical impedance spectroscopy (EIS) and acoustic wave sensing were used as sensing techniques to explore the ability of microalgae-EPS-functionalized sensors to detect cadmium and mercury as heavy metals. For impedimetric measurements, a two-dipole circuit was adopted and showed good-fitted results with a low total error. The acoustic sensor platforms showed good compatibility with EPS in adjacent water. For both EPS-functionalized sensors, metal ions (Cd2+, Hg2+) were successfully detected in the concentration range from 10−10 M to 10−4 M. Impedimetric sensor was more sensitive to Cd2+ at low concentrations before saturation at 10−7 M, while the acoustic sensor exhibited more sensitivity to Hg2+ over the full range. The results highlight a new potential alternative to use microalgae EPSs as a sensitive coating material for the detection of heavy metals. However, its use in a real liquid medium requires further investigation of its selectivity in the presence of other compounds.
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The Holes of Zn Phosphate and Hot Dip Galvanizing on Electrochemical Behaviors of Multi-Coatings on Steel Substrates
Article
Full-text available
  • May 2022
The aim of this investigation is focused on the evaluation of distinctive coatings commonly applied in the automotive industry. The resulting corrosion behavior is analyzed by using electrochemical impedance spectroscopy (EIS), equivalent circuit (EC) and potentiodynamic polarization curves. The novelty concerns a comparison between tricationic phosphate (TCP), cataphoretic electrodeposition (CED) of an epoxy layer, TCP + CED and HDG (hot-dip galvanized) + TCP + CED multi-coatings. Both the naturally deposited and defect-induced damage (incision) coatings are examined. The experimental impedance parameters and corrosion current densities indicate that multi-coating system (HDG + TCP + CED layers) provides better protection. Both planar and porous electrode behaviors are responsible to predict the corrosion mechanism of the majority of samples examined. Although induced-damage samples reveal that corrosion resistances decreased up to 10×, when compared with no damaged samples, the same trend of the corrosion protection is maintained, i.e., TCP < CED < TCP + CED < HDG + TCP + CED. It is also found that the same trend verified by using electrochemical parameters is also observed when samples are subjected under salt spray condition (500 h). It is also found that porous electrode behavior is not a deleterious aspect to corrosion resistance. It is more intimately associated with initial thickness coating, while corrosion resistance is associated with adhesion of the CED layer on TCP coating. The results of relative cost-to-efficiency to relative coating density ratios are associated with fact that a CED coating is necessary to top and clear coating applications and the TCP + CED and the HDG/TCP + CED coating systems exhibit the best results.
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Progress in Sensors for Monitoring Reinforcement Corrosion in Reinforced Concrete Structures—A Review
Article
Full-text available
  • Apr 2022
  • SENSORS-BASEL
Non-destructive monitoring methods and continuous monitoring systems based on them are crucial elements of modern systems for the management and maintenance of assets which include reinforced concrete structures. The purpose of our study was to summarise the data on the most common sensors and systems for the non-destructive monitoring of reinforced concrete structures developed over the past 20 years. We considered systems based on electrochemical (potentiometry, methods related to polarisation) and physical (electromagnetic and ultrasonic waves, piezoelectric effect, thermography) examination methods. Special focus is devoted to the existing sensors and the results obtained using these sensors, as well as the advantages and disadvantages of their setups or other equipment used. The review considers earlier approaches and available commercial products, as well as relatively new sensors which are currently being tested.
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Electrochemical Impedance Spectroscopy (EIS): Principles, Construction, and Biosensing Applications
Article
Full-text available
  • Oct 2021
  • SENSORS-BASEL
Electrochemical impedance spectroscopy (EIS) is a powerful technique used for the analysis of interfacial properties related to bio-recognition events occurring at the electrode surface, such as antibody–antigen recognition, substrate–enzyme interaction, or whole cell capturing. Thus, EIS could be exploited in several important biomedical diagnosis and environmental applications. However, the EIS is one of the most complex electrochemical methods, therefore, this review introduced the basic concepts and the theoretical background of the impedimetric technique along with the state of the art of the impedimetric biosensors and the impact of nanomaterials on the EIS performance. The use of nanomaterials such as nanoparticles, nanotubes, nanowires, and nanocomposites provided catalytic activity, enhanced sensing elements immobilization, promoted faster electron transfer, and increased reliability and accuracy of the reported EIS sensors. Thus, the EIS was used for the effective quantitative and qualitative detections of pathogens, DNA, cancer-associated biomarkers, etc. Through this review article, intensive literature review is provided to highlight the impact of nanomaterials on enhancing the analytical features of impedimetric biosensors.
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Overview on Lead-Cooled Fast Reactor Design and Related Technologies Development in ENEA
Article
Full-text available
  • Aug 2021
The next generation of nuclear energy systems, also known as Generation IV reactors, are being developed to meet the highest targets of safety and reliability, sustainability, economics, proliferation resistance, and physical protection, with improved performances compared with the currently licensed plants or those presently being built. Among the proposed technologies, lead-cooled fast reactors (LFRs) have been identified by nuclear industries in both Western and developing countries as being among the optimal Generation IV candidates. Since 2000, ENEA, the Italian National Agency for New Technologies, Energy, and Sustainable Economic Development is supporting the core design, safety assessment, and technological development of innovative nuclear systems cooled by heavy liquid metals (HLM) and, most recently, fully oriented on LFRs. ENEA is developing world-recognized skills in fast spectrum core design and is one of the largest European fleets of experimental facilities aiming at investigating HLM thermal-hydraulics, coolant chemistry control, corrosion behavior for structural materials, and material properties in the HLM environment, as well as at developing corrosion-protective coatings, components, instrumentation, and innovative systems, supported by experiments and numerical tools. Efforts are also dedicated to develop and validate numerical tools for specific application to HLM systems, ranging from neutronics codes, system and core thermal-hydraulic codes, computational fluid dynamics (CFD) and fuel pin performance codes, including their coupling. The present work aims at highlighting the capabilities and competencies developed by ENEA so far in the framework of liquid metal technologies for Generation IV LFRs. In particular, an overview on the ongoing R&D experimental program will be depicted considering the current fleet of facilities, namely: CIRCE, NACIE-UP, LIFUS5, LECOR, BID-1, HELENA, RACHEL, and Mechanical Labs. An overview on the numerical activities performed so far and those presently ongoing is also reported. Finally, an overview of the ENEA contribution to the ALFRED Project in the frame of the FALCON international consortium is reported, mainly addressing the ongoing activity in terms of core design, technology development, and auxiliary systems design.
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Phase Transformation by 100 keV Electron Irradiation in Partially Stabilized Zirconia
Article
Full-text available
  • Jun 2021
Partially stabilized zirconia (PSZ) is considered for use as an oxygen-sensor material in liquid lead-bismuth eutectic (LBE) alloys in the radiation environment of an acceleration-driven system (ADS). To predict its lifetime for operating in an ADS, the effects of radiation on the PSZ were clarified in this study. A tetragonal PSZ was irradiated with 100 keV electrons and analyzed by X-ray diffraction (XRD). The results indicate that the phase transition in the PSZ, from the tetragonal to the monoclinic phase, was caused after the irradiation. The deposition energy of the lattice and the deposition energy for the displacement damage of a 100 keV electron in the PSZ are estimated using the particle and heavy ion transport code system and the non-ionizing energy loss, respectively. The results suggest that conventional radiation effects, such as stopping power, are not the main mechanism behind the phase transition. The phase transition is known to be caused by the low-temperature degradation of the PSZ and is attributed to the shift of oxygen ions to oxygen sites. When the electron beam is incident to the material, the kinetic energy deposition and excitation-related processes are caused, and it is suggested to be a factor of the phase transition.
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Self-Healing Behavior of Oxide Layer in Liquid Metal
Article
  • Jun 2022
  • Plasma Fusion Res
The dynamic behaviors of oxide layer formation, growth and self-healing in liquid metal lead (Pb) were studied by on-line monitoring with electrochemical impedance spectroscopy (EIS). The metal rod made of zirconium (Zr) was immersed to liquid Pb at 773 K as the working electrode of EIS. The capacitance semicircle due to the oxide layer formation on the rod surface was detected by EIS after the immersion for 45 hr. The increase of impedance due to the growth of the oxide layer in liquid Pb was continuously detected. The rod was taken out from liquid Pb after the immersion for 150 hr, and the oxide layer formed on the rod was partially exfoliated using lathe to simulate the damage of the oxide layer. The rod was immersed again to liquid Pb for 200 hr. The capacitance semicircle due to the self-healing of the oxide layer was recognized after the immersion for 126 hr. The self-healing of the artificially damaged area on the oxide layer was recognized by the microscopic analysis.
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Measurement of Oxygen Concentration in Static and Flowing Liquid Pb-Bi by Using Zirconia Based Sensor
Article
  • Jan 2020
Liquid Lead-Bismuth Eutectic (LBE) has been proposed as one of the coolant for the Generation IV nuclear reactor. However, the oxygen should be controlled adequately to suppress the corrosion rate of the LBE. A device called oxygen sensor made of zirconia as solid electrolyte has been used to monitor the oxygen concentration online in order to control the oxygen concentration. The principle of this sensor is based on electrochemistry method where the difference oxygen activitiy between reference electrode (RE) and working electrode (in liquid LBE) can make potential difference. The potential difference is measured by electrometer and can be converted into oxygen concentration based on Nernst equation. Iron (Fe)/Magnetite (Fe3O4) was used as material for RE in this study. Measurement of oxygen concentration was conducted at 450 - 600°C for the static condition of LBE and around 390°C for the dynamic condition of flowing LBE. The oxygen concentration for both two experiment conditions were set in oxygen saturated condition of Pb-Bi. The oxygen sensor based on zirconia solid electrolyte with Fe/Fe3O4 as RE can measured the oxygen concentration in liquid LBE. The results showed that the measurement was agreed with the Nernst equation theoretical calculation.