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Issues and considerations for fuel cladding materials of LFR reactor

Issues and considerations for fuel cladding materials of LFR reactor. P. Agostini, A. Gessi, D. Rozzia , M.Tarantino – ENEA Contributions by participants of MATTER Project LEADER Meeting Petten , February 2013. Overview of damage modes in a LFR. Primary Vessel Tnom : 380-430°C

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Issues and considerations for fuel cladding materials of LFR reactor

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  1. Issues and considerations for fuel claddingmaterialsof LFR reactor P. Agostini, A. Gessi, D. Rozzia, M.Tarantino – ENEA Contributions by participants of MATTER Project LEADER Meeting Petten, February 2013

  2. Overview of damage modes in a LFR Primary Vessel Tnom : 380-430°C Damage modes: corrosion LM embrittlement, ratchetting ,fatigue, creep Pump Tnom : 380-480°C Damage modes: Erosion-corrosion, ratchetting, fatigue SteamGenerator Tnom : 380-480°C (Pb) – 450°C (steam) Damage modes: corrosion, LM embrittlement, ratchetting, fatigue, creep-fatigue, buckling, Inner Vessel Tnom : 380°C- 480°C Damage modes: corrosion, ratchetting, buckling, creep-fatigue FuelAssemblycladdings Tnom : 380°C- 550°C Damage modes: irradiation damage (swelling, creep, embrittlement), thermal creep, thermal fatigue, LM corrosion, LM embrittlement FuelAssemblyStructures Tnom : 380°C- 530°C Damage modes: irradiation damage (swelling, creep, embrittlement), LMcorrosion, thermal creep, LM embrittlement

  3. FUEL CLADDING CONDITIONS Fuelcriteriadefined in ELSY EU Project Max allowed peak linear power 32kW/m; Max clad and fuel temperatures of 560 °C and 2100 °C, respectively; Max neutron flux 2.4*1015 n/cm2s Peak clad damage of 100 dpa, in correspondence of a fuel burn-up of 100 MWd/kgHM (200 dpa are assumed as a long term option); Fuel pin OD 10.5 mm, overall length 2520 mm Hoop stress to be examined for creep 160 MPa (200 Mpa as long term option)

  4. Neutronspectrumof LFR core

  5. Irradiation swelling of the cladding tubes • Excessive swelling of the claddingtubes : • prevents and distorts the adequatecoolant flow • generatescontact stress at interactionwithfuelassemblystructures (e.g. grids). • In a first approximation a swelling limitof 6% isallowed Phenix experience on cladding materials exposed at high neutron flux 9 Cr F/M steel is the best one, neverthelessalso 15/15 Ti hasacceptable swelling at 150 dpa

  6. Swelling: Comparisonofprovenmaterials Austenitic steels are proven materials by FR technology The swelling performance dominates the qualification CW 15-15Ti Si enriched highlights good swelling performance  demonstrated at 160 dpa with possibility to reach 200dpa Swelling of Ferritic-Martensitic steels  the evolution of swelling with dose is slow  The swelling rates are much smaller than those for austenitic steels

  7. Advancedausteniticsteelsfor low swelling

  8. Thermal Creep resistanceofaustenitic vs. ferritic/martensitic steels Comparisonof creep resistance at 600°C betweenaustenitic and ferritic/martensitic steels The creep resistanceisanimporantparameterforcladding material selection. For ELSY a hoop stress of 160 MPaisenvisaged . In suchconditions, if the cladding temperature unexpectedlyrises up to 600 °C, the rupturetimebecomesvery short. The thermal creep resistanceof T91 at 600°appears toopoor. Neverthelessreliable creep data of 15/15 Ti havetoberecovered and re-measured.

  9. Irradiation Creep: Comparisonofprovenmaterials Austenitic steel  The creep vary close to linear with respect to the applied load The creep is proportional to the irradiation dose The creep proportionality to the dose is valid only in the domain of the swelling incubation period The creep performance is not largely dependent from alloying elements Comparison with Ferritic-Martensitic steel For high temperature or high stresses,  the creep do not vary linearlywith respect to the applied load  The thermal creep greatly contributes to dimensional changes  Where the creep is proportional to the irradiation dose, the creep/swelling correlation is similar to that for austenitic  At 520°C the creep behavior is acceptable, at 590 °C is no more acceptable. 40 dpa

  10. Creepruptureof F/M steels in HLM Creeptorupturetestsof T91, 10-6wt% oxygenperformed at Prometey St. Petersburg – V. MarkovA. Jianu, G. Mueller, A.Weisenburger LBE 160 MPa 3107h Ø ~ 2.5mm In LBE cracksin andthroughoxidesscale The lowerthe stress the larger thecracks Significantreductionof creep strengthof T91 in contactwith liquid LBE. Thisexperimentshowsthenecessitytoprotectthecladdingsteelby a compliantlayer different fromtheoxideslayer

  11. ModellingofTertiary Creep of F/M steels • Tertiary stage switched on by the threshold strain eth • Threshold strain appears time dependent, decreasing during thermal exposure due to the precipitation and coarsening of Laves phases • Damage strongly depends from accumulated strain

  12. Microstructureobservations • Correlation between the behavior of the threshold strain and the evolution of Laves dimension. • evolution of eth is proportional with the inverse of evolution of mean Laves radius during ageing • voids formation close to Laves • The threshold strain for tertiary creep of F/M is associated with Laves phase and voids formation Normalized mean Laves radius Normalized eth function P91 micrographic analysis VoidsformationclosetoLavesnucleation Fe2(Mo,W) C.Testani “MATTER workshop 2012”

  13. Fatigueresistanceof F/M steel Severaltestsofthermalfatiguewereperformed on Eurofer 97 by ENEA in the frame of the FusionPrograms .The studies are reported in: G. Filacchioni, The Thermo-Mechanical Fatigue Testing Facility of Casaccia’s Laboratories, MAT TEC, March 2002 The softeningeffectof strain controlledfatigueisevidentafterfewcycles. Eurofer (low activation Ferritic /martensitic) 316 L steel forfatiguecomparison Euroferchemicalcompositionis 9Cr and 1 W insteadof 9Cr and 1Mo as T91

  14. IrradiationEmbrittlement: comparisonofprovenmaterials In CW steels hardening at irradiation temperatures <450°C and ductility increaseat higher irradiation temperatures is observed. Loss of ductility is observed at higher irradiation conditions It has been proved that the enhancements that lead to higher swelling resistance also have beneficial effects on mechanical properties Embrittlement of Ferritic-Martensitic steel  DBTT value for T91 and EM10 after irradiation remains below room temperature  Martensitic steels behave better than ferritic steels

  15. HLM Embrittlement of grade 91 steel FerriticmartensiticsteelspresentLiquid Metal Embrittlement in the temperature range 300 – 420 °C whenexposedto HLM. Similarresultswhereobtainedby PSI for T91 and byPrometeyInstitutefornotched 10Ch9NSMFB steel (9.4 Cr, 1.3 Si, 0.84 Ni) Necking in air Necking in Pb Resultsby PROMETEY Institutefor 10Ch9NSMFB based on % neckingtorupture Resultsby PSI for T91 based on Total elongation

  16. Liquid Metal Embrittlement comparison LME observed in T91 under specific conditions and after UTS Tests performed in LBE at 350° 5×10-5 s-1 Observationsby SCK-CEN No LME observed in 316L Tests performed in LBE at 350° 5×10-5 s-1

  17. WELDING ISSUES OF Grade91 BM WM HAZ CEA experimentsto account for the reducedfatigueresistanceofwelded P91 At low cycles the type IV crackswereobserved At high cycles the cracks in the base metal wereoserved The determinationof the weldingcoefficientfor P91 deservesadditionalefforts. The filler metal, the weldingmethod and the post weldheat treatment are under study. BM WM HAZ

  18. HLM corrosion • The HLM presents high solubilityof the chemicalelementsofstructuralsteels: Fe, Cr and mainly Ni • In bothaustenitic and ferritic martensitic steels, a partialprotectionvs. dissolutionisachievedbyformationofprotectiveoxides • Nevertheless at 550 C and 10-6 wt% O2 (high oxygen) the dissolutionisnotcompletelyprevented • As shown, the protectiveoxides are ruptured under stress • Moreoverthe picture shows that for T91 in lead at 500°C, the oxide layer looses its adherence to the matrix and is fractured and removed by the Pb flow. T 91 AISI 316

  19. HLM CORROSION 316 @500°C , O2 10-6 wt% 10000h FlowingPb(ENEA) 316 @ 500°C, O2 10-6 wt% 10000hstagnantPbBi • Temperature limits for corrosion (dissolution) of steels in Pb/PbBi • 316 type steels: Tlimit < 450°might be 500°in Pb – to be assured • T91 type F/M steels Tlimit< 550 °C The oxide scale of austenitic steel is thinner and more stable than that of T91. The additional material protection appears to be necessary to face the corrosion by flowing lead. The suitable coating must be: Resistant to neutron irradiation Resistant to mechanical stress Thin to reduce risk of rupture (about 40 microns)

  20. Comparisonofmaterialsfor ALFRED cladding Austenitic Ferritic/Martensitic • Swelling performance ofgrade 91 isbetterthanthatofausteniticsteels: advancedaustenitichavetobedeveloped • Thermal creep resistanceofgrade 91 ispoor and Irradiation creep isnotlinearwithload • Grade 91 issubjecttofatiguesoftening • CyclicstrengthofGrade 91 is 50% lowerthanthatof 15-15 Ti • Irradiation embrittlement forboth 15-15 Ti and Gr.91 isacceptable • Gr.91 issubjectto HLM embrittlement at T< 420 C. • Gr.91 welds are subjecttotype IV rupture and requirespecialheat treatment • Both 15-15Ti and Gr.91 are subjectto HLM corrosion (elementaldissolution). • The onlyoxides scale isnotaneffectivecorrosionbarrier: ruptured under stress, spalled at highertemperatures

  21. Considerations on materialsfor ALFRED cladding • Itisconfirmedthat the fuelcladdingof the first coreof ALFRED willnotbemadeofGr.91 steel, sinceitsmechanicalproperties (creep, fatigue, HLM embrittlement, welds) appeartoopoor and subjecttoageing. • An intensive R&Disbeingaddressed in France forausteniticsteelsresistanttoirradiation swelling. ENEA alsoisverymuchinterestedtothisresearchline • Itisconfirmedthat the weakpointof LFR technologyisrepresentedby the dissolutionofmain steel elements. • The naturallyformedoxides scale, althoughmitigating the dissolutioneffect, cannotrepresentaneffectiveprotectionfor long time in stressedcondition and high temperature. • In short term, the reference material for ALFRED fuelcladdingis 15-15 Ti, Si stabilized, protectedby a wellqualifiedcorrosionbarrier. • The potentialcandidatesforcorrosionbarriers include : Fe-Al, TiN (BLUE), Al oxide, GESA, Ta and possiblyothers. • In the long term, corrosionresistantausteniticsteelshave to be selected and qualified for fuelcladding: Si or Al containingsteels

  22. Coatings under test: T91 “BLUE” coated Exposedfor 2000h in Pb Exposedfor 4000h in Pb • No apparent damages on the layer • No lead penetrations are observed

  23. Coatings under test: T91 “SS39L” coated 5000 hours of exposure for SS39L, the last CHEOPEIII run. The coatingappearsheavilydamaged, with random thicknessOxygeninnerprecipitation.

  24. Coatings under test: T91 “FeAl” coated Inner Oxygenprecipitation in conjuction with defects, near the limit of the coated area Pefectresult 5000 hours of exposure of FeAl, the last CHEOPEIII run. The coatingappearsuntouchedwhereitsoriginalqualityisgood, locallydamaged with Oxygenprecipitationwheredetachments are present. No changes in chemicalcomposition

  25. Coating under test: AISI 316 Ta coated Ta coating Successfullytestedas bulk material in PbBi. Successfullytestedwith plastic deformation in roomconditions. Notyettested in creep-rupturetests. The use in the corehastobeclarified due to high neutroncapture and transmutationto W 1µm

  26. Furthersteps • Extensivetestingcampaignof steel corrosionbarriers in controlledcorrosionconditions • Extensivetestingcampaignof steel corrosionbarriers in HLM under stress and strain conditions • PIE afterirradiationtestsperformed in BOR 60 at 16 dpa • Developmentofadditionalcorrosionbarriersforaustenitic and F/M steels • Qualification of corrosionresistantsteels for cladding • Collaborations to getirradiation data on advancedausteniticsteels • 253 MA (21wt% Cr, 11wt% Ni, 2wt% Si) 253MA Averagethickness < 1 µm 1µm

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