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Issues and considerations for fuel cladding materials of LFR reactor

P. Agostini, A. Gessi, D. Rozzia, M.Tarantino – ENEAContributions by participants of MATTER Project LEADER MeetingPetten, February 2013

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Primary VesselTnom : 380-430°CDamage modes: corrosionLM embrittlement, ratchetting ,fatigue, creep

PumpTnom : 380-480°CDamage modes:Erosion-corrosion,  ratchetting, fatigue

Steam GeneratorTnom : 380-480°C (Pb) – 450°C (steam)Damage modes:corrosion,  LM embrittlement, ratchetting, fatigue, creep-fatigue, buckling, 

Inner VesselTnom : 380°C- 480°CDamage modes: corrosion,  ratchetting, buckling, creep-fatigue

Overview of damage modes in a LFR

Fuel Assembly claddingsTnom : 380°C- 550°CDamage modes: irradiation damage (swelling, creep, embrittlement), thermal creep, thermal fatigue, LM corrosion,  LM embrittlement

Fuel Assembly StructuresTnom : 380°C- 530°CDamage modes: irradiation damage (swelling, creep, embrittlement), LMcorrosion,  thermal creep, LM embrittlement 

FUEL CLADDING CONDITIONS

Fuel criteria defined 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)

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Neutron spectrum of LFR core

Irradiation swelling of the cladding tubes

Phenix experience on cladding materials exposed at high neutron flux

Excessive swelling of the cladding tubes :• prevents and distorts the adequate coolant flow • generates contact stress at interaction with fuel assembly structures (e.g. grids). In a first approximation a swelling limit of 6% is allowed

9 Cr F/M steel is the best one, nevertheless also 15/15 Ti has acceptable swelling at 150 dpa

Swelling:Comparison of proven materials

Austenitic steels are proven materials by FR technologyThe swelling performance dominates the qualificationCW 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

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Advanced austenitic steels for low swelling

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Thermal Creep resistance of austenitic vs. ferritic/martensitic steels

Comparison of creep resistance at 600°C between austenitic and ferritic/martensitic steels

The creep resistance is an imporant parameter for cladding material selection.For ELSY a hoop stress of 160 MPa is envisaged . In such conditions, if the cladding temperature unexpectedly rises up to 600 °C, the rupture time becomes very short.The thermal creep resistance of T91 at 600°appears too poor. Nevertheless reliable creep data of 15/15 Ti have to be recovered and re-measured.

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Austenitic steel The creep vary close to linear with respect to the applied loadThe creep is proportional to the irradiation

doseThe 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

40 dpa

Comparison with Ferritic-Martensitic steelFor high temperature or high stresses, the creep do not vary linearly with 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.

Irradiation Creep: Comparison of proven materials

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Creep rupture of F/M steels in HLM

0 1000 2000 3000 4000 5000 6000 70000

1

2

3

4

5

6

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140MPaPbBi 160MPaPbBi 140MPaLuft 160MPaLuft

stra

in in

%

time in h

creep of T91 in LBE and air at 550°C

Creep to rupture tests of T91, 10-6wt% oxygen performed at Prometey St. Petersburg – V. MarkovA. Jianu, G. Mueller, A.Weisenburger

Significant reduction of creep strength of T91 in contact with liquid LBE. This experiment shows the necessity to protect the cladding steel by a compliant layer different from the oxides layer

LBE  160 MPa3107h   Ø ~ 2.5mm

In  LBE cracks in and through oxides scaleThe lower the stress the larger the cracks

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

0 2000 4000 6000 8000 10000 12000 140000.015

0.018

0.021

0.024

0.027

0.030

0.033

0.036

0.039

0.042

0.045

e th

Time [h]

P92 600°C P92 650°C

Modelling of Tertiary Creep of F/M steels

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 200000.0

0.2

0.4

0.6

0.8

1.0

Normalized mean laves dimention Normalized e

th function

Time [h]

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 LavesThe threshold strain for tertiary creep

of F/M is associated with Laves phase and voids formation

P91 micrographic analysis

Normalized eth function

Normalized mean Laves radius

Microstructure observations

Voids formation close to Laves  nucleation Fe2(Mo,W)   C.Testani “MATTER workshop 2012”

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Fatigue resistance of F/M steelSeveral tests of thermal fatigue were performed on Eurofer 97 by ENEA in the frame of the Fusion Programs .The studies are reported in: G. Filacchioni, The Thermo-Mechanical Fatigue Testing Facility of Casaccia’s Laboratories, MAT TEC, March 2002

The softening effect of strain controlled fatigue is evident after few cycles.

Eurofer chemical composition is 9Cr and 1 W instead of  9Cr and 1Mo as T91

Eurofer (low activation Ferritic /martensitic)

316 L steel for fatigue comparison

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Embrittlement of Ferritic-Martensitic steel DBTT value for T91 and EM10 after irradiation remains below room temperature Martensitic steels behave better than ferritic steels

Irradiation Embrittlement: comparison of proven materials

In CW steels hardening at irradiation temperatures <450°C and ductility increase at 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

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100 150 200 250 300 350 400 450 500 5502468

101214161820222426 in Ar

in LBE

TOTA

L ELO

NGAT

ION

(%)

TEST TEMPERATURE (oC)100 150 200 250 300 350 400 450 500 550

2468

101214161820222426 in Ar

in LBE

TOTA

L ELO

NGAT

ION

(%)

TEST TEMPERATURE (oC)

Results by PSI for T91 based on Total elongationResults by PROMETEY Institute for 10Ch9NSMFBbased on % necking to rupture

Ferritic martensitic steels present Liquid Metal Embrittlement in the temperature range 300 – 420 °C  when exposed to HLM. Similar results where obtained by PSI for T91 and by Prometey Institute for  notched 10Ch9NSMFB steel (9.4 Cr, 1.3 Si, 0.84 Ni)

HLM Embrittlement of grade 91 steel

Necking in air

Necking in Pb

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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 

No LME observed in 316L Tests performed in LBE at 350° 5×10-5 s-1 

Observations by SCK-CEN

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WELDING ISSUES OF Grade91

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1000 10000 100000

Cycles number

Strain range (%)

Weld Metal

Base Metal

bimaterial

Weld Metal law

Base Metal law

CEA experiments to account for the reduced fatigue resistance of welded P91At low cycles the type IV cracks were observedAt high cycles the cracks in the base metal were oserved 

HAZBM WM

BMHAZWM

The determination of the welding coefficient for P91 deserves additional efforts.The filler metal, the welding method and the post weld heat treatment are under study.

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HLM corrosion• The HLM presents high solubility of the chemical 

elements  of structural steels: Fe, Cr and mainly Ni• In both austenitic and ferritic martensitic steels, a 

partial protection vs. dissolution is achieved by formation of protective oxides

• Nevertheless at 550 C and 10-6 wt% O2 (high oxygen) the dissolution is not completely prevented

• As shown, the protective oxides are ruptured under stress

• Moreover the 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

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HLM CORROSION

316 @ 500°C, O2 10-6 wt% 10000h  stagnant PbBi

316  @500°C , O2 10-6 wt%  10000h Flowing Pb (ENEA)

• Temperature limits for corrosion (dissolution) of steels in Pb/PbBi316 type steels: Tlimit < 450°might be 500°in Pb – to be assuredT91 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 irradiationResistant to mechanical stress Thin to reduce risk of rupture (about 40 microns)

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Comparison of materials for ALFRED cladding

• Swelling performance of grade 91 is better than that of austenitic steels: advanced austenitic have to be developed

• Thermal creep resistance of grade 91 is poor and Irradiation creep is not linear with load

• Grade 91 is subject to fatigue softening• Cyclic strength of Grade 91 is 50% lower than that of 15-

15 Ti• Irradiation embrittlement for both 15-15 Ti and Gr.91 is 

acceptable• Gr.91 is subject to HLM embrittlement at T< 420 C.• Gr.91 welds are subject to type IV rupture and require 

special heat treatment• Both 15-15Ti and Gr.91 are subject to HLM corrosion 

(elemental dissolution). • The only oxides scale is not an effective corrosion barrier: 

ruptured under stress, spalled at higher temperatures

Austenitic    Ferritic/Martensitic

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Considerations on materials for ALFRED cladding

It is confirmed that the fuel cladding of the first core of ALFRED will not be made of Gr.91 steel, since its mechanical properties (creep, fatigue, HLM embrittlement, welds) appear too poor and subject to ageing.

An intensive R&D is being addressed in France for austenitic steels resistant to irradiation swelling. ENEA also is very much interested to this research line 

It is confirmed that the weak point of LFR technology is represented by the dissolution of main steel elements. 

The naturally formed oxides scale, although mitigating the dissolution effect, cannot represent an effective protection for long time in stressed condition and high temperature. 

In short term, the reference material for ALFRED fuel cladding is 15-15 Ti, Si stabilized, protected by a well qualified corrosion barrier.

The potential candidates for corrosion barriers include : Fe-Al, TiN (BLUE), Al oxide, GESA, Ta and possibly others.

In the long term, corrosion resistant austenitic steels have to be selected and qualified for fuel cladding: Si or Al containing steels

Exposed for 2000h in Pb

Coatings under test: T91 “BLUE” coated 

No apparent damages on the layer

No lead penetrations are observed

Exposed for 4000h in Pb

Coatings under test: T91 “SS39L” coated

5000 hours of exposure for SS39L, the  last CHEOPEIII run. The coating appears heavily damaged, with random thickness Oxygen inner precipitation.

Coatings under test: T91 “FeAl” coated

5000 hours of  exposure of  FeAl,  the  last CHEOPEIII  run.  The  coating appears untouched where  its  original  quality  is  good,  locally  damaged  with  Oxygen  precipitation  where detachments are present. No changes in chemical composition

Inner Oxygen precipitation in conjuction with defects, near the limit of the coated area

Pefect result

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Coating under test: AISI 316 Ta coated

Ta coating Successfully tested as bulk material in PbBi. Successfully tested with plastic deformation in room conditions.Not yet tested in creep-rupture tests.The use in the core has to be clarified due to high neutron capture and transmutation to W

1µm

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Further steps• Extensive testing campaign of 

steel corrosion barriers in controlled corrosion conditions

• Extensive testing campaign of steel corrosion barriers in HLM under stress and strain conditions

• PIE after irradiation tests performed in BOR 60 at 16 dpa

• Development of additional corrosion barriers for austenitic and F/M steels 

• Qualification of corrosion resistant steels for cladding

• Collaborations to get irradiation data on advanced austenitic steels

1µm

253MA

Average thickness < 1 µm

253 MA (21wt% Cr, 11wt% Ni, 2wt% Si)