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
1
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
Steam Generator
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
Fuel Assembly claddings
Tnom : 380°C- 550°C
Damage modes: irradiation damage (swelling,
creep, embrittlement), thermal creep, thermal
fatigue, LM corrosion, LM embrittlement
Fuel Assembly Structures
Tnom : 380°C- 530°C
Damage modes: irradiation damage (swelling,
creep, embrittlement), LMcorrosion, thermal
creep, LM embrittlement
2
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)
Neutron spectrum of LFR core
4
Irradiation swelling of the cladding tubes
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
Phenix experience on cladding materials exposed at high neutron flux
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
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
Advanced austenitic steels for low swelling
7
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.
8
Irradiation Creep:
Comparison of proven materials
Austenitic steel
Comparison with Ferritic-Martensitic steel
 The creep vary close to linear with respect to
the applied load
For high temperature or high stresses,
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
dependent from alloying elements
largely
 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.
40 dpa
9
Creep rupture of F/M steels in HLM
Creep to rupture tests of T91, 10-6wt% oxygen performed at Prometey St. Petersburg – V. Markov
A. Jianu, G. Mueller, A.Weisenburger
LBE 160 MPa
3107h Ø ~ 2.5mm
7
creep of T91 in LBE and air at 550°C
6
In LBE cracks in
and through
oxides scale
The lower the
stress the larger
the cracks
strain in %
5
140MPaPbBi
160MPaPbBi
140MPaLuft
160MPaLuft
4
3
2
1
0
0
1000
2000
3000
4000
5000
6000
7000
time in h
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
10
Modelling of Tertiary Creep of F/M
steels
0.045
P92 600°C
P92 650°C
0.042
0.039
0.036
eth
0.033
0.030
0.027
0.024
0.021
0.018
0.015
0
2000
4000
6000
8000
10000
12000
14000
Time [h]

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
Microstructure observations
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
1.0
Normalized mean Laves radius
0.8
0.6
Normalized eth function
0.4
0.2
Normalized mean laves dimention
Normalized eth function
0.0
0
2000
4000
6000
8000 10000 12000 14000 16000 18000 20000
Time [h]
Voids formation close to Laves nucleation
Fe2(Mo,W)
C.Testani “MATTER
workshop 2012”
P91
micrographic
analysis
Fatigue resistance of F/M steel
Several 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 (low activation Ferritic /martensitic)
316 L steel for fatigue comparison
Eurofer chemical composition is 9Cr and 13
1W
instead of 9Cr and 1Mo as T91
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
Embrittlement of Ferritic-Martensitic steel
 DBTT value for T91 and EM10 after
irradiation remains below room temperature
 Martensitic steels behave better than
ferritic steels
14
HLM Embrittlement of grade 91 steel
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)
26
24
in Ar
in LBE
TOTAL ELONGATION (%)
22
20
18
Necking in air
16
14
12
10
8
6
Necking in Pb
4
2
100 150 200 250 300 350 400 450 500 550
o
TEST TEMPERATURE ( C)
Results by PSI for T91 based on Total elongation
Results by PROMETEY Institute for 10Ch9NSMFB
based on % necking to rupture
15
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
Observations by SCK-CEN
No LME observed in 316L
Tests performed in LBE at
350° 5×10-5 s-1
16
WELDING ISSUES OF Grade91
1.2
Weld Metal
Strain range
(%)
Base Metal
bimaterial
1
Weld Metal law
0.8
Base Metal law
0.6
0.4
0.2
Cycles number
0
1000
10000
100000
BM
CEA experiments to account for the reduced fatigue
resistance of welded P91
At low cycles the type IV cracks were observed
At high cycles the cracks in the base metal were oserved
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.
WM
WM
HAZ
HAZ
BM
17
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
18
HLM CORROSION
316 @500°C ,
O2 10-6 wt%
10000h Flowing Pb (ENEA)
316 @ 500°C,
O2 10-6 wt%
10000h stagnant PbBi
•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
19
Thin to reduce risk of rupture (about 40 microns)
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
20
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
21
Coatings under test: T91 “BLUE” coated
Exposed for 2000h in Pb
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
Inner Oxygen
precipitation in
conjuction with
defects, near the limit
of the coated area
Pefect result
5000 hours of exposure of FeAl, the last CHEOPEIII run. The coating appears untouched
Figura
5. Micrografie
SEM relative
alle zone
di testa dove
il film è stato mascherato
e schiacciato dall’azione
where its original quality is good,
locally
damaged
with
Oxygen
precipitation
where
meccanica del supporto.
detachments are present.
No changes in chemical composition
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
25
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
253 MA (21wt% Cr, 11wt% Ni, 2wt% Si)
• Qualification of corrosion
resistant steels for cladding
• Collaborations to get irradiation
data on advanced austenitic
steels
253MA
Average thickness < 1 µm
1µm
26