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The Materials challenge for LFR core design
Giacomo Grasso Pietro Agostini
ENEA Bologna ENEA Brasimone
Responsible of ALFRED Head of Technical Unit for and ELFR Core Design Experimental Engineering
IAEA TM on “Liquid Metal Reactor Concepts: Core Design and Structural Materials” IAEA HQ, Vienna, June 12-14, 2013
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
LFR core design issues
The typical issues of all LMFBR (e.g.: high number of critical masses immobilized in the core, need to maintain the metal coolant liquid in all plant conditions, etc.) must be faced together with the specific issues of LFRs (related to core design), due to the physical and chemical properties of lead:
• lead is erosive on the structures the surfaces of the structures exposed to lead are
subject to mechanical damaging;
• lead is corrosive on the structures lead brings into solution some elements the steels are
composed of, dissolving the structures by chemical corrosion.
All these issues must be faced while trying to target the aimed goals, and not going to the detriment of the aimed performances!
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
The coolant must remain liquid in normal operation.
This means that its temperature must remain between the melting and boiling limits:
Constraints for coolant
327 °𝐶 = 𝑇𝑐𝑜𝑜𝑙𝑎𝑛𝑡𝑀𝐸𝐿𝑇𝐼𝑁𝐺 < 𝑇𝑐𝑜𝑜𝑙𝑎𝑛𝑡< 𝑇𝑐𝑜𝑜𝑙𝑎𝑛𝑡
𝐵𝑂𝐼𝐿𝐼𝑁𝐺 = 1740 °𝐶
Constraints for fuel
Because of the high number of critical masses immobilized in the core of every fast reactor, it is mandatory for the fuel to remain solid during operation.
𝑇𝑓𝑢𝑒𝑙< 𝑇𝑓𝑢𝑒𝑙𝑀𝐸𝐿𝑇𝐼𝑁𝐺 ≈ 2800 °𝐶
Putting all together:
Temperature constraints
Coolant Structures Fuel
T [oC]
327 --
2800 --
1740 --
A margin against lead solidification is required to ensure the coolant remains liquid everywhere in the primary circuit. Accordingly, the higher of the limits on the minimum temperature is assumed for all materials.
Temperature constraints
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Temperature constraints To allow extreme power excursion to happen without melting the fuel, a wide margin has to be imposed to house the corresponding temperature excursions. The limit on the max fuel temperature is therefore lowered.
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Mar
gin
to
m
elt
ing
2000 --
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
Fuel cladding conditions
Harshest corrosion (and erosion) conditions
The integrity of a resisting minimum thickness must be guaranteed for the entire fuel life
Most intense radiation damaging
The mechanical properties at end of life must ensure cladding integrity
The radiation-induced swelling must not lead to excessive distortion (preventing the adequate coolant flow) nor interaction with fuel assembly structures (limiting contact stresses)
Most intense hoop stresses
The thermal creep of the cladding must not undermine its integrity even in DEC (grace time)
Protection against swelling
Accordingly, two options are considered:
• Ferritic/Martensitic (grade 91) steels
• advanced Austenitic steels (15-15Ti)
From Phénix experience:
9 Cr F/M steels are the best performing, nevertheless also 15/15Ti has acceptable swelling up to 130 dpa (average 15/15Ti) or 150 dpa (advanced 15/15Ti)
(in a first approximation an acceptable limit of 6% swelling is assumed)
Protection against failing
The goals of
• cladding integrity at End of Life
• a sufficient grace time in DEC (cladding not suffering thermal creep)
can be achieved by a proper combination of the choice of a proper material and the design of a sufficient resisting thickness.
Nevertheless, due to the other issues depending only on the properties of the cladding material, it is decided to cope with the mechanical integrity of the cladding only through its thorough design.
Protection against corrosion
All the steels must be protected against corrosion, which is favored by high temperatures.
The two most diffused corrosion protection strategies are
1. the self-passivation of the outer surface of the steel by the formation of a protective oxide scale;
2. the coating of the outer surface of the steel by a protective adherent layer.
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
1. Passivation by oxide scale
Both candidate cladding materials form oxide scales, whose thickness depends on the oxygen concentration in the molten and the coolant flow velocity.
316 SS @ 500 oC, O2 10-6 wt% 10000 h in flowing Pb (ENEA)
1. Passivation by oxide scale
Nevertheless, for the passivation to be effective:
• a minimum oxygen concentration is required to ensure an oxide layer of minimum thickness is effectively formed (due to its continuous erosion from the surface);
• a maximum oxygen concentration is imposed to avoid formation of insoluble lead oxide (slugs) which might cause plugging, hence flow blockage.
The keeping of the oxygen concentration within the operating range is a challenge in a pool system
From experimental tests, the following relations are assumed as a good practice:
𝑇𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒𝑠 ≤ 𝑇316−𝑡𝑦𝑝𝑒 𝑠𝑡𝑒𝑒𝑙𝑠𝐶𝑂𝑅𝑅𝑂𝑆𝐼𝑂𝑁 = 500 °𝐶
𝑇𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒𝑠 ≤ 𝑇𝑇91−𝑡𝑦𝑝𝑒 𝑠𝑡𝑒𝑒𝑙𝑠
𝐶𝑂𝑅𝑅𝑂𝑆𝐼𝑂𝑁 = 550 °𝐶
Constraints for clad passivation
In order to maintain the oxide scale in place, standing the erosion operated by the flowing lead, the coolant flow velocity should be limited. From experimental tests, the following relation is assumed as a good practice:
𝑣𝑐𝑜𝑜𝑙𝑎𝑛𝑡 ≤ 𝑣𝑐𝑜𝑜𝑙𝑎𝑛𝑡𝐸𝑅𝑂𝑆𝐼𝑂𝑁 = 2 𝑚/𝑠
Constraints for clad passivation
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Mar
gin
to
m
elt
ing
500 --
2000 --
Temperature constraints: 1.316 Putting all together:
Temperature constraints: 1.316
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Mar
gin
to
m
elt
ing
Margin for uncertainties
A sufficient margin (few oC) on the most stringent constraint (clad temperature for lead corrosion) is introduced, to accommodate all the uncertainties coming from data, methodology and codes.
500 --
2000 --
Temperature constraints: 1.316 To respect the max clad temperature (according to the peak linear power rating assumed to protect the fuel), also a limit on the max coolant temperature must be considered, providing the latter a margin against boiling.
Coolant Structures Fuel
T [oC]
327 -- 400 -- 500 --
2800 --
1740 --
2000 --
Mar
gin
to
bo
ilin
g
Margin to solidification
Margin for uncertainties
Mar
gin
to
m
elt
ing
Impact on core design: 1.316
According to the scheme shown before, the viability domain for core design has a very narrow window for the coolant temperatures:
to target a peak cladding (316-type) temperature of 485 ÷ 490 oC, the maximum coolant temperature must not exceed 455 ÷ 460 oC
this means – even assuming a thorough neutronic and thermal/hydraulic design of the core – an average outlet at about 445 oC against 400 oC at the inlet.
Such a narrow range poses severe limits on the generation of superheated steam, impairing the efficiency of the secondary cycle.
Temperature constraints: 2.T91
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Mar
gin
to
m
elt
ing
Margin for uncertainties
550 --
Through analogous considerations, for T91-type steels a higher cladding temperature is allowed, resulting in a potentially wider viability domain for designing the system.
2000 --
Mar
gin
to
bo
ilin
g
When exposed to HLM, F/M steels exhibit Liquid Metal Embrittlement (LME) in the temperature range 300 ÷ 420 oC.
Impact on core design: 2.T91
100 150 200 250 300 350 400 450 500 5502
4
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10
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14
16
18
20
22
24
26 in Ar
in LBE
TO
TA
L E
LO
NG
AT
ION
(%
)
TEST TEMPERATURE (oC)
100 150 200 250 300 350 400 450 500 5502
4
6
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14
16
18
20
22
24
26 in Ar
in LBE
TO
TA
L E
LO
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AT
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(%
)
TEST TEMPERATURE (oC)
Results by PSI for T91 based on Total elongation
Results by PROMETEY Inst. for 10Ch9NSMFB based on % necking to rupture
Necking in air
Necking in Pb
Impact on core design: 2.T91
To protect the steels against LME, the coolant inlet temperature (hence the minimum one for the cladding) has to be raised up to – at least – 430 oC.
Consequently, as in the case for 316-type steels, the margin between the inlet and outlet temperatures is reduced (to about 55 oC), impairing once again the efficiency of the secondary cycle.
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
2. Coating by adherent layer
Due to the scarce resistance against corrosion of both cladding candidates, a layer of a different material is applied to the surface:
• the base material is kept to provide only the aimed mechanical strength to guarantee the integrity of the pin against stresses and irradiation;
• the surface material is added to complement the cladding with the corrosion resistance missing to the base material.
The potential candidates for corrosion barriers include: TiN, FeAl, FeCrAl, GESA and possibly others.
2. TiN coating on 316 SS
• TiN coating was applied to four steel substrates: P91, 304, 316, 441. • Good adherence in all cases. • Uniform thickness. • Thicknesses range from 3 to 5 microns.
TiN coating applied to 316 SS by PVD: Arc Ion Plating Technique
2. TiN coating on 316 SS
f
No decohesion at f=1.5 mm (e=2.25%)
Decohesion starts at f=2.0 mm (e=3.0%)
3 P bending test on TiN coating applied to 316 SS by PVD
2. TiN coating on P91 in Pb Exposed for 2000 h in Pb
No apparent damages on the layer
No lead penetrations are observed
Exposed for 4000 h in Pb
2. FeAl coating
• FeAl coating was applied to four steel substrates: P91, 304, 316, 441. • Good adherence in all cases. • Non uniform thickness. • Thicknesses of about 2 microns. • Presence of micro droplets on the surface.
2. FeAl coating on 316 SS
f
3 P bending test on FeAl coating applied to 316 SS
No decohesion at f=2 mm (e= 3.0%)
2. FeAl coating on P91 in Pb
Pefect result
• 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.
11
Figura 5. M icrografie SEM relative alle zone di testa dove il film è stato mascherato e schiacciato dall’azione meccanica del supporto.
Figura 6. M icrografia SEM relative alle zone di testa non r icoperta.
Inner Oxygen precipitation in conjuction with defects, near the limit of the coated area
2. FeCrAl coating on 316 SS
f
FeRcAl coating shows signs of decoesion at f=2mm (e=3%)
2. Coating by adherent layer
Besides the corrosion(/erosion) tests in stagnant and flowing lead at different temperatures (and velocities), several mechanical tests have been performed on coatings as well as on coated specimens to check:
• the quality and uniformity of the coating layer;
• the mechanical resistance of the coating;
• the affinity of the base and coating materials;
• the ability of the coating to maintain the adhesion on the base material despite differential deformations.
Other tests for the behavior of the coatings under irradiation are ongoing.
From experimental tests, the following relations are assumed as a good practice:
𝑇𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒𝑠 ≤ 𝑇𝑠𝑡𝑒𝑒𝑙𝑠𝐶𝑂𝑅𝑅𝑂𝑆𝐼𝑂𝑁 = 550 °𝐶
Constraints for clad coating
* Preliminary tests show promising results at even higher temperatures.
*
From experimental tests, the following relation is assumed as a good practice:
𝑣𝑐𝑜𝑜𝑙𝑎𝑛𝑡 ≤ 𝑣𝑐𝑜𝑜𝑙𝑎𝑛𝑡𝐸𝑅𝑂𝑆𝐼𝑂𝑁 = 3 𝑚/𝑠
Constraints for clad coating
*
* This limit refers to the component of the velocity normal to the surface of exposed structures.
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
In order to deploy a commercial fleet of LFRs in 2050, a demonstrator reactor (ALFRED) is needed at first to prove the technology is viable, followed by a system of intermediate size (PROLFR) for proving the up-scaling of the concepts towards the industrial reactor (ELFR).
The LFR technology chain
Outline
• Core design issues for a LFR • Fuel and coolant protection • Protection of the cladding
• Corrosion protection by passivation • Corrosion protection by coating
• The LFR technology chain
• Final ALFRED core design
Temperature constraints
Coolant Structures Fuel
T [oC]
327 -- 400 --
2800 --
1740 --
Margin to solidification
Mar
gin
to
m
elt
ing
Margin for uncertainties
550 --
Given the short term for ALFRED realization, it was decided to rely on almost mature technology, so to reduce the time required for qualification. Accordingly, it was decided to use for the cladding advanced 15/15Ti protected by coating.
2000 --
Mar
gin
to
bo
ilin
g
The respect of the maximum fuel temperature requires the targeting of a maximum linear power rating of about 330 W/cm.
From this value, and according to the aimed maximum clad temperature (considering also the uncertainties), the diameter of the pin has been fixed.
2 mm 9 mm 9.3 mm 10.5 mm
Final ALFRED core design
Final ALFRED core design The active height (hence the length of the whole pin) and the lattice pitch are set searching for optimization, to ensure both good natural circulation performances and good criticality while targeting the aimed outlet temperature, according to the allowed coolant flow velocity.
The core is then arranged with a sufficient number of FAs, CRs, SRs and dummy elements.
The active region is apportioned in two enrichment zones to achieve a power/FA distribution flattening ensuring the peak outlet temperature complies with the maximum cladding temperature.
Final ALFRED core design
A gagging scheme has been introduced to help flattening the outlet coolant temperature according to the power/FA distribution.
56
110
170
2
16
34
58
88
112
133
100
73
46
25
5
1
7
15
33
57
87
111
153
134
101
74
47
26
12
6
14
32
86
152
135
75
48
27
13
30
31
55
85
151
171
163
136
102
76
49
28
29
53
54
84
150
164
137
77
50
51
52
82
83
109
149
169
165
138
103
78
79
80
81
107
108
148
139
104
105
106
146
147
140
141
142
143
144
145
166
167
168
3
8
17
35
59
89
113
132
99
72
45
24
11
4
9
18
36
60
114
131
71
44
23
10
20
19
37
61
90
115
154
162
130
98
70
43
22
21
39
38
62
116
155
161
129
69
42
41
40
64
63
91
117
156
160
128
97
68
67
66
65
93
92
118
127
96
95
94
120
119
126
125
124
123
122
121
159
158
157
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180
SA number
80
120
160
200
240
280
Coola
nt
flow
rate
(kg/s
)
CG1
CG2
CG3
CG4
Reflector
All CAs
Bypass
btw FAs
Cooling group Power [MW] Average flowrate
per channel
[kg/s]
Total flowrate
per cooling
group [kg/s]
16 Fuel SA – I
294
172.3 14990
90 Fuel SA – II 145.2 3484
115 Fuel SA – III 117.5 4231
156 Fuel SA – IV 93.4 2241
Control assemblies 1.7 261 261
Reflector 3.1 143 143
Bypass between fuel SA 1.2 110 110
Sum 300.0 25460
Final ALFRED core design
For the hottest FA and the FA with the hottest pin, a complete thermal-hydraulic analysis has been performed to verify the respect of the limits on the maximum cladding and fuel temperature.
Final ALFRED core design
Hottest FA FA with hottest pin
For the hottest FA and the FA with the hottest pin, a complete thermal-hydraulic analysis has been performed to verify the respect of the limits on the maximum cladding and fuel temperature.
Final ALFRED core design
The peak cladding temperature is reduced even further to 539 oC (achieving the design margin for uncertainties) once the gagging for outlet temperature flattening is considered.
Conclusions
• LFR share the main issues of all Fast Reactors, while presenting specific issues due to the use of lead as coolant
• A number of constraints impairs the design of a LFR core, possibly resulting in a viability domain not exploitable for producing electricity in an efficient (hence economic) way
• In particular, the most restrictive issues to be faced pend on the cladding
Conclusions
• The selection of proper cladding materials provides the solution for the issues impairing the resistance of the cladding against stresses and irradiation effects
• On the other hand, the protection of the cladding requires surface protections like oxide scales (passivation) or adherent layers (coating)
• Oxide scales seem not sufficient for a stable and effective protection of the base material
Conclusions
• The application of adherent layers seems the only promising solution for protecting the cladding against corrosion
• For the short term (i.e.: ALFRED), advanced 15/15Ti with coating is the reference solution for the cladding, allowing a core design complying with all the design constraints and goals
• The candidate coatings are already being tested under irradiation to proceed towards qualification
Conclusions
• In parallel, new base materials and/or coatings are presently under investigation
• For the long term (i.e.: ELFR), the availability of such advanced materials/coatings might allow the extension of the viability domain towards higher and broader ranges (temperature, dpa, etc.), extending the fields of applications of LFRs and resulting in higher performances