The Materials challenge for LFR core design · The Materials challenge for LFR core design Giacomo...

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

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





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.



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.


T [oC]

327 -- 400 --

2800 --

1740 --


Mar

gin

to

m

elt

ing

2000 --

Outline





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





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


T [oC]

327 -- 400 --

2800 --

1740 --


Mar

gin

to

m

elt

ing

500 --

2000 --

Temperature constraints: 1.316 Putting all together:

Temperature constraints: 1.316


T [oC]

327 -- 400 --

2800 --

1740 --


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.


T [oC]

327 -- 400 -- 500 --

2800 --

1740 --

2000 --

Mar

gin

to

bo

ilin

g



Mar

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m

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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


T [oC]

327 -- 400 --

2800 --

1740 --


Mar

gin

to

m

elt

ing


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

6

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10

12

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

8

10

12

14

16

18

20

22

24

26 in Ar

in LBE

TO

TA

L E

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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





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





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







T [oC]

327 -- 400 --

2800 --

1740 --


Mar

gin

to

m

elt

ing


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

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to

bo

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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.


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

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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

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93

92

118

127

96

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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


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.


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.


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

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