Fuel assembly

1DEN/DANS/DPC/SCCMELaboratoire d’Etude de la Corrosion Aqueuse

April 19, 2023

Review: chemical compatibility of SiC/SiC Review: chemical compatibility of SiC/SiC composites with the GFR environmentcomposites with the GFR environment

C. CabetC. Cabet

Laboratoire of Non Aqueous Corrosion, CEA Saclay, FRANCELaboratoire of Non Aqueous Corrosion, CEA Saclay, FRANCE

2DEN/DANS/DPC/SCCMELaboratoire d’Etude de la Corrosion Non Aqueuse

Fuel assembly

HeateXchanger

GFR and SiC/SiC compositesGFR and SiC/SiC composites

850°C

Helium

Introduction


Needle conceptcompositeSiC-SiCfibers

Fission gas

Actinide compound :

UPuC or UPuN

(56%vol of fuel)

diffusion barrierrefractory metal : We, Mo, Cr,…

compositeSiC-SiCfibers

Fission gas

Actinide compound :

UPuC or UPuN

(56%vol of fuel)

diffusion barrierrefractory metal : We, Mo, Cr,…

Concepts of fuel assemblyConcepts of fuel assembly

Plate concept

Introduction


Requirements on material for fuel assemblyRequirements on material for fuel assembly

•Containment of fuel and FP

•Refractory behaviour

– Resistance to normal operating temperatures (about 900°-1200°C) on extended lifetimes

– Confining of FP during a transient incident up to 1600°C

– Mechanical integrity after a major accident up to 2000°C

•High thermal conductivity (>10 W/m.K)

•Transparency to fast neutrons

•Mechanical strength and creep resistance

•Ability to dissolve in nitric acid

•Workability and assemblage

•Resistance to corrosion/ oxidation

Best candidate material : SiCSiCff/SiC/SiCmm

Introduction


GFR environment GFR environment

•High temperature: 900-1200°C

+ short term transitory up to 1600°C (confining) and accident up to 2000°C (integrity)

•Long in-core times•No inspection, no repair•Cooling gas

Introduction

: impure helium

Helium

refuelingmaintenance

H2 ?

degassing

CO, CH4 ?

secondary circuitcooler

air, H2O

Helium+ traces air, H2O

air, H2O,


SiCSiCff/SiC/SiCmm usual applications usual applications

Rocket engines

Aircraft engines

Turbines

Introduction

•High temperature•Oxidative atmospheres•Inspection and repair• Short term


SiCSiCff/SiC/SiCmm compatibility with GFR physico-chemical compatibility with GFR physico-chemical

conditions over long term ?conditions over long term ?

•Thermal stability

•Oxidation resistance

•Consequences of thermal aging and oxidationon the mechanical (and confining) properties

•Improvement strategies

Lifetime prediction

Introduction


ContentContent

•Introduction on the GFR application

•SiCf/SiCm structure and fabrication


•Oxidation propertis

•Composite resistance

•R&D needs to qualify SiC/SiC for GFR applications


SiCSiCff/SiC/SiCmm structure structure

SiC-basedfibre ~10µm

interphase (C) ~0.1µm

SiC-based matrix

crack

SiCf/SiCm structure and fabrication

Fibres in bundleUD or 1D

2D3D


SiC based matrix (SiC + Si)SiC based matrix (SiC + Si)

•CVI Chemical Vapor Impregnation

•PIP Polymer Impregnation and Porolysis

•RMI Reative Melt Infiltration

•SI-HPS Slurry Infiltration and High Pressure Sintering

additives

porosity

pyrolysis

preceramic

Carbon coated fibre tows

Pre-forming Polymer infiltration

Pyrolysis


SiC-based fibres: fabricationSiC-based fibres: fabrication

•2nd generation

– cure by electron beam in inert atm at T~1400°C– Si-C + C (+ 0.5% O)

•3rd generation or nearly stoichoimetric

– cure at 1800°-2000°C + optimization– thin C layer on the surface

SiCf/SiCm structure and fabrication

PCS

Weak fibres Dense fibres

spinning curing

•1st generation

– cure in oxygen at T~1200°C– Si-C-O: 2nm SiC + C + SiCxOy


SiC-based fibres: 3 generationsSiC-based fibres: 3 generations

•Exemple of the development of the Nicalon fibres by Nipon Carbon


InterphaseInterphase

•Compliant material•Thin layer ~100nm•« leaf » structure

– pyrocarbon– hex-BN– Multilayer


ContentContent




– Monolithic SiC

– Matrix

– Fibres

•Oxidation properties




SiC phase diagramSiC phase diagram

•Stoichoimetric•no other intermediate

compound

•SiC (SiC)(l) + C2540°C

Thermal stability


Thermal stablity of SiCThermal stablity of SiC

•Thermodynamic calculation

SiC C + Si(g) + recrystalisation

•Kinetic factors: SiC stable up to ~1600°C

104/T (K)

Thermal stability

in vacuum or inert atmopsherein vacuum or inert atmopshere

SiC + Si


Thermal Stability of the matrixThermal Stability of the matrix

•SiC and SiC/C matrixes are stable up to about 1600°C

in vacuum or inert atmopsheresin vacuum or inert atmopsheres

Thermal stability


Thermal Stability of fibresThermal Stability of fibres

•Basically instable à T>1200°C

•(SiC, C, SiC2xO1-x) w SiC + x C + y CO(g) + z SiO(g)

Fibres of the 1st generation: Si-C-O Fibres of the 1st generation: Si-C-O

Porous C/SiC (large grains)

Mass loss

Decrease the creep strength

Thermal stability

Creep curves for Nicalon fibres tested in pure Ar under 0.7 GPa Mass loss for Nicalon fibres tested in

pure Ar

1200°C

1300°C

[Bodet et al. J Amer Ceram Soc 79 (1996) 2673]



•Stable up to 1350°C

•(SiC, C) + Otrace(g,s) SiC + CO(g) +C

Fibres of the 2nd generation: Si-C(0.5% O) Fibres of the 2nd generation: Si-C(0.5% O)

Large grains Mass loss

Si-C-O Nicalon NL202 and Si-C Hi-Nicalon (as-received and heat treated) fibres under 100kPa Ar (heating rate: 10°C/min) [Chollon et al., J Mater Sci 32 (1997) 333]

= r

Thermal stability

Tensile strength and Young’s modulus at RT of Si-C Hi-Nicalon after annealing under 100kPa Ar for tp=1hrs exept *tp=10hrs) [Chollon et al., J Mater Sci 32 (1997) 333]



•Stable up to very high temperatures 1800°-2000°C

•Some SiC grain growth

•Good mechanical properties up to 1400°-1500°C

Nearly stoichiometric fibresNearly stoichiometric fibres

Strengh as a function of temperature for 3rd gen fibres with a 250mm gauge length[Bunsell and Piant, J Mater Sci 41 (2006) 835]

Thermal stability


ContentContent




•Oxidation properties– Monolithic SiC

• passive oxidation• active oxidation

– Matrix– Fibres– Interphase




Oxidation of SiC at high PoOxidation of SiC at high Po22: passive oxidation: passive oxidation

•Same mechanism that the oxidation of Si and other ceramics

– SiC(s) + 3/2 O2(g) = SiO2(s) + CO(g)

– SiC(s) + 2 O2(g) = SiO2(s) + CO2(g)

– Linear-parabolic kinetics

Oxidation - SiC

Very protective

)t(x

K

x

K L2P

Parabolic rate constant

linear rate constant

Scale thickness

T>800°CMonolithic SiC

-SiC in 1 atm air [Costello & Tressler, J Am Ceram Soc 64 (1981) 327]


Oxidation of SiC at high PoOxidation of SiC at high Po22: mechanism: mechanism

RT

Eexp.BK a

p

T>800°CMonolithic SiC

Growth rate = oxygen transport through the SiO2 scale

Oxidation - SiC

MEB image of sintered -SiC 6hrs at 1400°C in 1 atm air [Costello & Tressler, J Am Ceram Soc 64 (1981) 327]

90µm

T<1400°CEa 300 kJ/mole

molecular diffusionamorphous SiO2

T>1400°CEa 150-300 kJ/mole

atomic diffusioncristobalite

KP for the oxidation of single-crystal SiC under 0.001 atm O2 [Zheng, J Electrochem Soc 137 (1990) 854]


Oxidation at high PoOxidation at high Po22: polycrystalline SiC: polycrystalline SiC

•Determining factors for Kp

– Polytype

– Porosity (fabrication process)

– Additives and impurities

• Formation of a silicate with a lower viscosity( transport of O )

• Modify the crystallisation

Oxidation - SiC

Kp from the literature for different type of SiC[Narushima et al., J Am Ceram Soc 72 (1989) 1386]

RT

Eexp.BK a

p

HP SiC with different %Al2O3 at 1370°C in 1 atm O2 [Opila & Jacobson, in Materials science and technology Vol. 19, RW. Cahn et al. Ed. (2000)]


Oxidation at high PoOxidation at high Po22: effect of water vapour: effect of water vapour

•Passive oxidation by water vapour

SiC + 2 H2O(g) SiO2 + CH4(g)

SiC + 3 H2O(g) SiO2 + CO2(g) + 3 H2(g)

Oxidation - SiC

CVD-SiC at 1200°C in pure CO2, pure O2 and 50%H2O/50%O2

[Opila & Nguyen., J Am Ceram Soc 81 (1998) 1949]

T<1400°C

T>1400°C

•Some water vapour increases the oxidation rate•Higher oxidation rate in pure water vapour

SiO2(s) + H2O(g) = SiO(OH)2(g)

SiO2(s) + 2 H2O(g) = Si(OH)4(g)


Oxidation of SiC at low PoOxidation of SiC at low Po22: active oxidation: active oxidation

•Same mechanism that the oxidation of Si and other ceramics

– SiC + O2(g) = SiO (g) + CO(g)

Oxidation - SiC

Volatilization

CVD-SiC in 0.1 MPa at 1600°C – Po2 in Ar from 0 to 160Pa

Corresponding rate constant for active oxidation at two gas flow rates

[Goto et al., Corrosion in advanced ceramics, KG Nickel Ed. (1993) 165]

Mass Change ka


Oxidation of SiC at low PoOxidation of SiC at low Po22: active oxidation: active oxidation

•Transition point between active and passive oxidation

Oxidation - SiC

Theory (Wagner)

Theory (Volatility diag.)

•Determining factors for transition

– Temperature

– Po2

– SiC purity

– Vgas

– Total pressure

Active to passive transitions from the literature for different types of SiC[Opila & Jacobson, in Materials science and technology Vol. 19, RW. Cahn et al. Ed. (2000)]


Oxidation at low PoOxidation at low Po22: effect of water vapour: effect of water vapour

•Active oxidation by water vapour

SiC + 2 H2O(g) = SiO(g) + CO(g) + 2 H2(g)

Oxidation - SiC

PLS α-SiC at 1300° and 1400°C 10min in H2 with different P(H2O)[Opila & Nguyen., J Am Ceram Soc 81 (1998) 1949]

Corrosion rate Flexural strength at RT

Active to passive transition

active passive

1400°C


Oxidation of SiC-based matrixes at high PoOxidation of SiC-based matrixes at high Po22

•Under oxidizing atmosphere CVD-SiC (representative of CVI-SiC: Passive oxidation

Oxidation - matrix

Amorphous SiO2

Crystallisation

CVD-SiC representative of CVI-SiC at 1000°C and 100 kPa[Naslain et al. J Mater Sci 39 (2004) 7303]

Thickness of the SiO2 scale


Oxidation of fibres: passive mode at high PoOxidation of fibres: passive mode at high Po22

•Growth of silica around the fibre surface (2nd and 3rd generation fibres)

Oxidation - fibres

Hi-Nicalon fibres (SiC-C) in Ar-O2 at 1300°C[Shimoo et al. J Mater Sci 35 (2000) 3301)]

Hi-Nicalon fibres (SiC-C) in Ar-25%O2

Flexural strengthMass change in Ar-25%O2

SiO2

Oxidation in Ar-O2 at 1500°C[Shimoo et al., J Ceram Soc Japan 108 (2000) 1096)]

Nicalon

Hi-Nicalon

Hi-Nicalon S

Mass change at 1300°C


Oxidation of fibres: active mode at low PoOxidation of fibres: active mode at low Po22

•Volatilization of SiO(g)

SiC(s) + O2(g) = SiO(g) + CO(g)

+ recrystallisation of SiC

Oxidation - fibres

Lox M fibres in Ar-O2 at 1500°C[Shimoo et al. J Mater Sci 37 (2002) 4361)]

Mass change at 1500°C

Passive oxidation

Active oxidation

SiO2

SiC

RT tensile strength

RT tensile strength for fibres heated for 20hrs in Ar-O2 at 1500°C


Oxidation of fibres: case of 1st generationOxidation of fibres: case of 1st generation

•Active oxidation

SiC + O2(g) = SiO(g) + CO(g)


Oxidation - fibres

Nicalon CG fibres in Ar-O2 at 1500°C[Shimoo et al. J Amer Ceram Soc 83 (2000) 3049]

Mass change

•Thermal decomposition of Si-C-O

SiCO = SiO(g) + CO(g) + SiC + C


•Passive oxidation with SiO2 growth

SiC + 3/2O2(g) = SiO2 + CO(g)

No thermal decomposition of Si-C-O


Oxidation of fibres: active to passive transitionOxidation of fibres: active to passive transition

Fibres heated 72 ks in Ar-O2 at 1500°C[Shimoo et al. J Mater Sci 37 (2002) 1793] Po2 for active to passive transition

Mass changeActive to passive transition

•As for pure SiC, there is an active to passive transition

Oxidation - fibres


Oxidation of fibres: effect of water vapor at high PoOxidation of fibres: effect of water vapor at high Po22

•As for pure SiC, H2O increases the oxidation rate

Kp for Hi-Nicalon fibres tested in N2/O2/ H2O under 100 kPa and Po2=20 kPa[Naslain et al. J Mater Sci 39 (2004) 7303]

Oxidation - fibres

Tensile strength of SiC fibres after 10h at 1400°C in dry or wet (2%H2O) air[Takeda et al. J Nucl Mater 258-263 (1998)1594]

Ln

(K

p) (h

-

1)


Oxidation of the interphase at any PoOxidation of the interphase at any Po22

•Carbon is highly oxidizable at T>600°C

– C + O2(g) = CO2(g)

– C + ½ O2(g) = CO(g)

– C + 2 H2O(g) = CO2(g) + 2 H2(g)

– C + H2O(g) = CO(g) + H2(g)

•Oxidation rate is dertermined by – Temperature

– Po2

– Total pressure– Gas flow rate

Oxidation - interphase


ContentContent




•Oxidation properties


– Thermal aging

– Oxidation

– Improvement of the HT oxidation resistance



Thermal aging of UD SiCThermal aging of UD SiCff/SiC (inert gas)/SiC (inert gas)

•UD-SiCf/C/PIP-SiCm

•Nicalon CG - 1st generation SiCO : thermal decomposition

•Hi-Nicalon - 2nd generation SiC-C (0.5% O) : stable up to 1350°C

•Hi-Nicalon S - 3rd generation: nearly stoichiometric

composite – thermal aging

Mass changeFracture strength

UD SiCf/C/PIP-SiCm 3.6ks in vacuum [Araki et al. J Nucl mater 258-263 (1998) 1540]

Residual oxygen


Thermal aging of 2D SiCThermal aging of 2D SiCff/SiC (inert gas)/SiC (inert gas)

•2D Nicalon CG/C/CVI-SiC

•1st generation SiCO : thermal decomposition

SiCO = SiO(g) + CO(g) + SiC + C

•Interaction with the interphase

SiO(g) + 2 C = SiC + CO(g)

composite – thermal aging

Stress-strain curves of 2D Nicalon/C/SiC composite at RT after thermal aging in vacuum under various conditions[Labrugère et al. J Eur Ceram Soc 17 (1997) 623]

coarse SiC

Tensile strength

Interfacial decohesion (weakening of the fibre-matrix bounding)

Partial consumption of the interphase with formation of coarse surface SiC-grains (weakening of the fibres)

Total consumption of the interphase with decomposition/crystallisation (fully brittle)


Passive oxidation of model SiCPassive oxidation of model SiCff/SiC/SiCmm (high Po (high Po22))

•Passive oxidation of fibres and matrix

– SiC + 3/2 O2(g) = SiO2 + CO(g)

– SiC + 2 O2(g) = SiO2 + CO2(g)

•Oxidation of the interphase

– C + O2(g) = CO2(g)

– C + ½ O2(g) = CO(g)

•Model UD Nicalon/C/CVI-SiC no coating on the back and front surfaces gas phase diffusion of O2 and CO in the pore

reaction of O2 with the C interphase

diffusion of O2 in SiO2 and reaction with SiCf

diffusion of O2 in SiO2 and reaction with Sim

[Filipuzzi et al. J Amer Ceram Soc 77 (1994) 459]

composite – oxidation


Passive oxidation of 2D SiCPassive oxidation of 2D SiCff/C/SiC/C/SiCmm (high Po (high Po22))

2D Nicalon / C (δ=0.1 µm)/ CVI-SiC without an anti-oxidation coating heated for 35hrs in air at different temparatures [Huger et al. J Amer Ceram Soc 77 (1994) 2554]

•Oxidation of the interphase

– C + O2(g) = CO2(g)

– C + ½ O2(g) = CO(g)

•Passive oxidation of fibres and matrix

– SiC + 3/2 O2(g) = SiO2 + CO(g)

– SiC + 2 O2(g) = SiO2 + CO2(g)

•Sealing of the pore•Passive oxidation of the matrix

– SiC + 3/2 O2(g) = SiO2 + CO(g)

– SiC + 2 O2(g) = SiO2 + CO2(g)


Residual Young’s modulusMass change


Active oxidation of 2D SiCActive oxidation of 2D SiCff/C/SiC/C/SiCmm (low Po (low Po22))

•SiC-based fibers are basically instable

SiC + O2(g) = SiO(g) + CO(g) + recrystallisation of SiC

•Strong impact on the fibre strength that provides the mechanical properties of the composite

– Surface flaws cracks


RT tensile strength of fibres heated for 3.6ks in Ar-O2 at 1500°C[Shimoo et al. J Mater Sci 37 (2002) 4361)]

Fully brittleno test


Oxidation of composites under loadOxidation of composites under load

•Even for coated specimens

•At >0-100MPa matrix cracking

•At 500-1000°C

•Jones et al. proposed a Po2/T map


Fibre creep only

Interphase removal

SiO2 on the fibres

Crack velocity for model composite with Nicalon fibres at 1100°C[Jones et al. Mater Sci Eng A198 (1995) 103]

[Jones et al. J Amer Ceram Soc 83 (2000) 1999]


Improvement of the oxidation resistance: EBCImprovement of the oxidation resistance: EBC

•Environmental Barrier Coating


CVD SiCSiO2

Time at 1000°C in air (h)

r

(MP

a)

RT flexural strength of a 2D-Nicalon/C/CVI-SiC with and without a CVD-SiC seal coat after oxidation in air at 1000°C[Lowden, in Designing Ceramic Interfaces II, Peteves Ed. (1993) 157]

•Boron forms an oxide with a low melting point [Tf(B2O3)=450°C]

2B + O2 B2O3

2BN + O2 B2O3 + N2(g)

B4C + 4 O2 2 B2O3 + CO2(g)

SiB6 + 11/2 O2 3 B2O3 - SiO2

•Fusible boron oxide or boron silicate seal the porosity and the crack tips

SiC

Si or SiC bound coat

B-based phaseSiO2


Improvement of the oxidation resistance:Improvement of the oxidation resistance:

•Matrix with dispersed particles

– Boron-based particles: B4C, BN, SiB6

– Forms a healing oxide

– Matrix fabricated by PIP

self-healing matrixesself-healing matrixes

Time (h)

Ap

pli

ed

s

tre

ss

Fatigue life (tensile) at 900°C in air[Steyer et al., J Amer Ceram Soc 81 (1998) 2140]

Nicalon fibres

2D-Nicalon/C/SiC

2D-Nicalon/C/SiC+C-B

•Multilayer matrix

– Low melting phase X: B, B4C, Si-B-C

– Compliant material Y: PyC, C(B), hex-BN

– Matrix fabricated by P-CVI: (X-Y-X-Y’)n

[Lamouroux et al., Composites Sci Technol 59(199) 1073]



•B-based interphases: hex-BN or C(B)

2BN + O2 B2O3 + N2(g)

2B + O2 B2O3

Forms a healing oxide

•Multilayer interphase

– Oxidation resistant material: SiC, TiC

– Compliant material Y: PyC, hex-BN

– Deposition by P-CVI: (X-Y-X-Y’)n


Improvement of the oxidation resistance:Improvement of the oxidation resistance:

alternative interphasesalternative interphases

Fatigue life (4-point bending) of 2D-Nicalon/PyC or BN/CVI-SiC in air at 600° and 950°C[Lin et al., Mater Sci Eng A321 (1997) 143)]


ContentContent




•Oxidation




R&D needs for qualifing SiC/SiC composite for GFRR&D needs for qualifing SiC/SiC composite for GFR

Corpus of data on the thermal aging and oxidation behaviour of composites

• All studies are on a very short term!

• For monolithic SiC: wide ranges of temperature and P(O2) were covered

– Widespread results (strong dependence to SiC purity and nature)

– Few data on the effect of water in relevant ranges

• For components: some domains of temperature and P(O2) were investigated

– Strong influence of chemistry, structure and fabrication processes

Pre-selection of candidate technologies and systematic study

• For whole composites: some particular studies at high P(O2)

Helium +O2, H2O

900°-1200°CVery long times

+Short time at 1600°C

(even 2000°C)

conclusion


R&D needs for qualifing SiC/SiC composite for GFRR&D needs for qualifing SiC/SiC composite for GFR

Choice of best state of the art materials

• Stoichiometric fibres

• Low-porosity matrix (+dispersed particles) or multilayer matrix

• Environmental Barrier Coating

• Multilayer interphase

Acceptability ofadditives and B ?

Control of the environment

• Control of the Po2 (lower and upper limit)

• Control of the PH2O

(upper limit)

• Limit on the temperature

• Design

Helium +O2, H2O

900°-1200°CVery long times

conclusion

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