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Fuel assembly

Review: chemical compatibility of SiC/SiC composites with the GFR environment C. Cabet Laboratoire of Non Aqueous Corrosion, CEA Saclay, FRANCE. GFR and SiC/SiC composites. Fuel assembly. 850°C. Heat eXchanger. Helium. Introduction. Concepts of fuel assembly. Needle concept. Plate concept.

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Fuel assembly

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  1. Review: chemical compatibility of SiC/SiC composites with the GFR environmentC. CabetLaboratoire of Non Aqueous Corrosion, CEA Saclay, FRANCE

  2. GFR and SiC/SiC composites Fuel assembly 850°C HeateXchanger Helium Introduction

  3. Concepts of fuel assembly Needle concept Plate concept Introduction

  4. Best candidate material : SiCf/SiCm Requirements 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 Introduction

  5. secondary circuit cooler H2 ? Helium Helium + traces air, H2O air, H2O CO, CH4 ? air, H2O, refuelingmaintenance degassing 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 : impure helium Introduction

  6. SiCf/SiCm usual applications • High temperature • Oxidative atmospheres • Inspection and repair • Short term Turbines Rocket engines Aircraft engines Introduction

  7. SiCf/SiCm compatibility with GFR physico-chemical conditions over long term ? • Thermal stability • Oxidation resistance • Consequences of thermal aging and oxidationon the mechanical (and confining) properties • Improvement strategies Lifetime prediction Introduction

  8. Content • Introduction on the GFR application • SiCf/SiCm structure and fabrication • Thermal stability • Oxidation propertis • Composite resistance • R&D needs to qualify SiC/SiC for GFR applications

  9. SiCf/SiCm structure SiC-basedfibre ~10µm SiC-based matrix crack interphase (C) ~0.1µm Fibres in bundle UD or 1D 2D 3D SiCf/SiCm structure and fabrication

  10. preceramic pyrolysis Polymer infiltration Pre-forming Carbon coated fibre tows Pyrolysis 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 porosity additives

  11. spinning curing Weak fibres Dense fibres PCS SiC-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 • 1st generation • cure in oxygen at T~1200°C • Si-C-O: 2nm SiC + C + SiCxOy SiCf/SiCm structure and fabrication

  12. SiC-based fibres: 3 generations • Exemple of the development of the Nicalon fibres by Nipon Carbon

  13. Interphase • Compliant material • Thin layer ~100nm • « leaf » structure • pyrocarbon • hex-BN • Multilayer

  14. Content • Introduction on the GFR application • SiCf/SiCm structure and fabrication • Thermal stability • Monolithic SiC • Matrix • Fibres • Oxidation properties • Composite resistance • R&D needs to qualify SiC/SiC for GFR applications

  15. SiC phase diagram • Stoichoimetric • no other intermediate compound • SiC (SiC)(l) + C 2540°C Thermal stability

  16. Thermal stablity of SiC in vacuum or inert atmopshere • Thermodynamic calculation SiC C + Si(g) + recrystalisation • Kinetic factors: SiC stable up to ~1600°C SiC + Si 104/T (K) Thermal stability

  17. Thermal Stability of the matrix in vacuum or inert atmopsheres • SiC and SiC/C matrixes are stable up to about 1600°C Thermal stability

  18. Thermal Stability of fibres Fibres of the 1st generation: Si-C-O • Basically instable à T>1200°C • (SiC, C, SiC2xO1-x)  w SiC + x C + y CO(g) + z SiO(g) Porous C/SiC (large grains) Mass loss Decrease the creep strength 1300°C 1200°C Creep curves for Nicalon fibres tested in pure Ar under 0.7 GPa Mass loss for Nicalon fibres tested in pure Ar [Bodet et al. J Amer Ceram Soc 79 (1996) 2673] Thermal stability

  19. Thermal Stability of fibres Fibres of the 2nd generation: Si-C(0.5% O) • Stable up to 1350°C • (SiC, C) + Otrace(g,s)  SiC + CO(g) +C Large grains Mass loss = r 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] 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] Thermal stability

  20. Thermal Stability of fibres Nearly stoichiometric fibres • Stable up to very high temperatures 1800°-2000°C • Some SiC grain growth • Good mechanical properties up to 1400°-1500°C 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

  21. Content • Introduction on the GFR application • SiCf/SiCm structure and fabrication • Thermal stability • Oxidation properties • Monolithic SiC • passive oxidation • active oxidation • Matrix • Fibres • Interphase • Composite resistance • R&D needs to qualify SiC/SiC for GFR applications

  22. Oxidation of SiC at high Po2: 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 Very protective T>800°C Monolithic SiC Parabolic rate constant linear rate constant Scale thickness a-SiC in 1 atm air [Costello & Tressler, J Am Ceram Soc 64 (1981) 327] Oxidation - SiC

  23. Oxidation of SiC at high Po2: mechanism T>800°C Monolithic SiC Growth rate = oxygen transport through the SiO2 scale T>1400°C Ea 150-300 kJ/mole atomic diffusion cristobalite T<1400°C Ea 300 kJ/mole molecular diffusion amorphous SiO2 90µm MEB image of sintered a-SiC 6hrs at 1400°C in 1 atm air [Costello & Tressler, J Am Ceram Soc 64 (1981) 327] KP for the oxidation of single-crystal SiC under 0.001 atm O2 [Zheng, J Electrochem Soc 137 (1990) 854] Oxidation - SiC

  24. Oxidation at high Po2: 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 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)] Kp from the literature for different type of SiC [Narushima et al., J Am Ceram Soc 72 (1989) 1386] Oxidation - SiC

  25. Oxidation at high Po2: 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) 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) CVD-SiC at 1200°C in pure CO2, pure O2 and 50%H2O/50%O2 [Opila & Nguyen., J Am Ceram Soc 81 (1998) 1949] Oxidation - SiC

  26. Oxidation of SiC at low Po2: active oxidation • Same mechanism that the oxidation of Si and other ceramics • SiC + O2(g) = SiO(g) + CO(g) Volatilization Mass Change ka 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] Oxidation - SiC

  27. Oxidation of SiC at low Po2: active oxidation • Transition point between active and passive oxidation • Determining factors for transition • Temperature • Po2 • SiC purity • Vgas • Total pressure Theory (Wagner) 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)] Theory (Volatility diag.) Oxidation - SiC

  28. Oxidation at low Po2: effect of water vapour • Active oxidation by water vapour SiC + 2 H2O(g) = SiO(g) + CO(g) + 2 H2(g) Active to passive transition Corrosion rate Flexural strength at RT active passive 1400°C PLS α-SiC at 1300° and 1400°C 10min in H2 with different P(H2O) [Opila & Nguyen., J Am Ceram Soc 81 (1998) 1949] Oxidation - SiC

  29. Oxidation of SiC-based matrixes at high Po2 • Under oxidizing atmosphere CVD-SiC (representative of CVI-SiC: Passive oxidation Thickness of the SiO2 scale Crystallisation Amorphous SiO2 CVD-SiC representative of CVI-SiC at 1000°C and 100 kPa [Naslain et al. J Mater Sci 39 (2004) 7303] Oxidation - matrix

  30. Oxidation of fibres: passive mode at high Po2 • Growth of silica around the fibre surface (2nd and 3rd generation fibres) SiO2 Flexural strength Mass change at 1300°C Mass change in Ar-25%O2 Hi-Nicalon S Hi-Nicalon Oxidation in Ar-O2 at 1500°C [Shimoo et al., J Ceram Soc Japan 108 (2000) 1096)] Nicalon Hi-Nicalon fibres (SiC-C) in Ar-25%O2 Hi-Nicalon fibres (SiC-C) in Ar-O2 at 1300°C [Shimoo et al. J Mater Sci 35 (2000) 3301)] Oxidation - fibres

  31. RT tensile strength SiO2 RT tensile strength for fibres heated for 20hrs in Ar-O2 at 1500°C SiC Oxidation of fibres: active mode at low Po2 • Volatilization of SiO(g) SiC(s) + O2(g) = SiO(g) + CO(g) + recrystallisation of SiC Mass change at 1500°C Passive oxidation Active oxidation Lox M fibres in Ar-O2 at 1500°C [Shimoo et al. J Mater Sci 37 (2002) 4361)] Oxidation - fibres

  32. Oxidation of fibres: case of 1st generation • Active oxidation SiC + O2(g) = SiO(g) + CO(g) + recrystallisation of SiC Mass change • Passive oxidation with SiO2 growth SiC + 3/2O2(g) = SiO2 + CO(g) No thermal decomposition of Si-C-O • Thermal decomposition of Si-C-O SiCO = SiO(g) + CO(g) + SiC + C + recrystallisation of SiC Nicalon CG fibres in Ar-O2 at 1500°C [Shimoo et al. J Amer Ceram Soc 83 (2000) 3049] Oxidation - fibres

  33. Oxidation of fibres: active to passive transition • As for pure SiC, there is an active to passive transition Mass change 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 Oxidation - fibres

  34. Oxidation of fibres: effect of water vapor at high Po2 • As for pure SiC, H2O increases the oxidation rate Ln (Kp) (h-1) 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] 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

  35. Oxidation of the interphase at any Po2 • 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

  36. Content • Introduction on the GFR application • SiCf/SiCm structure and fabrication • Thermal stability • Oxidation properties • Composite resistance • Thermal aging • Oxidation • Improvement of the HT oxidation resistance • R&D needs to qualify SiC/SiC for GFR applications

  37. Thermal aging of UD SiCf/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 Mass change Residual oxygen Fracture strength UD SiCf/C/PIP-SiCm 3.6ks in vacuum [Araki et al. J Nucl mater 258-263 (1998) 1540] composite – thermal aging

  38. Thermal aging of 2D SiCf/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) Tensile strength coarse SiC 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) 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] composite – thermal aging

  39. Passive oxidation of model SiCf/SiCm (high Po2) • 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

  40. Passive oxidation of 2D SiCf/C/SiCm (high Po2) • 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 modulus Mass change 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] composite – oxidation

  41. Active oxidation of 2D SiCf/C/SiCm (low Po2) • 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 Fully brittleno test RT tensile strength of fibres heated for 3.6ks in Ar-O2 at 1500°C [Shimoo et al. J Mater Sci 37 (2002) 4361)] composite – oxidation

  42. Oxidation 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 SiO2 on the fibres Interphase removal Fibre creep only 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] composite – oxidation

  43. SiO2 SiO2 B-based phase SiC CVD SiC Si or SiC bound coat Improvement of the oxidation resistance: EBC • Environmental Barrier Coating • 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 r (MPa) 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] Time at 1000°C in air (h) composite – oxidation

  44. Nicalon fibres Applied stress 2D-Nicalon/C/SiC+C-B Fatigue life (tensile) at 900°C in air [Steyer et al., J Amer Ceram Soc 81 (1998) 2140] 2D-Nicalon/C/SiC Time (h) Improvement of the oxidation resistance: self-healing matrixes • Matrix with dispersed particles • Boron-based particles: B4C, BN, SiB6 • Forms a healing oxide • Matrix fabricated by PIP • 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] composite – oxidation

  45. Improvement of the oxidation resistance: alternative interphases • 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 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)] composite – oxidation

  46. Content • Introduction on the GFR application • SiCf/SiCm structure and fabrication • Thermal stability • Oxidation • Composite resistance • R&D needs to qualify SiC/SiC for GFR applications

  47. R&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°C Very long times + Short time at 1600°C (even 2000°C) conclusion

  48. R&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 ? Helium +O2, H2O Control of the environment • Control of the Po2 (lower and upper limit) • Control of the PH2O (upper limit) • Limit on the temperature • Design 900°-1200°C Very long times conclusion

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