NEA/NSC/DOC(2013)9
Advanced fuel technologies at General Atomics
Christina A. Back General Atomic (GA), US
General Atomics (GA) has made significant contributions since its founding in the 1950s to develop nuclear power for peaceful means. With the conception and construction of the TRIGA reactors and research on TRISO particles, GA has long recognised the importance of “accident-tolerant” materials. Before the accident at Fukushima Daiichi, GA had already initiated work on silicon carbide (SiC) and SiC-related technologies for application in nuclear reactors. At that time, the work was initiated in support of the GA advanced gas-cooled fast reactor concept called the Energy Multiplier Module, EM2. This work continues, however, the reasons that make SiC materials attractive for fast reactor concepts also make them attractive for advanced light water reactors. These include superior performance over zircaloy for high-temperature strength, especially above 1 500°C, and significantly reduced hydrogen production in accident scenarios. The current focus on “accident-tolerant” components is to develop cladding made of silicon carbide fiber and silicon carbide matrix, SiC-SiC composites. The goal for this work is to produce a cladding that provides strength and impermeability to meet reactor performance and safety requirements. To date, GA has examined the trade-offs between processing time and infiltration uniformity to reduce fabrication time, fabricated cylindrical prototypes, and refined material properties for fracture toughness, impermeability, and thermal conductivity. Generally, the GA programme is developing innovative fuel elements that employ both high density uranium-bearing fuels that enable longer lifetime with higher burn-up, and claddings that are more resistant to neutron damage. In addition to fabrication, significant effort is devoted to measuring the critical parameters, such as thermal conductivity, mechanical strength and component performance at reactor-relevant operational conditions, using a mix of commercial equipment, customised in-house test rigs, and specialised fixtures. Furthermore, GA strives to iteratively refine models and simulations with benchtop experimental data to accelerate process development and optimise component design. Throughout the programme, GA maintains active collaborations with industry, universities, and national laboratories. This work has been supported by General Atomics internal funding.
34 INCREASED ACCIDENT TOLERANCE OF FUELS FOR LIGHT WATER REACTORS, © OECD 2013 Advanced Fuel Technologies at General Atomics
By Dr. Christina A. Back
Presented at OECD/NEA Workshop on Accident Tolerant Fuels of LWRs
Paris, France
10 December 2012
OECD/NEA Workshop on ATF 1 SiC Composite Clad for Light Water Reactors Can Make a Major Improvement in Safety
• Zr + 2H2O è ZrO2 + 2H2 + 595 kJ/g-mole è • SiC + 4H2O SiO2 + CO2 + 4H2 + 264 kJ/g-mole Fukushima Daiichi
2.0 Zircaloy 100%)consumed Eliminate hydrogen explosions 1.5 9.5 9.5 Zircaloy SiC-SiC 1400oC mm mm clad clad rod surface
2 o 1100 C 0.57 mm 1.0 1.0 mm
8.19 7.5 mm mm
generated/m 0.5
2 o 1400 C UO2 UO2 H 3 o m 1100 C SiC _ <1%)consumed
0.0 β 0 1 2 /SiC β reaction)time)(days) SiC ZIRCALOY ZIRCALOY
OECD/NEA Workshop on ATF 2 β-SiC Composite (SiCβ/SiCβ) Is an Attractive Material for Nuclear Applications
β-phase (diamond) compared to α-phase (hexagonal) SiC • Much better resistance to irradiation damage • Higher strength in irradiated material at high temps (>1200oC) • Better corrosion resistance
crystalline SiCβ fiber pyrocarbon interface SiCβ matrix infiltration SiCβ/SiCβ component a,b c Properties Zircaloy-4 SiCβ/SiCβ Density @ 25oC, g/cc 6.56 2.8 – 3.0 Design tensile stress @316oC, MPa ~450 120-250 Usable tensile strength above 800oC, MPa None 120-250* Irradiated fracture toughness @316oC, Mpa-m0.5 < 45 25-37 Reaction rate with air at 1200oC, mm/s0.5 0.45 0.0026 o 0.5 Reaction rate with H2O at 1200 C, mm/s 5.9 0.005 Thermal absorptions per source neutron, barns/n 7.21E-04 3.27E-04 DPA limit ~10 > 40 Unirradiated thermal cond. at 316oC, W/m-K 18 ┴ 25
a - F Azzarto, JNM (1969) OECD/NEA Workshop on ATF b – E Ibrahim, JNM (1984) 3 c – Y Katoh, JNM (2007) SiC-SiC Improves LWR Safety Through High Temperature Strength
• CVD SiC, stoichiometric SiC fiber, and SiC-SiC composites can hold fission gas pressure beyond 1500°C and shape beyond 2000oC • Zircaloy shows ~90% drop in strength at 800°C SiC-SiC composites Both Zircaloy and SiCβ/SiCβ meet design condition maintain mechanical properties at high temperatures
SiCβ-SiCβ • Strength
• Stiffness
• Toughness
Geelhood, et al., PNNL-17700 (2008) Zircaloy-4 Gulden, J. Amer. Ceram. Soc. (1969)
Katoh, et al., J. Nuc. Mat. (2010)
Hironaka, et al., J. Nuc. Mat. (2002)
Hasegawa, et al., J. Nuc. Mat. (2000)
Snead, et al., J. Nuc. Mat. (2007)
OECD/NEA Workshop on ATF 4 GA has a 40 Year History of Nuclear Fuel Development with Emphasis on Use of SiC for "Accident-Tolerance"
• Improving Nuclear Fuel Clad via
Silicon Carbide Composite (SiCβ/SiCβ)
• Developing SiCβ/SiCβ clad fuel rod for a new reactor concept, EM2 TRISO fuel • Making substantial investment in
people & equipment to develop SiCβ/ SiCβ fab processes EM2 SiC/SiC fabrication • Vested interest in LWR fuel supply fuel development lab – mining, U3O8 supply and UF6 conversion
Rio Grande Resources Heathgate Resources Nuclear Fuel Services EM2 reactor U.S.A and Canada Australia
OECD/NEA Workshop on ATF 5 GA Sol-Gel Fabrication Laboratory Produces High-density Uranium Fuels
Sol-gel column drying calcining sintering UC pellets
gel
UC kernels UN kernels Sol-Gel particles with
carbon
- Materials achieve highly uniform composition - Kernel size and morphology can be controlled
OECD/NEA Workshop on ATF 6 GA SiCβ/SiCβ Development Addresses All Aspects of Fuel Cladding Fabrication
GA SiC lab
End plug fabrication
Composite tubes
Joint sample Monolith fragment
Infiltration Prototypes High strength joint is β-SiC
OECD/NEA Workshop on ATF 7 Many Resources Are Used to Measure Critical Parameters
Commercial analysis equipment Custom test rigs Load Thermal conductivity End plug push-out test assembly
Specialized
fixtures Iosipescu mechanical testing
OECD/NEA Workshop on ATF 8 Advanced Reactor Concepts Pose Research Challenges in Cladding Development and Testing
EM2 reactor Challenges: example • Survive high dpa • Achieve high thermal conductivity • Retain structural integrity with joints • Withstand fuel swelling and thermo-chemical interactions Vented composite endcap • Coated 10um fibers in EM2 bundle • Bundle GA modeling capabilities assembly accelerate process Core development
OECD/NEA Workshop on ATF 9 Development of SiC-SiC Composites Is GA’s Primary Focus for Accident Tolerant Fuel Work
• Design to provide strength, impermeability stress – Meet performance and safety requirements
• Research to accelerate SiC-SiC fabrication time – Reduce fabrication cost – Achieve high density for improved material properties – Model fabrication to aid process optimization
• Research to produce irradiation resistant joints – Ensure joint material is compatible with the parent composite material – Improve irradiation resistance, thermal expansion, relative density, mechanical properties, etc
• Measure and characterize materials and parts
OECD/NEA Workshop on ATF 10 CVI Processing Parameters Are Being Optimized for Composite Fabrication
• Diffusion of MTS between fibers – Affected by sample geometry, depletion, and spatial variation • Trade-off between processing time and uniformity 3 Start of Process 2.5 Mass gain 2
1.5 30% fiber
1 35% fiber
0.5
Normalized mass and mass rate Normalized rate 0 Start Normalized Infiltration time → End End of Process
OECD/NEA Workshop on ATF 11 Cylindrical Prototypes Have Been Infiltrated and Polished
Bi-axial braid Fibers inside tow
5 µm 1/2 foot foot 1/2
Polished Tube
OECD/NEA Workshop on ATF 12 GA Experiments Show Interfaces Have Strong Effect on Thermal Conductivity
• Multi-layer interface showed lower thermal conductivity compared to thin or regular pyrolytic carbon interface – Normalized to density, multi-layer conductivity is ~24% lower than the regular interface; the thin interface is ~9% higher • Density differences do not account for the effect
25 Regular Mul -layer Thin-layer
20
15
10 Conduc vity (W/m-K) 5
0 processingSlow-Fast 1 processingSlow-Slow 2
OECD/NEA Workshop on ATF 13 GA Makes Robust Joints With Polymer-derived β-SiC Material • Larger crystal size undergo fewer irradiation induced structural changes and amorphization than finer crystal structures • XRD analysis and peak broadening evaluation
Scherrer: D=λ/(βcosθ)
D=Crystal grain size λ=wavelength of radiation β=integral breadth of peak – Control of crystal size
~10 nm at TB to ~100 nm at TC – Tyranno SA3 SiC fibers
Davg~100-200nm exhibit good irradiation performance
100 nm grain size is targeted for nuclear applications
OECD/NEA Workshop on ATF 14 The Strength of Polymer-derived Joints Can Be Enhanced by Improving Joint Density Base polymer
10 µm
Slurry
50 µm
• Use of polymer alone leads to high porosity and low joint strength • Addition of SiC powder decreases porosity, crack nucleation sites
OECD/NEA Workshop on ATF 15 GA is Drawing from Industry, Academia and National Labs to Meet the Technological Challenges
• University of California, Berkeley – Use of Focused Ion Beam and Transmission Electron Microscopy techniques to study matrix cracking
• San Diego State University - SiC joining through spark plasmas sintering
• Matech – SBIR involvement to perform characterization of fibers
• University of California at San Diego – Development of fatigue testing
• Brookhaven National Laboratory - 3-D X-ray tomography of SiC
OECD/NEA Workshop on ATF 16 Contact Information
Christina A. Back
(858) 455-2025
This work supported by General Atomics internal funding
OECD/NEA Workshop on ATF 17