Advanced nuclear fuels by spark plasma sintering � fuel properties and accident tolerance

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Gong, Bowen
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Electronic thesis
Mechanical engineering
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This thesis mainly focuses on the development of advanced fuel fabrication technologies to manufacture oxide fuels and new strategies of improving oxidation and corrosion resistance of high-density uranium silicide fuels as the leading concept of accident tolerant fuels (ATFs) for safer and more effective nuclear energy systems. Spark plasma sintering (SPS), a field-assisted fuel sintering technology, has been applied as the advanced fuel manufacturing approach to fabricate oxide and silicide fuels. The properties of the SPS-densified nuclear fuels were characterized symmetrically with the focus on fuel microstructure, microchemistry, thermal-mechanical properties, and oxidation/corrosion resistance. Commercial-size UO2 fuel pellets with a theoretical density of 95% were consolidated by SPS at 1600 C for 5 minutes. Uniform densification and stoichiometric UO2 with an ideal fluorite structure across the commercial-size fuel pellet were identified, but with a distributed grain structure because of the non-uniform distribution of temperature during sintering. Nanoindentation and microindentation testing was performed at different temperatures. The mechanical properties of the sintered fuels were characterized as functions of grain structure and temperature. Nanocrystalline UO2 displays higher hardness than the microcrystalline counterpart, consistent with the Hall-Petch strengthening mechanism. Greater Young’s modulus and fracture toughness are identified for the nanocrystalline UO2. Hardness and Young’s modulus decreased with temperature, suggesting better ductility of oxide fuels at a greater temperature and a small length scale. Hyper-stoichiometric UO2 specimen displays higher hardness and fracture toughness than stoichiometric UO2 due to the impediment of the crack propagation by the oxygen interstitial atoms. The results are useful in understanding the mechanical properties of the high burn-up structure (HBS) formed in nuclear fuels during reactor operation and also provide critical experimental data as the input for the development and validation of the MARMOT fracture model of nuclear fuels. Monolithic dense U3Si2 pellets with controlled grain structure and enhanced thermal-mechanical and oxidation properties were also synthesized with SPS. A dominant phase of distorted U3Si2 was identified with lattice expansion due to residual thermal stress upon SPS consolidation and rapid cooling processes. Both micron-sized and nano-sized pellets show exceptional thermal transport properties, simultaneously high hardness, fracture toughness, as well as exceptional oxidation performance with extended onset oxidation temperature and reduced oxidation kinetics. A new concept of strain engineering was proposed for properties optimization, enabling the development of potential oxidation and corrosion-resistant silicides with extended performance. High-temperature creep behavior of dense U3Si2 pellets was assessed using an SPS apparatus at elevated temperatures and pressures under vacuum conditions. The stress exponent was subsequently derived to be 3.21 at 1173 K and 2.17 at 1223 K, indicating a grain boundary sliding creep mechanism. The creep activation energy was subsequently determined to be 203.6 ± 19.0 kJ/mol, which agrees well with the literature. Microstructure characterizations indicate that the main phase of the specimens after creep tests remains U3Si2. The successful conduct of creep experiments demonstrates the great potential of SPS to perform high-temperature mechanical testing of nuclear fuels under vacuum conditions. The subsequent finite element modeling exhibits excellent capabilities for accurately predicting material performance in the creep tests and provides a practical tool in evaluating nuclear fuels’ performance for a much-extended time scale. The oxidation kinetics of the SPS-densified U3Si2 with controlled microstructure were investigated as the high-density fuel suffers from severe oxidation and steam corrosions and thus structural disintegration. In addition, the detailed oxidation kinetics of U3Si2 is scarcely reported, and the oxidation mechanisms have not been fully elucidated. Therefore, the oxidation behavior of microcrystalline (mc-) and nanocrystalline (nc-) U3Si2 have been systematically investigated using a thermogravimetric analyzer (TGA) through a series of isothermal and non-isothermal kinetic studies. The isothermal kinetic study with a model-fitting approach indicates an activation energy of 85 kJ/mol for mc-U3Si2 and 96.4 kJ/mol for nc-U3Si2; while the isoconversional approach leads to an activation energy in the range of 70 to 85 kJ/mol for mc-U3Si2 and 75 to 86 kJ/mol for nc-U3Si2. The higher energy barriers for oxidation of nc-U3Si2 is consistent with the improvement of the oxidation resistance, as evidenced by a slower oxidation rate. This can be attributed to a strong strain effect in the nc-U3Si2 pellets densified by spark plasma sintering, highlighting the important impacts of microstructure controls on the performance of nuclear fuels. We also explored different strategies of using composite fuels, e.g., U3Si2-UO2 and U3Si2-UN composites and metallic additives, to further improve the oxidation and corrosion resistance of high-density silicide fuels. Dense Cr-doped U3Si2 composite fuels were manufactured by SPS, and the effects of Cr addition on mechanical properties and oxidation resistance were investigated. Dynamic oxidation testing by TGA revealed significantly improved oxidation resistance of U3Si2 with a minimal doping amount of 3 wt% Cr. The onset oxidation temperature increased to above 550 ºC in air and ~520 ºC in steam conditions for 5 wt% and 10 wt% Cr-doped composites. Steam corrosion testing under 360 ºC for 24 hours indicated well-maintained pellet integrity without pulverization for the 10 wt% Cr-doped U3Si2 pellet, showing only minor surface oxidation. The first promising results open up the possibility of designing and manufacturing metal additive-doped U3Si2 composite fuels with significantly improved corrosion resistance as a potential candidate of accident tolerant fuels. The U3Si2 and UO2 composites with different silicide and oxide ratios were sintered by SPS at temperatures from 1,000 to 1,300 °C. The microstructure and phase composition of the SPS densified composite fuels were characterized with scanning electron microscopy, X-ray diffraction (XRD), and energy dispersed spectroscopy (EDS). A systematic study of the thermal and mechanical properties was conducted, along with oxidation resistance measurements using TGA. Improved physical density generally leads to improved hardness, fracture toughness, thermal diffusivity, and onset temperature during the oxidation process. The composite with 50 wt% UO2 sintered at 1300 °C displayed an onset oxidation temperature of 500 °C by dynamic oxidation testing at a ramp degree of 10 °C/min. The composite also achieved a high fracture toughness of ~3.5 MPam½. These results highlight the potential of composite fuel forms densified by SPS with simultaneously enhanced fissile element density, fracture toughness, thermal transport properties, and oxidation resistance. Both UN and U3Si2 are potential candidates for accident tolerant fuels due to their high fissile element density and exceptional thermal conductivity. However, they display a high susceptibility to oxidation and corrosion in steam environments. UN-U3Si2 composites were sintered by spark plasma sintering, and the micro-cracks can be significantly mitigated by controlling the cooling. The composite with 50 wt% UN and 50 wt% U3Si2 displays simultaneously enhanced strength and fracture toughness and possesses excellent thermal conductivity. The onset temperatures of all composites tested through dynamic oxidation testing are close to 540 ºC, suggesting significantly improved oxidation resistance than the monolithic UN. These results highlight the important application of using SPS as an advanced manufacturing technology to fabricate nuclear fuels with controlled microstructure and fuel chemistry, and effective strategies of designing composite fuels and using metallic additives that can greatly improve fuel properties and accident tolerance. Further evaluations are needed to assess and verify the behavior and performance of the advanced fuels in reactor operation environments.
May 2022
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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