Uranium silicides, such as U3Si2, USi, U3Si5 and USi3, have attracted considerable interest due to increasing demands for accident-tolerant fuels (ATFs), especially in the aftermath of the Fukushima disaster. Uranium dioxide, UO2, is currently the major nuclear reactor fuel, but its thermal conductivity is undesirably low, resulting in large temperature gradients and therefore excessive stored energies. By contrast, the U-Si binary compounds possess much higher thermal conductivities, and further, the U-rich compositions (such as U3Si2) contain higher amounts of U and are thus more favored for nuclear fuel applications. However, to utilize U-Si compounds as high-efficient ATFs, it is essential to investigate their structure, stability and properties at high temperature (operational) and hydrothermal (accident) conditions. In this study, we employed high-temperature or high-pressure synchrotron X-ray diffraction to characterize crystal structural variations of these compounds as a function of temperature or pressure. Rietveld analyses of the obtained data allowed determination of unit-cell parameters, atomic positions and atomic displacement parameters at various temperatures or pressures, from which the thermal expansion coefficients and bulk moduli were derived. Second, we studied the thermodynamic stability of these phases using high-temperature transposed temperature drop calorimetry under an oxygen atmosphere. With increasing U/Si ratio, the determined enthalpies of oxidation become more exothermic while standard enthalpies of formation become less exothermic, indicating decreased stability. Finally, we examined the behavior of U3Si2 in pressurized water at temperatures up to 350 °C using autoclave methods. Solubility measurements yielded the Gibbs free energy of formation of the silicate phase formed as coating layers of U3Si2 from the hydrothermal reaction. These results have important implications for the utilization of U-Si compounds as nuclear fuels at reactor operational and accidental conditions.

Structure and stability of uranium silicides for nuclear fuel applications

2019 Spring National Meeting

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