Thermal Oxidation and High Temperature Structures of Uranium Carbide—New Insights into the U-C-O System from In Situ X-Ray Diffraction Studies

Uranium carbide (UC) shares a number of characteristics with industry standard uranium oxide, that make both ceramics well-suited to nuclear fuel applications, including high melting points and high tolerance to irradiation. In addition to this, UC, along with other non-oxide ceramic fuel candidates, boasts higher thermal conductivity and higher fissile density than conventional uranium oxide fuels, making it a strong candidate for use in emerging Generation IV advanced reactors as both a pristine fuel and a component in accident tolerant TRISO fuels. For fuel qualification, it is critical to evaluate thermal oxidation behavior, phase changes and thermodynamics of UC under normal and off-normal reactor conditions. A rigorous understanding of U-C-O phase equilibria is also a requisite for assessing possible disposal routes, where oxidation to less reactive oxides is widely considered the necessary first step prior to storage. In this work, UC was characterized with in situ high temperature synchrotron x-ray diffraction under low and 0.21 atm O₂ partial pressure conditions, with temperatures up to 960 K and 770 K respectively.

Among the two in situ studies, we were able to determine the temperature dependent thermal expansion function of UC from room temperature to 960 K from the low oxygen partial pressure conditions. The mean coefficient of thermal expansion at 960 K was found to be 11.5 ● 10^-6 K^-1, in good agreement with previously reported values. Of particular interest in this experiment was the observed formation of the tetragonal I4/mmm α-UC₂ phase around 800 K. Thermodynamically α-UC₂ is not favored to form at this temperature but may be stabilized as oxygen substituted UC_2-xO_x under low O₂ conditions. Until now the lower temperature boundary of the region of stability for this phase has not been identified. Herein, from its observed formation at 800 K, we report a CALPHAD optimized Gibbs molar energy for the α-UC_2-xO_x phase. Furthermore, the second experiment conducted in open-air provided a clear bulk thermal oxdiation pathway of UC: UC → UO₂ → U₃O₈, which was confirmed by reitveld refinement of diffraction patterns.

New insights and discoveries from this work emphasizes the incomplete nature of our understanding of U-C-O phase equilbria, highlighting the importance of continued investigation into this critical system for the deployment of fuels in advanced reactors both during operation and for permanent spent fuel disposal.