Thermal oxidation and high temperature structural behavior of uranium carbide

Uranium monocarbide (UC) exhibits physiochemical characteristics well-suited for nuclear fuel applications in Generation IV reactors, but its high susceptibility to oxidation remains a major barrier to deployment. A detailed understanding of the U-C-O system, including UC thermal oxidation, crystal chemistry, and thermodynamic/kinetic properties, is essential to predict its behavior under normal and off-normal reactor conditions. In this work, in situ high temperature synchrotron X-ray diffraction was conducted under sealed and open-air conditions to characterize UC thermal expansion and oxidation behaviors. From the sealed experiment, the mean coefficient of thermal expansion of UC was determined to be 9.8 × 10⁻⁶K⁻¹ from room temperature to 970 K. Open-air experiments conducted from room temperature to 773 K revealed the oxidation sequence UC → UO₂ → U₃O₈. Notably, a tetragonal U(C_1-xO_x)₂ phase, absent from current thermodynamic predictions, was observed at 840 K, lower than previously considered, suggesting potential relevance for advanced reactor fuel applications. These findings reveal ambiguities in existing knowledge of the U-C-O system, emphasizing the need for continued investigation to facilitate the use of UC-based TRISO and other carbide fuels in emerging reactor designs.