Atomistic investigation of uranium oxycarbide (UCO) phase stability at finite temperatures: DFT+U and AIMD+U approaches

Materials properties of uranium oxycarbide (UCO), a fuel candidate for high temperature gas reactors (HTGRs), are investigated using the Hubbard corrected density functional theory (DFT+ U ) and ab initio molecular dynamics (AIMD+ U ). To accurately describe UCO systems within the supercell approach, we have evaluated the structural disorder of the ion sublattice within the special quasirandom structures (SQS) approach. The ground-state of UC _xO _2-x and UC _1-xO _x systems was found by examination of the electronic configuration, the occupation matrix and the Hubbard U parameter. We obtained the electronic properties, lattice structure, formation enthalpy and free energy of mixing of the stoichiometric UC _xO _2-x and UC _1-xO _x phases. Subsequently, we have determined the preferential phase of UCO (UC _xO _2-x or UC _1-xO _x) for low carbon and oxygen concentrations, respectively. In particular, UC _xO _2-x was found stable for carbon content lower than 50%, with carbon atoms in the first coordination shell out of the perfect fluorite positions, driven by the formation of C ₂ groups as in α -UC ₂ . Moreover, the analysis of the radial distribution functions shows that structures with C ₂ groups remain stable in the range of temperatures from 300 to ∼ 2500 K.