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

Abstract

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 x O 2-x and UC 1-x O 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 x O 2-x and UC 1-x O x phases. Subsequently, we have determined the preferential phase of UCO (UC x O 2-x or UC 1-x O x ) for low carbon and oxygen concentrations, respectively. In particular, UC x O 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.