Structure, mechanical and thermo-physical properties of lanthanide fission product doped UO2 in U(V) state: A density functional study

Abstract

During the nuclear fission of UO2 fuel, many fission products are generated which are trapped in the fuel matrix. These fission products can amend the mechanical and thermo-physical properties of UO2 fuel, which are a prime concern for nuclear safety. In this regard, we investigate the effect of different lanthanide (Ln) fission products on structural, mechanical, electronic and thermal properties of UO2. The substitution of U4+ ions by Ln3+ ions creates a charge imbalance in the lattice which will be neutralized by either oxidation of U atoms from +4 to +5 state (U2Ln2O8) or creation of oxygen vacancies (U2Ln2O7). Here, we study the former case, where the U(IV) ions are oxidized to U(V) state in the Ln substituted UO2 (Ln- UO2) fuel using density functional theory with Hubbard-U corrections. The volume of different Ln-UO2 structures is calculated and compared with undoped UO2. The mechanical properties such as bulk modulus and Young's modulus are calculated from the optimized structures of Ln-doped UO2 and show deviation from undoped UO2 and Ln-doped UO2 with oxygen vacancies. The total density of states and projected density of states indicate that the presence of Ln fission products alters the electronic properties of UO2. The thermal property such as specific heat capacity is evaluated at different temperatures using quasi-harmonic approximation applied to phonon frequencies obtained from density functional perturbation theory. We do observe an excellent agreement of the calculated specific heat capacity with the experimental values for the pure UO2. The counter balancing effect of heat capacity values of U2Ln2O8 and U2Ln2O7 systems may control the net heat capacity of the UO2 fuel matrix.