A first principle based study on the mechanical and thermal properties of UO2: Effect of La and Dy fission product concentrations

Abstract The amount of fission products substitution in the uranium dioxide (UO2) fuel matrix and subsequently their interaction with the host is important in the perspective of safe and efficient operation of nuclear reactors as fission products can change the structural, mechanical and thermo-physical properties of the host fuel significantly. Therefore, structural, elastic, mechanical and thermo-physical properties of lanthanide (Ln) fission products doped UO2 with different doping concentrations of Ln atoms are studied by employing density functional theory with Hubbard U correction. In this study, two Ln atoms (Ln = La and Dy) and different doping concentrations in UO2 e.g. 6.25%, 12.5%, 25%, and 50% are considered. The mechanical property such as bulk modulus of Ln doped UO2 are directly related to the Ln doping concentration. Expressions are generated to extract various properties at very low and high Ln doping concentrations. The thermo-physical properties like heat capacity and coefficient of thermal expansion are also evaluated based on quasi-harmonic approximation to the phonons frequencies which are obtained from density functional perturbation theory. The UO2 lattices with higher Ln atom concentrations exhibit new electronic energy bands near the band gap region, which are dominated by Ln atomic orbitals. From the present study, we derived correlations between La and Dy doping concentration and various thermo-physical properties of UO2. Moreover, the properties obtained from our first principle calculations show a good agreement with the available experimental and theoretical values. Thus, our systematic study with varying fission product concentrations will provide insight to the UO2 fuel properties under reactor conditions.