Phonons and thermophysical properties of U1−yPuyO2 mixed oxide (MOX) fuels

Abstract

The phonon and thermophysical properties of uranium-plutonium mixed oxides (MOX) fuels are investigated. We have applied Hubbard corrected density functional theory (DFT + U) and empirical potentials (EP) coupled to the Boltzmann transport equation (BTE) to investigate the harmonic and anharmonic lattice vibrational properties. The specific heat, entropy and thermal conductivity, were analyzed to investigate the effect of actinide composition on the thermal properties in pure and mixed oxides. Our results indicate a relative increase in the anharmonicity of phonons in mixed oxides compared to the parent oxides. We discuss the decrease in the lattice thermal conductivity in the MOX in terms of the increase in the Grüneisen parameter, group velocity and phonon-phonon scattering, in comparison to the parent oxides. Furthermore, we demonstrate that the differences in the specific heat of actinide oxides, and thus the Neumann-Kopp rule for MOX, which is the rule of mixtures, are originated from the contribution of the crystal electric field at the actinide cations.