Unravelling the effect of fission gases on the thermal transport and high-temperature structural stability of UO₂ using atomistic simulations

Abstract Radioactive fission gases such as Xe and Kr play a decisive role in the thermal transport of UO 2 under nuclear reactor operating conditions. Here, systematic molecular dynamics simulations are performed to understand the effect of fission gases on the thermal conductivity and structural properties of UO 2 . The conductivity deteriorates significantly in the presence of fission gases. The strong phonon-disorder scattering leads to overdamping of the phonon modes, particularly impacting the midfrequency region, leading to substantial decay in conductivity. We unveil the interplay of anharmonicity and disorder in conductivity. Significant decay in conductivity is attributed to the phonon-disorder scattering at low temperatures and anharmonic phonon-phonon scattering at high temperatures. To analyze the high-temperature structural stability, we compute the radial distribution function, mean square displacements, and diffusion coefficients; an early onset of superionic diffusion is observed in UO 2 with fission gases compared to pristine UO 2 .