Thermal Conductivity of UO2 with Defects via DFT+U Calculation and Boltzmann Transport Equation

Accurate evaluation of the thermal conductivity of UO2 with defects is very significant for optimizing fuel performance and enhancing the safety design of reactors. We employed a method that combines the Boltzmann transport equation with DFT+U to calculate the thermal conductivity of UO2 containing fission products and irradiation-induced point defects. Our investigation reveals that the thermal conductivity of UO2 is influenced by defect concentration, defect type, and temperature. Fission products and irradiation defects result in a decrease in thermal conductivity, but they have markedly different impacts on phonon scattering mechanisms. Metal cations tend to scatter low-frequency phonons (less than 5.8 THz), while the fission gas xenon scatters both low-frequency and high-frequency phonons (greater than 5.8 THz), depending on its occupancy at lattice sites. Uranium vacancies scatter low-frequency phonons, while oxygen vacancies scatter high-frequency phonons. When uranium and oxygen vacancies coexist, they scatter phonons across the entire frequency spectrum, which further results in a significant reduction in the thermal conductivity of UO2. Our calculated results align well with experimental data across a wide temperature range and provide fundamental insights into the heat transfer mechanisms in irradiated UO2. These findings are essential for establishing a thermal conductivity database for UO2 under various irradiation conditions and benefit the development of advanced high-performance UO2 fuel.