Phonon Thermal Conductivity of UO2 with Defects via DFT+U Calculation and BTE

Accurately evaluate the thermal conductivity of UO2 with defects is very significant for the superior safety design of reactors and optimize fuel performance. The DFT+U combining with the phonon Boltzmann transport equation to obtain applied to calculate electronic structure, phonon spectrum and the thermal conductivity of UO2 with fission products and irradiation point defects. The calculated thermal conductivity of UO2 has a good agreement with the experimental data over a wide temperature range. Our investigation indicates that the thermal conductivity of UO2 is a function of defect concentration, defect type, and temperature. Fission products and irradiation defects lead to a decrease of thermal conductivity, but show distinctions on scattering mechanisms for phonons. Metal cations scatter low-frequency phonons, while fission gas xenon not only scatter low-frequency phonons but also high-frequency phonons depending on the lattice site it occupied. U vacancies scatter low-frequency phonons, O vacancies scatter high-frequency phonons, while U and O vacancies jointly scatter full-frequency phonons which will further reducing thermal conductivity of UO2. Our findings provides a fundamental insight to heat transfer in irradiated UO2 which is crucial for the development of high-performance UO2 fuel.