Experimental Validation of First-Principles Calculation on Fission Product Effect in ThO₂ Thermal Transport

The degradation of thermal conductivity in nuclear fuels under irradiation poses significant challenges, necessitating a thorough understanding of thermal transport influenced by fission products for the safety and efficiency of the nuclear reactors. This study investigates the thermal conductivity of thorium dioxide and zirconium-doped thorium dioxide crystals, synthesized using the hydrothermal method, using the spatial-domain thermoreflectance (SDTR) technique. Zirconium-93 is one of the major long-lived fission products in thorium-based fuels that scatter heat-carrying phonons in the crystalline lattice of the fuel. Therefore, thermal property measurements in zirconium-doped thorium dioxide single crystals provide access to the effects of substitutional zirconium doping without the complexities of extrinsic factors such as grain boundary scattering. The experimental results are compared with first principles calculations of lattice thermal conductivity of thorium dioxide, employing an iterative solution of the Peierls-Boltzmann transport equation. Additionally, the non-perturbative Green’s function methodology is utilized to compute phonon-point defect scattering rates, accounting for local distortions around point defects, including mass difference changes, interatomic force constants, and structural relaxation. The congruence between the predicted results from first-principles calculations and the measured temperature-dependent thermal conductivity validates the computational methodology. Moreover, the methodology employed in this study will enable systematic investigations of the thermal conductivity reduction by fission products, potentially leading to the development of efficient fuel performance codes.