Dependence of thermal conductivity on radiation defects in ThO2 investigated by molecular dynamics method

Nuclear fuel performance would be deteriorated due to radiation defects. Therefore, investigating the effect of irradiation-induced defects on nuclear fuel properties is essential. Thermal conductivity is an important property of nuclear fuel. In this work, the influence of radiation defects on the thermal conductivity of ThO2 within 600-1500 K has been studied using molecular dynamics (MD) method. Three types of point defects have been investigated in the present work: Frenkel pairs, substitutional Xe and vacancies with concentrations from 0 to 1 %. The results indicate that these irradiation-induced point defects increase the lattice parameter (L) at all studied temperatures. The strength of the dependence of Xe atoms on L is the highest. The analytical models for pure ThO2 and defected ThO2 are developed and there is a good agreement between the MD derived results and the model. The thermal conductivity of ThO2 systems is decreased due to Frenkel pairs, substitutional Xe and vacancies. The thermal resistance to thermal conductivity due to three types of defects are different. The degree of reduction in thermal conductivity by Xe is the largest. The dependence of vacancy cluster size on thermal conductivity is analyzed. For the ThO2 system with 0.5 % porosity, the effect of vacancy cluster size on thermal conductivity is weak. For the ThO2 system with a fixed porosity of 2 %, there exists a critical cluster radius of about 0.6 nm, below which the thermal conductivity increases with the cluster size and above which the thermal conductivity almost remains unchanged. Finally, the thermal conductivity of the amorphous ThO2 is calculated and effects of the amorphous structure and Frenkel pairs on thermal conductivity are compared. The result shows that the thermal conductivity of ThO2 systems can be further degraded by the amorphous structure. All these results indicate irradiation-induced defects could degrade the thermal properties of ThO2 systems and should be considered seriously for estimation of radiation damages in nuclear fuels used in nuclear reactors.