Dependence of thermal conductivity on fission-product defects and vacancy concentration in thorium dioxide

The dependence of thermal conductivity on fission generated products (FPs) and vacancies in ThO ₂ within the temperature range 300–1500 K has been investigated using molecular dynamics (MD) simulations. The effect of defect on specific heat and enthalpy increment is also studied. Two typical FPs: Xe and Kr (0–1.02% defect) and vacancies with 0–5% concentration are examined. The relative change in enthalpy increment due to defects is small (<2%) for lower concentration at all temperatures; however, for higher defect concentration, this parameter decreases with temperature. The vacancies reduce specific heat in the low temperature range, while the reduction is negligible at elevated temperature. In contrast, considering all temperatures, a maximum of ∼2% reduction in specific heat is observed for ThO ₂-FP systems with any concentration of defect. Thermal conductivity of ThO ₂ degrades significantly by the FPs and vacancies and the degradation decreases with the increase in temperature. For both pure and defective ThO ₂, the scattering parameters are derived by fitting a Callaway and Analytical model to MD data. The percentage of reduction ( R ) by defects in thermal conductivity follows the trend R F P > R V a c a n c y , where R X e is somewhat higher than R K r for the studied concentrations of FPs. The reduction rate is higher for the systems with smaller concentration of defect and vice versa. Our results report that the degree of reduction in conductivity of defected ThO ₂ is lower compared to that of UO ₂ . We also observed that the clustered defects reduce thermal conductivity more than that by individually distributed defects and conductivity decreases almost linearly with the increase in cluster size.