Thermal expansion and thermal conductivity of (Th,U)O2 mixed oxides: A molecular dynamics and experimental study

Abstract

Thermal expansion and thermal conductivity of ThO2 and (Th,U)O2 mixed oxides (MOX) have been investigated by using molecular dynamics (MD) simulation and experimental study. MD simulation have been performed to determine lattice thermal expansion of Th1−xUxO2 (x = 0 to 0.3125) solid-solutions in the temperature range of 300–3000 K using a combination of Coulomb-Buckingham-Morse and many-body potential model with partial ionic charges. Special quasi-random structures were employed to establish solid-solution configurations. Experimental verification of the lattice thermal expansion behavior is carried out on polycrystalline sample of ThO2 -x wt.% UO2 (x = 0, 6, 13, 25 and 30) MOX from room temperature to 1273 K in vacuum using high temperature X-ray diffraction (HT-XRD). Our combined MD simulations and HT-XRD measurements indicate that incorporation of UO2 in ThO2 systematically increases coefficient of thermal expansion but the rate of increase is higher in low UO2 composition range (≤13 wt.%). A comparative evaluation of the thermal properties of ThO2-6wt.% UO2 pellets prepared by coated agglomerate pelletization (CAP) process and powder metallurgy routes (sintered in Ar-8%H2 atmosphere) have been performed with respect to micro-inhomogeneity, porosity, O/M and trace impurities. The thermal conductivity of ThO2 and ThO2-6 wt.% UO2 calculated under equilibrium condition by Green–Kubo formalism show good agreement with our experimental measurements.