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

Abstract

Thermal expansion and thermal conductivity of (Th,Pu)O2 mixed oxides (MOX) have been investigated using molecular dynamics (MD) simulations and experimental study. MD simulations have been performed to determine lattice thermal expansion of Th1−xPuxO2 (x = 0, 0.03125, 0.0625 and 0.09375) solid-solutions using a potential model which combines Coulomb–Buckingham–Morse and many-body functional forms. Special quasi-random structures have been employed to establish solid-solution configurations. Experimental investigations have been conducted to study the effect of homogeneity (PuO2 distribution in ThO2 matrix) on the thermal properties of ThO2–1wt.%PuO2 fuel pellets fabricated by conventional powder and pelletization (POP) process as well as coated agglomerate pelletization (CAP) processes. Dilatometry and laser flash techniques, respectively, have been employed for measuring thermal expansion and thermal diffusivity of the ThO2, ThO2–1wt.%PuO2 (both POP and CAP fabricated) and ThO2–6wt.%PuO2 POP pellets. The study shows a difference of <4% in the thermal conductivity values of ThO2–1wt.%PuO2 fuel pellets fabricated through CAP and POP techniques at 873 K and the difference decreases with increase in temperature. The thermal conductivity of ThO2 and ThO2–6.25 at% PuO2 calculated under equilibrium conditions by Green–Kubo formalism shows a good agreement with our experimental measurements. Our MD calculated melting temperatures of pure ThO2, PuO2 and Th0.9375Pu0.0625O2 using two-phase simulation are between 3650-3675 K, 2800–2825 K and 3575–3600 K, respectively, in good agreement with reported experimental observations.