Thermal diffusivity and conductivity of thorium–plutonium mixed oxides

The thermal diffusivity of (Th, Pu)O2 (with 3, 8 and 30 wt.% PuO2) mixed oxides and of the single oxides PuO2 and ThO2 was measured using the laser flash technique. In several samples, a further degradation in the thermal diffusivity due to auto-irradiation damage was noted, and a recovery could be achieved by thermal treatment. The ThO2 and (Th, Pu)O2 samples with 3 and 8 wt.% PuO2 were found to be semitransparent and had to be coated with C for the thermal diffusivity measurement. The experimental thermal conductivity of the fuels was calculated and analysed. The introduction of PuO2 in the ThO2 was found to have an effect similar to that observed in (U, Pu)O2 mixed oxides: small decrease in the range 3–8 wt.% PuO2 and higher decrease at 30 wt.% PuO2 content. Using the classical phonon transport model in crystal structures, a new correlation for the thermal conductivity of (Th, Pu)O2 mixed oxides is proposed and is compared to the results of another correlation built after the Klemens–Callaway theory.