Effect of cooling rate on achieving thermodynamic equilibrium in uranium–plutonium mixed oxides

In situ X-ray diffraction was used to study the structural changes occurring in uranium–plutonium mixed oxides U_1−yPu_yO_2−x with y = 0.15; 0.28 and 0.45 during cooling from 1773 K to room-temperature under He + 5% H₂ atmosphere. We compare the fastest and slowest cooling rates allowed by our apparatus i.e. 2 K s⁻¹ and 0.005 K s⁻¹, respectively. The promptly cooled samples evidenced a phase separation whereas samples cooled slowly did not due to their complete oxidation in contact with the atmosphere during cooling. Besides the composition of the annealing gas mixture, the cooling rate plays a major role on the control of the Oxygen/Metal ratio (O/M) and then on the crystallographic properties of the U_1−yPu_yO_2−x uranium–plutonium mixed oxides.