High temperature X-ray diffraction study of the kinetics of phase separation in hypostoichiometric uranium–plutonium mixed oxides

Uranium–plutonium mixed oxides incorporating high amounts of plutonium are considered for future nuclear reactors. For plutonium content above 20%, a phase separation occurs, depending on the temperature and on the oxygen stoichiometry. Here, using an in situ fast X-ray diffraction device dedicated to radioactive materials, we evidenced a phase separation occurring during rapid cooling from 1773 K to room temperature at the rates of 0.05 and 2 K s−1 for two (U1−yPuy)O2−x compounds, with y = 0.28 and 0.45, under a reducing atmosphere. Optical microscopy reveals that the cooling rate impacts the microstructure of the fuel pellet by inducing severe macroscopic cracks. These results are important for using uranium–plutonium mixed oxides with high plutonium content as nuclear fuels. Considering the associated issues, they dictate a cautious attitude when defining their conditions of fabrication.