Oxidation behavior of (U_1-yCe_y)O_2.00; (y = 0.21, 0.28 and 0.44) solid solutions under different oxygen potentials. Thermogravimetric and in situ X-ray diffraction studies

For the first time, oxidation behavior of (U_1-yCe_y)O_2.00; (y = 0.21, 0.28 and 0.44) was examined under various oxygen potentials ranging from 0 to −58 kJ/mol. Thermogravimetry (TG) and powder X-ray powder diffraction (XRD) were used as prime techniques for determination of oxygen stoichiometry and identification of different compounds formed. Rietveld analysis of XRD data of the oxidized products, formed under different oxygen potentials, showed the formation of different proportions of orthorhombic M₃O₈ (M = U + Ce) and face centered cubic (FCC) MO_2+x phases for y = 0.21 and 0.28 and a single FCC phase for y = 0.44. A novel method based on TG and XRD analysis was used to get the quantitative information on the distribution of uranium, cerium and oxygen between the product phases. It has been observed that lowering of oxygen potential resists the formation of M₃O₈ in the oxidized products and potential lower than −70 kJ/mol is required for maintaining single FCC phase during the storage of fast reactor fuel. The oxidation kinetics of (U_1-yCe_y)O_2.00; (y = 0.21, 0.28 and 0.44) was studied using model free iso-conversional method. High temperature X-ray diffraction (HT–XRD) studies in vacuum and oxygen atmospheres were used to separate out the oxidation effect from the combined effect of expansion and oxidation. For the first time, HT–XRD and TG studies were coupled together to correlate the change in lattice parameter of FCC phase (Δa) obtained in vacuum and oxygen atmosphere, with change in oxygen to metal ratio (ΔO/M). The influence of phase separation on the correlation of lattice parameter with O/M was also discussed.