Oxygen potential calculations for conventional and Cr-doped UO2 fuels based on solid solution thermodynamics

Abstract The effect of Cr-doping on the oxygen potential of UO2 fuels was studied through a comprehensive thermodynamic treatment. Berman-type sublattice solid solutions were formulated and calibrated for UO2 (with Pu, Am, Cm, Np, La, Nd, Ce and Cr as solutes) and for a five-component noble-metal phase (Mo, Pd, Ru, Rh, Tc). Ideal gas, a binary (Sr,Ba)ZrO3 ideal solid solution and a number of stoichiometric solids were also included. The equilibrium calculations were carried out using the GEM-Selektor code with the in-house Heracles database. Ellingham diagrams were calculated for burnups between 1 and 60 GWd/tiHM over the temperature range 400 °C–1400 °C. They indicate no significant effect of Cr-doping on the oxygen potential, except at very low burnup. Oxygen potentials appear however to depend sensitively on mole fraction and non-ideality of the Mo component in the noble-metal phase. In contrast, the effect of solutes in UO2 is small. Above about 40 GWd/tiHM, the fuel oxygen potential curves converge to values exceeding by 5–30 kJ/mol the pure Mo–MoO2 buffer. Further calculations indicate that oxidation of zirconium alloy at the inner side of the cladding could considerably lower the oxygen potential.