Oxygen potential of irradiated urania--plutonia fuel pins

Abstract The oxygen-potential gradient in irradiated urania-plutonia fuel pins has been estimated from the distribution of molybdenum between the noble-metal-alloy inclusions and the oxide matrix as measured using an electron microprobe. The values of the oxygen potential were found to be more negative than would be computed for the fuel on the basis of the initial O/M ratio, thus indicating that the overall O/M of the fuel was decreased by irradiation. The O/M gradient has been computed and an oxygen material balance was determined between fuel oxide, fission products, and cladding. The nearly identical oxygen pressure profiles found for fuel pins irradiated for a range of values of the burnup indicates that the oxygen-potential gradient is establsihed early in the irradiation period, probably at the time the structure has been established and then remains nearly constant. The oxygen lost during the restructuring period is determined by the initial O/M of the fuel, the smear density and the temperature gradient. The oxygen lost during the burnup period is proportional to the burnup.