OXYGEN POTENTIALS AND LATTICE-PARAMETER OF IRRADIATED BWR FUELS

The oxygen potentials and lattice parameter of oxide fuels (UO2 and UO2-2 w/0Gd2O3) irradiated in commercial BWRs have been measured by using a solid electrolyte galvanic cell technique and X-ray diffractometry. The fuel burn-up was 13 approximately 30 GWd/t for the former measurements and 6 approximately 35 GWd/t for the latter. The oxygen potentials at 750-degrees-C for irradiated UO2 ranged from -420 kJ/mol at the fuel rim to -480 kJ/mol at the fuel center. The corresponding values for irradiated UO2-2 w/0Gd2O3 ranged from -460 to -540 kJ/mol. The lattice parameter near the fuel rim in the irradiated UO2 increased with burn-up, and was distinctly larger than that at the fuel center. This lattice dilatation was mainly due to the accumulation of radiation defects, not to a decrease in O/M ratio, and was almost recovered after the annealing at 1,600-degrees-C for 5 h.