FISSION-GAS RELEASE DURING POST IRRADIATION ANNEALING OF BWR FUELS

Uranium dioxide fuels irradiated in a commercial BWR during 1 to 4 cycles (burn-up: 6-approximately-28 GWd/t) have been annealed in an out-of-pile condition and the release rate of Kr-85 was measured continuously. The annealing temperature ranged 1,500-approximately-1,800-degrees-C. The effects of temperature and burn-up have been studied on burst release during temperature ramp and on diffusional release during isothermal annealing. The burst release evolved on temperature ramp was mainly due to the rapid release of fission gas from grain boundaries, which had previously accumulated there during base irradiation. With increasing burn-up, the critical temperature for the onset of the burst release became lower and the fractional burst release increased. The number of fission gas atoms per unit area of grain boundary, which was calculated from the quantities of the burst released Kr-85, was about 2-approximately-4 times larger than saturation values estimated from theoretical considerations based on the assumption of mechanically equilibrated bubbles for 3 and 4 cycle irradiated fuels. Effective diffusion coefficient of Kr-85 obtained by the isothermal annealing tests appeared to be independent of burn-up (< 30 GWd/t).