FISSION-GAS BEHAVIOR DURING POSTIRRADIATION ANNEALING OF LARGE GRAINED UO(2) FUELS IRRADIATED TO 23-GWD T/

Fission gas behavior of UO2 fuel pellets with controlled microstructur e irradiated to 23 GWd/t in a test reactor has been studied by using a postirradiation annealing experiment. Four types of fuel pellets with or without additives were examined: (1) un-doped standard (grain size :16 mum), (2) un-doped large grained (43 pm), (3) 0.7 wt% Nb2O5-doped large grained (110 mum), (4) 0.2 wt% TiO2-doped large grained (85 mum) fuels. The annealing was conducted at 1,600 or 1,800-degrees-C for 5 h in reducing or oxidizing atmospheres. Fission gas release and bubble swelling caused by the high temperature annealing for the two un-dope d fuels were reduced to about 1/3 approximately 1/2 by increasing the grain size from 16 to 43 mum, which roughly corresponded to the ratio of their grain sizes. By contrast, the performance of the two large gr ained fuels doped with Nb2O5 or TiO2 was roughly equivalent to, or rat her inferior to that of the standard fuel, despite their large grain s izes of 110 and 85 mum. 'rhe fission gas behavior of un-doped fuels wa s aggravated by increasing the oxygen potential in the annealing atmos phere, while that of additive doped fuels did not depend on it. The ef fects of grain size, additive doping and oxygen potential on fission g as release and bubble swelling were discussed in connection with the d iffusivities of fission gas atoms and cation vacancies.