Fission Gas Behavior during Postirradiation Annealing of Large Grained UO₂ Fuels Irradiated to 23 GWd/t

Abstract

Fission gas behavior of UO₂ fuel pellets with controlled microstructure 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 μm), (2) un-doped large grained (43 μm), (3) 0.7 wt Nb₂O₅-doped large grained (110 μm), (4) 0.2 wt c), " TiO₂-doped large grained (85 μm) fuels. The annealing was conducted at 1, 600 or 1, 800°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-doped fuels were reduced to about 1/31/2 by increasing the grain size from 16 to 43 pm, which roughly corresponded to the ratio of their grain sizes. By contrast, the performance of the two large grained fuels doped with Nb₂O₅ or TiO₂ was roughly equivalent to, or rather inferior to that of the standard fuel, despite their large grain sizes of 110 and 85 μm. The fission gas behavior of un-doped fuels was aggravated by increasing the oxygen potential in the annealing atmosphere, while that of additive doped fuels did not depend on it. The effects of grain size, additive doping and oxygen potential on fission gas release and bubble swelling were discussed in connection with the diffusivities of fission gas atoms and cation vacancies.