Formation and growth of intragranular fission gas bubbles in UO₂ fuels with burnup of 6-83 GWd/t

The detailed characteristics of intragranular fission gas bubbles in UO₂ fuel pellets (burnup: 6-83 GWd/t) before and after postirradiation annealing at 1600 and 1800°C have been examined by TEM and SEM fractography. In the base-irradiated fuels, a high density of small bubbles of about 2 nm in diameter precipitated uniformly in the matrix. When increasing burnup above 44 GWd/t, larger bubbles of 10-20 nm newly appeared in addition to the small bubbles. On heating at high temperatures, bubble growth was saturated within a few minutes. Enhanced coarsening of bubbles was found preferentially near the grain boundaries in the middle burnup fuels of 16-28 GWd/t and throughout the grains in the high burnup fuels of 44 and 83 GWd/t. The bubble growth during annealing was associated with a remarkable decrease of the bubble number density, and the relationship between bubble density N_b and mean diameter d_b was expressed as N_b α d_b^-2.6. The coarsening was attributed to coalescence via bubble migration for moderately large bubbles of up to 50-60 nm, and to Ostwald ripening accompanied by a sufficient vacancy supply from external vacancy sources such as free surfaces, grain boundaries or irradiation-induced sub-grain boundaries for huge bubbles above 100 nm.