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 U O2 fuel pellets (burnup: 6-83 GWd/t) before and after postirradiation annealing at 1600 and 1800 degrees C have been examined by TEM and SEM fractography. In the base-irradiated fuels, a high density of small b ubbles of about 2 nm in diameter precipitated uniformly in the matrix. When increasing burnup above 44 GWd/t, larger bubbles of 10-20 nm new ly appeared in addition to the small bubbles. On heating at high tempe ratures, bubble growth was saturated within a few minutes. Enhanced co arsening of bubbles was found preferentially near the grain boundaries in the middle burnup fuels of 16-28 GWd/t and throughout the grains i n the high burnup fuels of 44 and 83 GWd/t. The bubble growth during a nnealing was associated with a remarkable decrease of the bubble numbe r density, and the relationship between bubble density Nb and mean dia meter was expressed as N-b proportional to d(b)(-2.6). The coarsening was attributed to coalescence via bubble migration for moderately larg e les of up to 50-60 nm, and to Ostwald ripening accompanied by a suff icient vacancy supply from external vacancy sources such as free surfa ces, grain boundaries or irradiation-induced sub-grain boundaries for huge bubbles above 100 nm.