Helium bubbles at grain boundaries of high-density 238PuO2 shards

Hydroxide- and oxalate-base 238PuO2 shards that were sintered to high density at 1200 or 16000C and then aged for 6 months or more exhibited grain-boundary gas bubbles due to agglomeration of alpha-decay helium when heated to 12000C and above. Conditions for bubble formation depended markedly on shard microstructure; large-grained shards with few large residual sintering pores formed gas bubbles at lower temperatures than small-grained shards with many pores. This behavior was especially apparent in oxalate-base 238PuO2, in which small-grained shards resisted bubble formation to above 15000C; small-grained hydroxide-base shards with less internal porosity than oxalate-base shards formed bubbles at lower temperatures. Helium is apparently released from aged 238PuO2 shards at high temperatures by bulk diffusion within grains to the grain boundaries, where bubbles are formed and interconnect into networks which permit helium escape. It is postulated that helium is released by grain boundary diffusion at temperatures below thresholds for grain boundary bubble formation. Small grain size and high residual porosity within grains inhibit the formation of the grain-boundary gas bubbles by reducing the concentration of helium gas at the grain boundaries. (10 fig)