EFFECT OF EXTERNAL RESTRAINT ON BUBBLE SWELLING IN UO2 FUELS

The effect of external restraint on bubble growth in unirradiated UO2 fuels has been examined using a high temperature and high pressure fur nace. The powder compacts of UO2 were sintered at 1800 degrees C in hi gh pressure Ar/0.2% HZ mixed gas of 50 of 100 MPa, and then the sinter ed pellets were annealed at lower ambient pressures than the sintering pressures. This treatment caused bubbles in the pellets becoming over pressurized; the excess pressure driving force caused bubble growth. T he growth rate significantly depended on the magnitude of the driving force, and the growth of high pressure bubbles was effectively suppres sed by very strong external restraints even at the high annealing temp eratures of 1600-1800 degrees C. The bubble growth data obtained under various excess pressures and annealing temperatures were analyzed by using a well known model for bubble growth rate, in which the grain bo undary diffusion coefficient was best-fitted based on the data. The bu bble size calculated by the above model was in good agreement with rha obtained data unless the swelling exceeded about 8%. Above this value , the measured swelling was saturated, due to interlinking of grown bu bbles and subsequent gas release. (C) 1997 Elsevier Science B.V.