Bubble migration in UO2—A study using a laser image furnace

The migration rate of bubbles in UO2 has been studied. Migration was induced by a temperature gradient caused by a laser beam focused on one surface of the UO2 sample. Temperatures within the specimen were computed from knowledge of the boundary conditions for heat flow and the temperature-dependent physical properties of the material. (The hot surface temperature was calculated from the rate of evaporation of the material, and the cold surface temperature was measured by a thermocouple.) The migration velocities were determined from the length of columnar grains lying behind lenticular bubbles. We found that bubble migration velocities were much slower than forecast by a diffusion-controlled migration mechanism. In the case of one bubble, we estimated that of a total driving force of 8.6 deg C, about 5.8 deg C was used to develop new ledges on the condensing surface, and only about 0.2 deg C was used to promote diffusion of UO2 molecules across the bubble. From this study we conclude that bubble migration in nuclear fuels is likely to be variable, and especially sensitive to impurities which might affect the processes involved in migration (particularly surface active materials, which grossly affect layer nucleation). Consequently, the response of the fuel to bubble migration under irradiation (swelling and gas release) is in principle subject to control.