An electron microscope examination of matrix fission-gas bubbles in irradiated uranium dioxide

Abstract Uranium dioxide irradiated over a wide temperature range to a dose of greater than ~ 3.2 × 1016fissions/ mm3 is always found to contain a high concentration of intragranular fission-gas bubbles which contain a constant amount of gas approximately independent of both irradiation temperature and burn-up. These bubbles, which are assumed to be nucleated heterogeneously upon the sites of the energetic fission fragments, have a lifetime which lies in the range of 4–40 h with thermal neutron fluxes of ~ 5 × 1011n/mm2 · sec. The collection of fission-gas at grain boundaries is controlled by resolution and atomic diffusion of gas atoms through the uranium dioxide lattice rather than by the migration of bubbles up the temperature gradient. The diffusion coefficient of the fission-gas atoms in irradiated uranium dioxide has been shown to be considerably enhanced at temperatures below ~ 1300 °C.