Release of xenon from sintered UO₂ at low temperatures

Release of Xe from lightly sintered UO₂ was measured by post-irradiation heating in vacuum at 400°∼800℃. The release rate after initial burst followed the diffusion model. An Arrhenius plot of the diffusion coefficients revealed a discontinuity at 600℃. Activation energies were calculated to be 3.4 kcal/mol at temperatures lower than the discontinuity, and over 36 kcal/mol in higher temperature range, which corresponded respectively to non-lattice and lattice diffusion. The discontinuity is explained by a changeover of the predominance of one process over another depending on temperature range. The initial burst could not be attributed to surface oxidation. Based on analyses of such factors as dependence of burst on temperature and on specific surface area, as well as percentage of burst release, a kind of activated extrication from superficial layer, approximately 5Å thick, was proposed as a concept for interpreting the burst phenomenon. (author)