The fractal nature of the surface of uranium dioxide: a resolution of the short-lived/stable gas release dichotomy

The framework for analysis of fission product release in the Halden gas flow rigs has been developed over a period of 20 years. The predominant mode of release is single gas atom diffusion to free surfaces with a small, but burn-up-dependent contribution from direct recoil. Measurements of longer-lived fission products indicated that their release appeared to be controlled by a diffusion coefficient, which, at low temperatures, was several orders of magnitude smaller than that of the shorter-lived fission products. The enhanced release of the shorter-lived products was attributed to a near-surface enhancement effect. However, a theoretical study of a two-zone diffusion model reveals that it is incapable of accounting for the observations. A model based on the fractal nature of the uranium dioxide surface circumvents these difficulties and provides a framework in which a single three-term diffusion coefficient can be used to describe the release of both short- and long-lived fission products.