On the release of fission gas from an evaporating fuel body

Abstract

Isothermal post-irradiation anneal experiments have been extensively used to determine diffusion coefficients for fission gases in nuclear fuel bodies. Several of these experiments have been carried out at temperatures sufficiently high for an appreciable evaporation rate of the fuel body, but the effect of this evaporation upon the release of the fission gas and upon the determination of diffusion coefficients has heretofore not been analyzed. In the following the time-dependent lattice diffusion of a radioactive gas contained in solids of simple shapes (sphere, slab, cylinder and certain partially unbounded media) is analyzed. The solid sublimes into a vacuum and as a result the boundary surface regresses. This regression is assumed to be a linear function of time. It is shown that the effect of the moving boundary increases the rate of diffusion of the gas through the boundary because the regression of the interface steepens the surface concentration gradient compared with a stationary interface. If surface evaporation is ignored, the predicted diffusion coefficient in a post-irradiation anneal experiment can be erroneously high by several orders of magnitude.