The diffusion coefficient of fission-product rare gases in single crystal uranium dioxide during irradiation

Abstract

In the present work it is assumed that gas in an intermediate state exists side by side with the diffusing gas, the dissolved fission gas and that trapped in bubbles. The dissolved fission gas is immobile-attached to defects in the lattice caused by irradiation damage and the diffusing gas behaves according to the rules of classical diffusion. As a result of an analysis of the physical phenomena connected with fission gas release from the UO 2 fuel during irradiation, it is shown that up to a certain value of fission fluence the concentrations of intermediate gas and diffusing gas dominates. Above this limiting value a more intensive process of irradiation-induced solution occurs until the diffusion of single gas atoms is insignificant. This means that diffusion of the rare gases is negligible in the presence of diffusion of bubbles for high fission damage (10 18 fissions/ cm 3). On the basis of this analysis there is found an expression for the apparent diffusion coefficient versus fission fluence. The dependence of the apparent diffusion coefficient on the fission rate and the decay constant of the isotopes is also interpreted.