THE DIFFUSION OF FISSION GAS ATOMS AND BUBBLES THROUGH URANIUM DIOXIDE

Abstract

We report first a study of the trapping and migration mechanisms of the fission gas atoms, Kr and Xe in UO2. Our approach is to combine the results of calculations of the energetics of the processes we consider, with the evidence from experimental studies of gas release — principally the data Felix and Miekeley. Our models differ markedly from those previously presented in discussions of rare gas diffusion in ionic crystals. We propose that trapping occurs at vacancy aggregates, and that Xe diffusion occurs predominantly by mechanisms which are not assisted by lattice defects ; also detrapping of gas atoms into interstitial sites is shown to be unimportant. Different mechanisms are suggested for the trapping and diffusion of the two atoms ; this difference arises from their different sizes. Our results rationalize the principal observations of the experimental study of Felix and Miekeley and emphasize the importance of the chemical composition of the fuel on gas diffusion coefficients. The contradictions between this work and the earlier studies summarized by Matzke is, we suggest, possibly due to the higher levels of radiation damage which may have been present in the crystals used in the latter work. Our proposals for the mechanisms of diffusion of single gas atoms are then used in a discussion of gas bubble migration. Here we attempt to reconcile apparent contradictions between different experimental studies, some of which indicate volume diffusion while others are compatible with a surface diffusion mechanism. We suggest that in some cases surface diffusion may be suppressed owing to the accumulation on the surface of the bubble of impurities which diffuse with the gas atoms to the bubble nuclei during bubble growth.