We present the results of a theoretical investigation of the trapping and migration of the fission gas atoms, Kr and Xe in UO
2
. Our models differ from those previously presented in discussions of rare gas diffusion in ionic crystals. We propose that trapping occurs at vacancy aggregates, and that detrapping into interstitial sites is unimportant; gas migration is effected by the intrinsic mobility of the gas-trap complexes. We suggest that the trapping and diffusion of the two gas atoms occur by different mechanisms, owing to their different sizes. Our results explain many of the observations of the experimental study of Felix & 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.
Protoplanetary disc evolution in magnetically layered disc models
