THE ROLE OF DIRECTED BUBBLE DIFFUSION TO GRAIN-BOUNDARIES IN POSTIRRADIATION FISSION-GAS RELEASE FROM UO2 - A QUANTITATIVE ASSESSMENT

In the analysis of post-irradiation fission gas release experiments on UO2, there have been difficulties in understanding the processes invo lved in the important step of intragranular fission gas transfer to th e grain boundaries. Recently, a new model has been introduced, based o n the strong directed bubble migration that must occur in the vacancy gradient induced between the grain boundary and overpressurised bubble concentrations within a grain during annealing. The present paper ext ends this work significantly by using numerical calculations to simula te these processes. This approach has allowed quantitative predictions to be made within the model, showing that large effects are possible, and providing information on the parameters of importance. Among thes e parameters are the accumulated fission gas content and the induced s welling adjacent to grain boundaries, while gas release kinetics are p redominantly controlled by self-diffusion parameters. In addition, the paper discusses several examples in which there is good agreement bet ween the results of the model and experimental data taken from the lit erature.