An assessment of fission gas release and the effect of microstructure at high burn-up

The report outlines a simple fission gas release model, which, when incorporated in a code, predicts the observed threshold for fission gas release by grain boundary inter linkage including new high burn-up - low temperature data. The model adopts a conventional approach of single gas diffusion to grain boundaries opposed by an irradiation induced re-solution and hence a counter flux of gas atoms away from the grain boundary. In its original form, the model predicts the well know Vitanza curve. In order to include the new data at high burn-up, it has proved necessary to include a burn-up dependence to the diffusion coefficient and re-solution parameter. In this matter, the changes were introduced empirically. The model is shown to reproduce the onset and kinetics of fission gas evolution in a number of well characterized experiments. The model has been applied successfully to experiments designed to investigate the effect of grain size on fission gas release. Evidence is investigated to support the theory that fission gas release is reduced by increasing the grain size of the fuel. With the help of the model, it is shown that this is most effective at modest powers and release values -10 percent and for high powers and high values of release, an increase in grain size becomes less effective. This is because at the high temperatures experienced during these conditions, the increased diffusion path of large grain material no longer restricts the number of gas atoms reaching the grain boundaries. Analysis of experiments where the fission gas release from different grain size material is compared shows that particularly at high powers, the effects may be influenced by grain growth of the smaller grain size material. This can lead to misleading conclusions regarding the benefits or otherwise of advanced fuels (author) (ml)