Analysis of fission gas release and gaseous swelling in UO2 fuel under the effect of external restraint

To analyze fission gas release and gaseous swelling in UO 2 fuel, a model has been developed that can be used under both steady-state and transient operating conditions up to high burnup. With emphasis on the effect of external restraint stress on the behavior of gas bubbles at grain boundaries, the gaseous swelling due to grain edge bubbles, which affects gas release rate through the formation of release tunnels at grain edge, is described. Gas release rate at the grain edge is assumed to be proportional to both the fraction of grain edge bubbles interlinked to open space of fuel and the rate of gas atoms arriving at the grain edge bubbles. The model was compared with the data obtained from commercial reactors, Risφ-III Project, isothermal irradiation and post-irradiation annealing experiments. It is shown that the model predicts well the fractional fission gas release as well as the radial distribution of Xe gas across fuel pellet under various operating conditions. This suggests that the present model can be used for the analysis of fission gas release at high burnup fuel where strong external restraint stress may develop due to pellet cladding interaction.