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

To analyze fission gas release and gaseous swelling in UO2 fuel, a model ha s been developed that can be used under both steady-state and transient ope rating conditions up to high burnup. With emphasis on the: effect of extern al 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 fr action 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 wi th the data obtained from commercial reactors, Ris phi-III Project, isother mal irradiation and post-irradiation annealing experiments. It is shown tha t the model predicts well the fractional fission gas release as well as the radial distribution of Xe gas across fuel pellet under various operating c onditions. This suggests that the present model can be used for the analysi s of fission gas release at high burnup fuel where strong external restrain t stress may develop due to pellet cladding interaction. (C) 2000 Elsevier Science B.V. All rights reserved.