A microstructure-dependent model for fission product gas release and swelling in UO 2 fuel

Abstract

A model for the release of fission gas from irradiated UO 2 fuel is presented. It incorporates the relevant physical processes: fission gas diffusion, bubble and grain boundary movement, intergranular bubble formation and interlinkage. In addition, the model allows estimates of the extent of structural change and fuel swelling. In the latter, contributions of thermal expansion, densification, solid fission products, and gas bubbles are considered. When included in the ELESIM fuel performance code, the model yields predictions which are in good agreement with data from UO 2 fuel elements irradiated over a range of water-cooled reactor conditions: linear power outputs between 40 and 120 kW m −1, burnups between 10 and 300 MW h(kg U) −1, and power histories including constant, high-to-low and low-to-high power periods. The predictions of the model are shown to be most sensitive to fuel power (temperature), the choice of diffusion coefficient for fission gas in UO 2, and burnup. The predictions are less sensitive to variables such as fuel restraint, initial grain size and the rate of grain growth.