An attempt to explain the high burnup structure formation mechanism in UO2 fuel

A formation mechanism of high burnup structure (HBS) or rim effect in UO2 fuel was proposed. The fission gas bubbles in the HBS region may be nucleated and stabilized at the threshold atomic concentration of gas atoms inside the grain of UO2 fuel. HBS formation in HBS region may be driven not by the conventional thermal diffusion process, which should increase with the temperature, but by the cascade of atomic movements by the fission fragments. Based upon the above mechanism, the threshold local burnup for HBS initiation was derived as a function of temperature, fission rate and grain size. The threshold burnup for HBS initiation was predicted to increase with temperature, and at high temperature, HBS would never form due to the fission gas release. Variations of HBS widths among the different irradiation tests could be explained by the proposed mechanism