An attempt to explain the high burnup structure formation mechanism in UO2fuel

A formation mechanism of high burnup structure (HBS) or rim effect in UO2 f uel was proposed. The fission gas bubbles in the HBS region may be nucleate d 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 tem perature, but by the cascade of atomic movements by the fission fragments. Based upon the above mechanism, the threshold local burnup for HBS initiati on was derived as a function of temperature, fission rate and grain size. T he threshold burnup for HBS initiation was predicted to increase with tempe rature, and at high temperature, HBS would never form due to the fission ga s release. Variations of HBS widths among the different irradiation tests c ould be explained by the proposed mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.