Postirradiation Behavior of UO₂ Fuel I: Elements at 220 to 250°C in Air

Irradiated UO₂ fuel elements with and without deliberately induced 0.8-mm-diam defects were heated in air at 220, 230, and 250°C for times up to 685 h. Pretest burnup was typically 190 MW·h/kg U (7600 MW·d/TeU) at a maximum linear power of 45 kW/m; cooling times were 1 to 3 yr. In fuel elements with six defects, diametral increases in excess of 5% and sheath splitting occurred after ∼200 h at 250°C, consistent with oxidation to U₃O₈. There was a maximum 2% increase in elements with one defect at 250°C, but no change in an irradiated but undefected control. Dimensional increases in defected elements at 220 and 230°C were typically <1% for times in excess of 600 h, consistent with oxidation primarily to nonswelling U₃O₇. The rate of weight change at 230 and 250°C was up to 50 times greater in defected irradiated elements, compared with that for defected unirradiated elements. There was no significant effect of power history or cooling time in the elements studied.