Bundle uranium content and performance of CANDU fuel

Between 1970 and 1988, the mass of uranium in CANDU bundles increased by 2 to 3%. To assess the effects of the increase, post-irradiation examination data for 1970 to 1996 fuel with a range of uranium contents were evaluated. The results show a sheath strain increase with increasing UO2 density and burnup. Other factors that affect the content of uranium, could not be quantitatively evaluated from the data. Bundle uranium mass is affected by several contributing parameters, which have either positive or negative effects on the fuel sheath strain and bundle subchannel cross-sections. The ELESTRES fuel modelling code has been used te determine the relative effect on sheath strain of the design parameters that control uranium mass, namely, pellet density, diametral clearance, axial gap, and pellet face geometry (chamfer, dish depth, and land width). The increases in uranium mass achieved between 1970 and 1988 caused increases in fuel element diameter which in turn has an effect on the margin to dryout. A limiting bundle mass was calculated for an overall average of zero element strain in a fuel channel in a previous study. For comparison, this paper presents a revised calculation by including midpellet as well as ridge strains to determine a weighted average sheath strain. We have re-evaluated the current fuel technical specifications with respect to the margin to dryout, and examined uranium mass as a simplified indicator for bundle acceptability. Links between uranium content and fuel performance are discussed. (author)