In 1969, analysis of fuel defects in the Douglas Point power reactor indicated that the power ramp defects tended to occur preferentially in fuel with high strength/low ductility sheathing. The sheathing specification for all CANada Deuterium Uranium (CANDU) reactor fuel was changed to a lower strength/higher ductility Zircaloy which was found to retain a greater proportion of its ductility after irradiation than the high strength/low ductility tubing. Subsequently, more detailed mechanical testing failed to substantiate the relationship between the mechanical properties and the defect behavior.

Experimental fuel irradiations were undertaken to investigate the ability of fuel with high ducitility sheathing to survive power ramps after undergoing significant burnup at low power. Defects occurred in the experimental fuel bundles regardless of the mechanical properties of the sheathing, even in bundles annealed after the low power soak, but before the power ramp. These defects are attributed to stress corrosion cracking at areas of localized stress, not tensile failure.

Brazing is used to attach appendages to the fuel sheaths. Defects have been found in the as-received material, but a majority of defects have occurred in the two braze heat affected zones near the element ends. Because nonbrazed bundles have experienced defects in similar locations, it appears the defect location is related primarily to end-flux peaking. The low overall defect rate in CANDU fuel (0.03 percent since 1972) shows that the brazing process is not affecting fuel performance adversely.

We conclude that the initial metallurgical properties of the tubing have little influence on the defect behavior of power ramped fuel; thus the limits on mechanical property requirements of the sheath have become less stringent. Good fuel management to limit maximum powers and power increases, and an interfacial layer (referred to as a CANLUB layer) between fuel and sheath have proved more effective in reducing defects.