An Assessment of the Structural Integrity of CANDU Fuel Bundles while Being Rotated during Closure of the Used Fuel Container

Abstract

After loading CANDU fuel bundles in the Used Fuel Containers (UFCs) for final storage in a DGR, the UFCs are rotated horizontally up to 2500 times at a rate up to 10 RPM during the UFC lid closure process. The impact of the rotations on the bundles physical integrity at a maximum temperature of 400°C was assessed.

During the rotation, the bundles rotate around the container central axis. Additionally, they might also rotate inside the storage tube if there is a gap sufficient for a bundle to rotate in a “walking” mode by having the weight of the bundle supported by only one outer element at a time as the bundle rotates. The abnormal condition of the bundles crashing against the storage tube due to a sudden stop was also considered.

Among possible failure mechanisms, failure of the outer elements endplate/endcap welds is critical to the bundle structural integrity since they support loads and maintain the fuel bundle array configuration. A weld fails if the weld stresses exceed its elastic material properties.

Based on a typical weld having a circumferential notch at the endplate face of the weld reducing the weld nominal diameter by 23%, it was shown that the endplate/endcap welds will not fail at the normal operational speed of 10 RPM and during the abnormal crashing event. The maximum allowable RPM was also estimated for all commercial CANDU bundle types and it was found that the fatigue strength of the weld during the bundle “walking” inside the storage tube became the critical mode for failure of the weld. The maximum allowed RPMs for the various CANDU fuel types were: 33.2 for Pickering, 34.7 for Bruce, 30.0 for Gentilly-1, 48.1 for NPD-7 and 54.3 for NPD-19 and Douglas Point CANDU fuels.