Post-test fuel basket evaluations of the CASTOR-V/21 cask

Following an extensive testing program of the CASTOR-V/21 cask at INEL, eight symmetrically positioned indications were observed on the fuel basket. Since the presence of fuel in the cask permitted only remote visual inspection, it could not be conclusively determined if the indications represented material failure. The cask was not functionally limited since vertical movement of various fuel assemblies was possible and the structure remained intact. The basket is a redundant structure and criticality safety is maintained by fluxtrap boxes which were not in affected regions of the basket. The indications were observed at plate joints, which are stitch-welded for basket-manufacturing purposes. An extensive analysis was made of the basket design, manufacture, and test sequence to determine the possible cause and nature of the indications. This test cask had been tested under stringent thermal operating conditions. The cask was held at a power level 45% over rated conditions (28.5 kW vs. 21 kW). Also, the cask was held for two days with a vacuum in the cavity rather than helium (a conductive, inert gas), which is used during fuel storage. An evaluation was performed which included the following considerations: history under similar conditions, unique aspects of the test, basket construction techniques, fatigue, metallurgy and welding, and thermal stress. The consensus of several experts was that high thermal stress due to constrained thermal expansion of the fuel basket components caused the indications. This situation was remedied for future baskets by ensuring that certain manufacturing tolerance be measured and controlled. These limiting dimensions were established to permit sufficient space for thermal expansion. An extensive stress analysis was performed to define the dimensional requirements and demonstrate that the resulting basket stresses are acceptably low