Apparent cause for leakage in a dry storage container

The last processing step before placing a Dry Storage Container (DSC) loaded with used fuel into interim storage at Ontario Power Generation (OPG) is the OPG-helium leak test. DSC 5149 failed its OPG-helium leak test with an unacceptable leak rate. After various methods of trouble shooting, it was diagnosed that the leak was from a weld defect on the outer shell. Leakage from outer shell indicates that there was also leakage from the containment boundary, which contains the used fuel and was backfilled with helium prior to leak testing. An Apparent Cause Evaluation (ACE) was performed; the results of the ACE investigation are presented in this paper. Of the five possible causes identified in the investigation, the ACE pointed to two apparent causes: (1) Manufacturer's helium leak test failed to detect the leak, and (2) Containment boundary was damaged during manufacturing, after the manufacturer's helium leak test. Investigation and validation included internal discussion and meetings with OPG staff involved with processing the DSC, meetings with the manufacturer to review details of the manufacturer-helium leak test and manufacturing process, and review of all relevant OPG and manufacturer documents and records for this DSC. The ACE investigation identified no high probability issue that could lead to this type of defect. The manufacturer's materials and manufacturing procedures have been examined, and deemed of sufficient quality and well planned. It was concluded that this was a 'one-off' instance, and leakage from a DSC containment boundary is not expected to reoccur. (author)