Helium Leak Testing Requirements and Experiences on Transport and Storage Casks for Interim Storage in Switzerland

This paper provides an overview of the actual surveillance requirements of the Swiss Federal Nuclear Safety Inspectorate, ENSI, relating to helium leak testing on dual-purpose casks for interim storage of nuclear waste in Switzerland.The requirements are summarized in the “Regulatory Guideline for Swiss Nuclear Installations” (ENSI-G05). The defined limit of standard helium leakage rate (SHeLR) for each barrier function seal of the containment system is 10-8 Pa m3/s. In addition, the ratio of the sum of all these SHeLRs to the volume of the monitored space must not exceed 2*10-6 (Pa m3/s)/m3.These values have to be verified for all casks by measurements during the final inspection at the manufacturers’ site as well as after loading and immediately before the interim storage. The supervision of these measurements and the review of the underlying test procedures according to national and international standards for leak testing on storage casks of radioactive materials have been delegated by ENSI to SVTI-N.In the last twenty years the test procedures have been improved, based on the experiences from measurements. For example, the leak testing requirements have been adapted to include the storage conditions. The typical storage cask used in Switzerland must have a double-lid system with metallic seals. Each lid forms a barrier and shall have the ability to provide the leak tightness of the cask.The inner lid closes the storage volume and has a metallic seal on the inner side and an elastomer O-ring outside. The volume between both seals is used for the measurements.The outer lid has the same sealing principle, which closes the cask from the environment.In the storage configuration, the volume between the lids is pressurized with helium (6.5 bar). This pressure is permanently monitored and an abnormal pressure loss indicates a failure.The nominal pressure in the main storage volume is below atmospheric pressure (0.2 bar helium). In our presentation we will cover the individual steps of the leak testing procedure and mention in particular the test with elevated pressure (7.0 bar), which turned out to reveal a significant difference between reliable and potentially error-prone sealing.