Criteria for and proof of leaktightness concerning casks for dry transport and intermediate storage of spent LWR-fuel elements

Using a model cask with typical dimensions, typical radioactive contents and a typical sealing system, some general aspects of its leaktightness assessment will be outlined quantitatively. Taking into account molecular as well as viscous-laminar gas flow it will be shown that under pessimistic assumptions standard helium leakage rates less than some 10-2 mbar . 1/s for transport and less than some 10-4 mbar . 1/s for accident conditions must be guaranteed in order to meet the activity release limits required by the 1973 IAEA Regulations. Realistic calculations of the tritium activity release show that the contribution of tritium permeation through the cask wall is negligibly small even for storage times up to 40 a. Finally some radiation protection considerations concerning the relationship between activity release and radiation exposure will be presented. On the basis of these considerations we recommend to retain the activity leakage rate limit of 10-3A2 per week after Type B tests as it is requested in the 1973 IAEA Regulations. 13 references, 10 figures