The design of bolted flange joints for the German double barrier closure system of spent fuel and high-level waste transport and storage casks is described. Additionally, the basic load assumptions are presented for service conditions and for hypothetical accident scenarios. The modelling details and the results of numerical analyses are discussed with respect to those of experiments. The treatment of the numerical results for the bolts is presented considering safety factors derived by nominal stress concepts. The lids of the double barrier system are designed for a specified helium leak tightness. In addition to interior pressure, the accelerations during usual service and in particular during accidental drop tests are considered. The sliding and the gap formation at the gasket surfaces is analysed with respect to drop test loads. The consequences for the containment analysis are discussed on basis of the tightness characteristics. Concerning the bolts, the transfer of local stresses provided by numerical analysis to nominal stress values is presented. This transfer enables the use of safety factors based on nominal stress concepts for the local stresses given by numerical analysis.

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