MLIFT – Leak rate after recompression of aged metal seals

Gaping in the sealing area as well as transverse lid displacement are ruled out under normal operating conditions and represent a generic accident scenario. Generic because the mechanical design of the casks is basically such that, even under maximum accident loads, the lifting of the lid is limited to values that remain below the useful elastic recovery of an aged metal seal. To cover longer periods of time, a scenario where the aged metal seal completely loses contact with the sealing surface during load application is considered here as a limiting case. The design of the bolting ensures that the gap created under load is completely closed by the remaining pretension of the bolts and the seal is recompressed. The recompression of the seal is ensured by the fact that the bolts of the lid system in accordance with the design requirements (BAM GGR 012 Guideline on the Assessment of the Lid Systems and Load Attachment Systems of Transport Packages for Radioactive Materials, edition 2020-12) may essentially only be elastically stressed, even under accident conditions, so that sufficient residual clamping force remains after the impact phase to close the lid again without a gap.

As the aim of the research project is to determine a covering leak rate for the load case described, the contact between the seal and the lid is artificially removed during the tests by completely opening the flange with the aged seal. One of the two flange parts is lifted off after the bolting has been loosened and, after a relative rotation (by one screw pitch circle), is then replaced and screwed back on. Twisting ensures that a completely new contact between the seal and the sealing surface must be established during compression.

As only main lids are potentially affected by the load case – the accident loads acting on small lids are orders of magnitude lower – only metal seals for the primary and secondary lids are tested accordingly. This means that ten silver-coated and ten aluminium-coated seals with corresponding cross-sections are compressed in the flanges and thermally aged in line with the results of the MSTOR project. After ageing, the flanges are opened and reassembled as described above. The leak rate is measured for at least one week following recompression. In this way, in addition to the maximum leak rate immediately after recompression, a prognosis can also be made about the development of leakage following a hypothetical incident.

In addition to the seals/flanges to be procured specifically for this project, it is planned to include the existing test flanges used in the MSTOR project in the MLift programme once the respective test series have been completed. This increases the statistical quality of the measurement programme and allows the effects of different ageing conditions or temperatures on the seal behaviour during recompression to be evaluated.