Compliance with the specified leak rate of 10-8 Pa m3/s is only ensured within the operating range of the seal, i.e. if the contact between the outer liner and the sealing surface, as set at the time of installation of the seal, is maintained unchanged.
In particular, the exposure to transverse forces on the seal, caused by a radial relative movement between the lid and cask, is not intended. Under normal operating and transport conditions, the seal is in a static state; the pre-tensioning of the bolting ensures that no relative movements, i.e. dislocations occur in the lid system. Under hypothetical accident or incident conditions, however, changes in the position of the lid system can no longer be completely ruled out. In order to include the shipment after extended interim storage in the ageing considerations, a drop in any direction from a height of 9 m onto an unyielding target must be assumed in accordance with the regulations. Due to their high inertia, the resulting loads can cause the main lids to shift, for example in the event of a horizontal drop. This transverse displacement can lead to changes in the contact surface between the seal and the flange and/or to deformation of the seal as a result of the transverse forces.
As the sealing effect is based on a complex interaction in the microscopic range between the ductile outer liner of the metal seal and the contact surface of the lid or cask, this cannot be derived from the characteristic curve of the seal, but must be determined experimentally for the respective load case. For this purpose, the change in the helium leak rate must be measured as a function of displacement of the lid.
In a drop test with a prototype cask in which the lid system is enclosed by a shock absorber as in the original, it is not possible to measure the leak rate at the moment of the impact due to the drop height and the inaccessibility of the lid system. In order to obtain valid measurement results nevertheless, the cask is not dropped from 9 m, but the “lid” is displaced by applying appropriate external forces. For this purpose, a pair of flanges – similar to the MSTOR tests – is held firmly in place with the seal to be tested. A guided falling mass then hits one of the two flange parts from a defined height, causing it to move transversely, while the other part, to which the leak rate measurement equipment is connected, remains in place. In this way, the leak rate can be determined over the entire duration of the dynamic transverse displacement as a function of the displacement. In addition, this generic test setup allows the setting of a transverse displacement that can be selected independently of the specific boundary conditions when testing an individual cask design.
In line with MShift’s objective of determining the leak rate of aged metal seals under dynamic transverse displacement, artificially pre-aged seals are used in the tests. In analogy to MSTOR, ageing is carried out by thermal ageing of the seals already compressed in the flange pair, whereby the ageing period and temperature are adapted to the storage periods to be taken into account. The MShift project tests both silver-coated and aluminium-coated metal seals, each with the cross-sectional dimensions of the original metal seals used in the CASTOR® cask main lids. In addition to tests at room temperature, tests are also carried out at –40°C to additionally cover the influence of the operating temperature. Following the transverse displacement, leak rate measurements are continued for up to two weeks in order to provide an indication of the development of the sealing behaviour in the longer term after an incident. Due to the pre-tensioning of the bolting, which ensures that the flange parts continue to be compressed tightly, in combination with the remaining elastic recovery of the metal seal, it is to be expected that the lake rate will subsequently decrease again after peaking during the dynamic displacement until a steady-state value is reached.
