| Subject matter: | MSTOR – Metal seals during long-term storag » Extension of the existing experimental basis for the temperature-dependent ageing behaviour of metal seals » Development of a forecast model for sealing parameters |
| Project partners: | » GNS (cask manufacturer, Germany) » Technetics (seal manufacturer, France) » EWN Entsorgungswerk für Nuklearanlagen GmbH (store operator) |
| Project period: | 2021 to 2031, longer if necessary |
Only Helicoflex® metal seals made by the French manufacturer Technetics are used in the sealing barrier. Helicoflex® type seals consist of a helical spring core and a stainless steel jacket covered by an outer liner made of aluminium or silver. The functioning of the seals is based on the elasticity of the helical spring (see Figure 3) that, when compressed, generates the restoring force that is necessary to maintain contact between the outer liner and the sealing surfaces. The plasticity of the outer liner also ensures that the surface of the seal is optimally adapted to the structure of the sealing surface. During compression, the liner material fills the flange imperfections in order to achieve a high degree of tightness with standard He leak rates of less than 10-8 Pa m3/s. The stipulated standard He leak rate of 10-8 Pa m3/s is therefore also a system-specific quality criterion for the required long-term functionality and is not a radiologically based tightness requirement, as this could also be met at higher leak rates.
Figure 3:
Functionality of Helicoflex® metal seals
During initial compression of the metal seal, a force-deformation curve is obtained as shown in Figure 4. At deformation e0, the required tightness is achieved for the first time, but only at deformation e2 is the seal at its operating point. The seal is compressed to the operating point thanks to the depth of the groove in which the seal is inserted in the lid, as the difference between the groove depth and the torus diameter of the seal corresponds exactly to the optimum compression e2.
If the seal is decompressed as a result of external loads, i.e. if a gap emerges between the lid and the mating surface, the required tightness is maintained until e1 is reached. The permissible gap between the sealing surface and the lid corresponds to the useful elastic recovery ru (see Figure 4). In particular, this parameter and the associated force Y1 are essential for evaluating the behaviour of the metal seal under operating and accident conditions.
In the assembly state, mechanical stress and temperature exposure lead to creep processes in the metal seal. These become noticeable in the form of relaxation. As shown in Figure 4, relaxation decreases the restoring force at the operating point to Y2r with unchanged deformation. Although the minimum force required to maintain the specified tightness Y1r decreases upon decompression, the deformation e1r also decreases accordingly, so that the remaining useful recovery rur for an aged seal decreases considerably compared to the assembly state.
Knowledge of the characteristic values Y1r and rur of an aged seal is therefore of great importance to reliably evaluate the long-term behaviour of the metal seals. In particular, the remaining useful recovery rur is a measure for assessing the robustness of the sealing barrier, i.e. the extent to which the seal is able to maintain the required tightness even under external mechanical influences.
Figure 4:
Change in characteristic values of metal seals due to ageing
The long-term tests carried out so far and the operating experience of more than 25 years demonstrate that the high tightness requirements are also met in the long term by the metal seals used. This can be expected to remain the case for interim storage of 40 years and longer. However, in order to validate and quantitatively predict the sealing behaviour over longer periods of time, further investigations are required to determine the change in sealing characteristics under the influence of temperature and time.
A study on the ageing behaviour of metal seals was carried out between 2013 and 2016 under the direction of GNS with RuDrift. The tests were carried out in the joint laboratory of Technetics and CEA at the Pierrelatte site (France). The stainless steel (SST) test flanges developed for the tests represent the standard sealing surface/lid combination (nickel-plated cask body/martensitic lid material). Aluminium- and silver-coated metal seals were used in the test flanges. These metal seals are of the kind used in the primary and secondary lids, the so-called main lids. The compressed metal seals used in the test flanges were stored for a total of two years at temperatures of 100°C and 130°C and for one year at 150°C to predict the sealing behaviour, particularly for the maximum design temperatures. Reference flanges were stored in parallel at room temperature. At different times, the test flanges were removed from the furnaces and the decompression curves were recorded with respect to Y1r and rur. The results describe the quantitative behaviour of the metal seals for a temperature level that corresponds to the maximum design thermal load.
In fact, the seal temperatures during interim storage are significantly lower than the test temperatures at RuDrift. This means that the design temperatures are not reached even at the time of loading and continue to drop due to the decreasing heat generation. Additional tests at lower temperature levels with a correspondingly extended test duration are also planned. These will be performed to obtain valid statements on the behaviour of metal seals at the sealing temperatures expected during extended interim storage. The study will also cover seals with a smaller torus diameter. These seals are used in the so-called small lids (closure lid, protective cap and pressure switch). There are also plans to study the transferability to the use of another sealing surface/lid combination. This is a combination of uncoated cast iron (DCI) with stainless steel (SST), which is only relevant for main lids with aluminium-coated metal seals. The test programme (EP) under MSTOR is therefore as follows (see Figure 5):
Figure 5:
Overview of the RuDrift/MSTOR test programme
EP1/2
Continuation of the artificial ageing of aluminium- and silver-coated metal seals in main lids at 130°C (plus one year) and 100°C (plus three years), which was started with RuDrift. The aim is to improve prediction accuracy in the selected temperature range and to carry out further reference testing of seals stored at room temperature (plus eight years).
EP3/6
Expansion of the database for aluminium- and silvercoated metal seals in main lids at ageing temperatures of 60°C and 80°C, in each case over a period of at least eight years.
EP4
Creation of a database for aluminium-coated metal seals in main lids for the flange combination DCI/SST at representative ageing temperatures of 130°C (one year), 100°C (six years) and at room temperature (six years) for reference purposes.
EP5/7
Creation of a database for aluminium- and silver-coated metal seals in small lids for the flange combination SST/ SST at ageing temperatures analogous to the large lid seals of 150°C (one year), 130°C (three years), 100°C (six years), 80°C (seven years) and 60°C (at least seven years) as well as at room temperature (at least seven years) for reference purposes.
Models for predicting the change in the characteristic values of the metal seals are generated based on the temperature and time-dependent measurement results. These models allow the long-term behaviour of metal seals to be evaluated, taking into account real thermal loads and decay behaviour. The results can also be used to specifically pre-age metal seals by artificial ageing, for example for further studies on aged seals. To verify the calculations, additional flanges (so-called travelling flanges) with silver- and aluminium-coated metal seals are aged at various temperatures (130°, 100°, 80° and 60°C) in the EP3/6 test programmes. The respective measurement results are compared with the temperature- and time-dependent calculations. The aim is to demonstrate that the prediction model found is able to correctly calculate ageing under realistic operating conditions at decreasing temperatures.
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