Ageing of metallic gaskets for spent fuel casks: Century-long life forecast from 25,000-h-long experiments

An experimental programme is being carried out that aims at quantifying the relaxation of four types of metallic HELICOFLEX^® seals during their use in spent nuclear fuel storage casks. Two types of lining are taken into account: aluminium and silver. Tests longer than 10,000 h are implemented only for silver. For each type of lining, two different section diameters are investigated. The work aims at evaluating the minimum residual linear load that can be guaranteed for a seal after a particular time of relaxation. This relaxation depends on the evolution of the seal temperature with time. Therefore, holds of seals tightened between two flanges have been performed at several constant temperatures, including 100 and 200 °C. Residual load and ‘useful’ recovery have been measured after the holds. Results are interpreted according to two methods: a time extrapolation, and a time–temperature equivalence parameter. Both methods are based on linear relationships and are assessed through a statistical analysis (calculation of scatter) which is also used to determine a minimum guaranteed residual load. Finite element simulations of the relaxation of a seal have also been performed in order to justify qualitatively that the time extrapolation method is safe. For silver lining seals, the use of a time–temperature equivalence parameter equal to T (11 + log₁₀ (t)) appears justified and this enables us to assess the maximum temperature at which seals can be ‘safely’ used ‘up to a century’.Using the available ageing results (longest holds: 25,000 h), and the proposed prediction method, it can be proven that the two types of silver lining seals which are evaluated will retain a residual linear load of at least 100 N mm⁻¹ of seal perimeter after one century of use in a cask, if the initial temperature of the seal after closing the cask is less than or equal to 100 °C.