Ageing of Helicoflex® metallic gasket for spent fuel cask: results and analysis of sealing performances of a 75,000h campaign

In the framework of CEA, TECHNETICS GROUP France, GNS and CRIEPI collaboration an experimental program is being carried out to assess the long-term use of HELICOFLEX® metallic seals in spent nuclear fuel storage casks. The harvested data on more than 60 mockups after 75,000h of ageing provide a consistent database to extrapolate the long-term sealing performances to lifetime exceeding 100 years. After a description of the HELICOFLEX® gasket and of the ageing program, the results of all the measurement campaign are discussed using an analytical model to interpret them from a statistical point of view. The paper then focuses on the finite element approach implemented to consolidate the experimental protocol and the statistical analysis hypothesis. A 3D-model of the mockup is first used to check and understand the mechanical behavior of the assembly (two bolted thick stainless steel blind flanges). Based on the comforting results of this analysis, a detailed 2Daxisymmetric model of the seal is then developed to assess qualitatively the time extrapolation method. In the numerical simulation time-dependent loads are applied on the seal to replicate the different loading cycles inherent to the experimental protocol. The non-linear behaviors of the seal materials are also considered (plasticity of spring and inner layer, plasticity and viscoplasticity of silver outer lining) to describe as precisely as possible the gasket load relaxation along the ageing. The detailed description of the most relevant features and outcomes of these calculations are presented. In the light of the statistical analysis of the experimental data and prediction methods, providing a maximum temperature of ‘safe’ use, the sealing performances of the HELICOFLEX® over duration exceeding a century can be assessed.