Numerical simulation of HELICOFLEX® metallic gasket ageing mechanism for spent fuel cask

In the framework of CEA, GNS and CRIEPI collaboration an experimental program is being carried out, in the CEA Marcoule - TECHNETICS GROUP France joint lab, to assess the long-term use of HELICOFLEX® metallic seals in spent nuclear fuel storage casks. Beside an experimental program of 100,000 hours accelerated ageing at different temperatures, a numerical model using finite element analysis has been developed to study seal ageing mechanisms and help the extrapolation of these behaviour to various ageing conditions and seal designs. The studies rely on use of the Larson-Miller parameter to define a time-temperature equivalence that has the advantages to be relevant to describe ageing mechanisms and to be easy to use to study different ageing scenarios. However, this analytical approach faces challenges when seal designs and materials change. Thus, to ensure the model reliability, any seal design change should require additional long term and expensive ageing tests. The presentation presents the latest developments related to HELICOFLEX® seals numerical simulation. Indeed, to help the seals behaviour extrapolation for different seal designs, mechanical tests and a specific 3D detailed numerical model has been developed to describe its relaxation mechanisms. This model details the different stages of live of the seal with compression, thermal transient and ageing itself and could be used to assess the evolution of the residual spring-back. The relevance of this simulation is assessed and discussed regarding the available database related to long term ageing tests at different temperatures.