Radiation effects on the mechanical properties and long term ageing of spent fuel storage containers

The CEA is presently studying a spent fuel container which has to insure the insulation of the radio-nucleides and the retrieval of the spent fuel during a storage period covering several centuries. The container is a 50 mm thick low carbon steel cylinder with a welded lid (by TIG or electron beam). An alternative solution is a ferritic ductile cast iron container. Each container contains seven 4 mm stainless steel 304L individual fuel holders. The temperatures at the beginning of the storage are about 250°C at the fuel holder and 150°C at the container. As the duration of the storage can exceed those that are generally accepted for industrial containers, investigations must be carried out to insure that the materials performance will be kept sufficiently high during the whole storage period. A ten years experimental program has been undertaken by the CEA in order to predict the behaviour of the container and of the fuel holder in storage conditions [1]. In order to model the long term behaviour of the materials, the radiation-induced damages in the materials must also be evaluated. In this paper, the damage flux from neutron and gamma radiation on the low carbon steel container are estimated. The influence of the irradiation on the container’s ageing is estimated by a Cluster-Dynamics Model, developed for the reactor vessel. This model takes into account the hardening due to point defects clustering (vacancies and self interstitial atoms (SIA) created by irradiation develop cavities and loops which harden the materials) and copper precipitation : the copper mobility is enhanced by DP in excess and then precipitates. The increase of the shear stress is calculated for different storage conditions. The influence of some parameters is discussed. Conclusions are drawn on the effects of the irradiation and the Cu content on the mechanical properties of the container.