Development of Aluminum Extruded Alloy for Basket of Transport/Storage Casks (1) - Strengthening mechanism after long term storage and design of chemical composition -

The basket of transport/storage casks of spent fuels must satisfy the structure strength corresponding to transport/storage conditions, the criticality prevention function and heat removal functions. Additionally, the casks should be reduced the weight as much as possible suiting to the design requirement to maximize the storage capacity of fuel assemblies in view of cost and efficiency. Aluminum alloys have the properties of lightweight and high thermal conductivity, which are desirable characteristics for the basket material. On the other hand, the aging degradation of the material must be evaluated properly, because the basket is exposed to the decay heat of spent fuels during the usage period up to 60 years.2. Development of Aluminum Extruded Alloy A3J04-OThe compositional design of the aluminum extruded alloy developed this time is based on A3004 alloys. Although some aluminum alloys decrease in strength after overaging, this potential has been almost eliminated and it simplifies the evaluation of aging degradation.Strength of the material is increased with dispersion hardening of manganese compound and solid-solution hardening of magnesium. Therefore, these elements have been focused on the evaluation of aging degradation.Regarding the dispersion hardening of manganese compound, it was confirmed that the strength property and microstructure did not change after the heat treatment of which condition was evaluated using the diffusion length of manganese in aluminum matrix to envelope the heat history for 60 years. As for the magnesium, the content with which the effect of solid-solution hardening is maintained for 60 years was determined using the evaluation method that was newly developed using supersaturation and diffusion length. These two main hardening mechanisms are reflected in the compositional design.In addition to the above two main hardenings, work-hardening, precipitation strengthening and grain boundary strengthening are also known as strengthen mechanisms of materials. Although these strengthen mechanisms are not used for the material, it was confirmed that these mechanism did not influence on the strength property from the viewpoint of the evaluation of aging degradation. Generalizing these evaluation results, the chemical composition of the new material was determined.