STUDY FOR EVALUATE METHOD OF DESIGN STRENGTH OF ALUMINIUM ALLOY FOR BASKET MATERIALS

Aluminium alloys are widely used as structural materials for basket of dry transportable storage (dual-purpose) casks because of its high thermal conductivity and low density. The baskets are required to keep structural integrity during whole service period of the cask. The temperature of basket is approximately 200oC at the beginning of its use and gradually becomes lower due to decrease of decay heat of spent fuel assemblies. Even under such long term exposure at the elevated temperature (thermal ageing), the basket material has to keep mechanical strength well over the values used for designing of cask. Therefore, application of aluminium alloys as structural material of basket requires assessment of effects of the thermal history on mechanical properties. Regulations and guidelines are not available or under development in most countries for the evaluation of thermal ageing of aluminium alloys for basket materials.In this paper, a thermal ageing evaluation methodology for safe use of aluminium alloys as structural material for basket is proposed. In this method, basket materials are subjected to heat treatments specially designed to simulate ageing effect taking account for the evolution of metallurgical microstructure that occur during the service period of the basket. Mechanical properties of the heat treated materials are then evaluated for determination of design strength.The generalized principle proposed here for determining the heat treatment conditions are following (1) through (3).(1)Temperature is in the range which causes the phase transformations or growth of precipitates expected at temperature of basket.(2)Holding time is long enough to verify that all changes in mechanical properties stabilize at the temperature.(3)After the holding, cooling rate is small enough not to cause unexpected strengthening from excess supersaturation or precipitationAn example of application of such methodology to an Al-Mn alloy basket material is explained in this paper. As a result, the design strength has been established for the material. It was also concluded that mechanistic study on microstructure evolution including coarsening of precipitates and grains, change in alloying element in the matrix and dislocations are crucially important to determine the temperature levels and cooling rate.