Properties of Aluminum Alloys for Transportable Storage Cask Basket After Long Term Storage

A basket for transportable storage cask for spent nuclear fuel is designed to maintain prescribed geometrical arrangements of spent fuel assemblies under accidents during transport and storage condition, and also designed for effective removal of decay heat from spent fuel assemblies. Various kinds of aluminum alloys are widely used for basket material as structural members of the transportable storage casks, for the high heat-transfer performance and material workability. Considering transportation to a reprocessing facility after long term storage at an interim storage facility, it is important to maintain integrity of basket structure under transport accidents. However, mechanical properties of these alloys are possibly changed by thermal aging during storage since cask service temperature may affect microstructure of aluminum alloys over the long time period. It is therefore essential in cask design qualification to identify degradation of mechanical strength of basket material both for normal storage and transport accident after long term storage. MHI has conducted evaluation of mechanical properties considering thermal aging of the aluminum alloys A6N01 containing boron carbide (B4C) and commercial A3004 without boron for the basket, which resulted in construction of a data base for assessing structural integrity of the basket [1–2]. In this paper, effects of thermal aging on mechanical properties of these materials are presented, and the microstructural strengthening mechanisms which play important roles in the evolution of mechanical properties are discussed.