Dry cask storage is a method of storing high-level radioactive waste, such as spent nuclear fuel that has already been cooled in the spent fuel pool for at least one year and often as much as ten years. Casks are typically steel cylinders that are either welded or bolted closed. The fuel rods inside are surrounded by inert gas. Ideally, the steel cylinder provides leak-tight containment of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other material to provide radiation shielding to workers and members of the public.

There are various dry storage cask system designs. With some designs, the steel cylinders containing the fuel are placed vertically in a concrete vault; other designs orient the cylinders horizontally. The concrete vaults provide the radiation shielding. Other cask designs orient the steel cylinder vertically on a concrete pad at a dry cask storage site and use both metal and concrete outer cylinders for radiation shielding. As of 2021 there is no long term permanent storage facility anywhere in the world; dry cask storage is designed as an interim safer solution than spent fuel pool storage.

Some of the cask designs can be used for both storage and transportation. Three companies – Holtec International, NAC International and Areva-Transnuclear NUHOMS – are marketing Independent Spent Fuel Storage Installations (ISFSI) based upon an unshielded multi-purpose canister which is transported and stored in on-site vertical or horizontal shielded storage modules constructed of steel and concrete.