A single nuclear fuel assembly spends around five years in the reactor of a nuclear plant, creating heat that is then turned into electricity. Typically, every 18 to 24 months, a nuclear plant stops generating electricity to replace a third of the fuel assemblies in the reactor with fresh ones. The assemblies removed from the reactor are then stored in a large body of water inside the nuclear plant, called the spent fuel pool, where they cool over time. The water also shields the workers from the radiation that comes from the fuel assemblies. Due to the use of water, this storage method is also often referred to as “wet storage”.
After the used fuel assemblies have cooled for at least one year, they may be moved from the pool to canisters made from stainless steel filled with an ‘inert gas,’ i.e. a gas that does not chemically react and prevents corrosion of the content of the canister, such as Helium. The steel canisters are strength welded closed in order to provide a leak-tight containment of the used nuclear fuel and are placed inside large robust casks made of steel and concrete.
The steel and concrete casks surrounding the canister provide radiation shielding to workers and the public from the stored used nuclear fuel, and physical protection of the fuel. The casks are then initially stored at the site of the nuclear plant. Since this storage method does not require any water, it is often referred to as “dry storage”. It also does not use any fans for cooling and electric power is not required.