After fuel is removed from a reactor, it remains highly radioactive and continues to generate heat through radioactive decay. It must therefore be cooled to prevent overheating. Spent fuel is initially stored in water pools, usually located at the nuclear power plant. The water removes decay heat and also provides effective shielding against ionising radiation. As the radioactivity and decay heat gradually decrease, the fuel may remain in wet storage or, after several years, be transferred to a dry interim storage facility, where it can safely remain for decades. Most interim storage facilities are located at nuclear power plant sites, although centralised facilities receiving spent fuel from several reactors also exist. Depending on national fuel-cycle policy, the stored fuel may eventually be transported for reprocessing or final disposal.

Video: Storage of spent fuel in wet interim storage pools.

Wet Interim Storage

Interim storage of spent nuclear fuel at the Temelín nuclear power plant.

For the first few years after removal from a reactor, spent fuel is stored in large water pools. Fuel assemblies are transferred from the reactor to the spent fuel pool using specialised fuel-handling equipment, usually while remaining under water. The advantages of wet storage include easy visual inspection and effective cooling by circulating water, which also provides radiation shielding. The disadvantages include the need to maintain the water level, circulation, cooling and water chemistry, resulting in higher operating and maintenance requirements than for dry storage. Most spent fuel pools are located at nuclear power plant sites. However, independent centralised wet storage facilities also exist and may receive spent fuel from several reactor units or nuclear power plants.

Dry Interim Storage

Dry interim storage generally has lower operating and maintenance requirements than wet storage. After sufficient cooling in a spent fuel pool, the fuel is transferred to sealed containers or vaults, usually filled with an inert gas such as helium. Decay heat is removed through the walls of the container and ultimately transferred to the surrounding air, typically by passive natural convection without the need for pumps or fans.

Several types of dry storage systems are used. In some designs, sealed canisters containing spent fuel are placed in concrete vaults or modules; in others, they are stored vertically or horizontally in massive concrete and steel casks. Spent fuel is also commonly stored in dual-purpose casks or canisters designed for both storage and subsequent transport. Their structure provides containment, radiation shielding and effective removal of decay heat. Dry storage systems may be located inside dedicated storage buildings, as at Ahaus in Germany and at the Dukovany and Temelín nuclear power plants in the Czech Republic, or outdoors on specially designed concrete pads, as at Surry in the USA.

At the Wylfa nuclear power plant, two stages of dry spent fuel storage were used: initial storage cooled by CO2, followed by longer-term storage cooled by air.

Sweden operates a centralised wet interim storage facility known as Clab, where spent fuel from the country's nuclear power plants is stored in water-filled pools in underground rock caverns before final disposal.