Spent nuclear fuel, also called the used nuclear fuel, is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor) and that must be replaced by a fresh fuel due to its insufficient reactivity.
An irradiated fuel is due to presence of high amount of radioactive fission fragments and transuranic elements very hot and very radioactive. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In nuclear power plants, spent nuclear fuel is usually stored underwater in the spent fuel pool on the plant. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged. At first glance, it is difficult to recognize a fresh fuel from an used fuel. From mechanical point of view, the used fuel (irradiated) is identical as the fresh fuel. In most PWRs, used fuel assemblies stand between four and five metres high, are about 20 cm across and weighs about half a tonne. In contrast to the fresh fuel, which are simply shiny, the oxide layer forming on the surface of used fuel assemblies during the four-year fuel cycle makes them dark. Moreover, Cherenkov radiation is typical only for spent nuclear fuel. The glow is visible also after the chain reaction stops (in the reactor). The cherenkov radiation can characterize the remaining radioactivity of spent nuclear fuel, therefore it can be used for measuring of fuel burnup.
Currently, spent fuel is being stored in at-reactor (AR) or away-from-reactor (APR) storage facilities and we can identify two basic solutions of interim storage:
- Dry Storage, where natural air circulation dissipates heat.
- Wet Storage, where water is used as the heat conductor.
Dry Storage of Spent Fuel
Dry storage is most often based on the use of spent fuel casks. As its name implies, dry storage of spent fuel assemblies differs from wet storage by making use of gas or air instead of water as the coolant and metal or concrete instead of water as the radiation barrier. In dry storage systems, sufficiently cooled spent fuel is not stored underwater, but it is loaded in these casks (or vaults or silos). If on-site pool storage capacity is exceeded, it may be desirable to store the spent fuel in modular dry storage facilities, which may be at the reactor site (AR) or at a facility away from the site (AFR). Spent fuel is transferred from spent fuel pools to thick metal casks or thinner canisters. These casks are then drained, filled with inert gas, and sealed. The thick casks can be placed directly on a concrete pad, while the thinner canisters are placed in concrete casks or vaults to provide radiation shielding. Many regulators have determined that dry storage of spent fuel at reactor sites is safe for at least 100 years, and generally considers dry storage safer than pool storage.
Initially dry storage were single purpose systems. They only provided storage without the capability or authorisation for eventual transport off site (without rehandling and reloading the fuel into transport casks). Vaults, silos and non-transportable casks are single purpose systems. Dual purpose casks were developed and they allow storage and transport to and from a storage facility without rehandling of fuel assemblies. The fuel containers of some storage systems may be used for transport and/or final disposal. Dry storage managing is less expensive since it provides all safety characteristics, doesn’t need electrical systems (necessary only in vault storage), periodic maintenance and a constant fuel monitoring, increasing the system reliability for longer periods. Because of their inherent flexibility, cask systems have proved popular with reactor operators.
Dry interim storages are practical and approved in many countries especially that have the “wait and see” philosophy (wait to see new technologies development). It must be added, any strategy for managing spent nuclear fuel will be built around combinations of many options, and all strategies must ultimately include permanent geological disposal.
Spent Fuel Cask – Dry Storage Cask
Dry storage casks or canisters are widely used for dry interim storage and for transportation of spent nuclear fuel. They can be oriented and stored vertically or horizontally. The casks provide both shielding and containment. The casks are typically steel cylinders that are either welded or bolted closed. 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 the public. This additional material serves as a barrier preventing physical damage that might result in a release of radiation.
- Metal Casks. Metal casks are massive containers used in transport, storage and eventual disposal of spent fuel. These casks usually consist of a monolithic body made of ductile cast iron, a basket for accommodating the fuel assemblies and a double-lid system (primary and secondary lid arranged one above the other). Internal basket or sealed metal canister provides provides structural strength as well as assures subcriticality.
- Concrete Casks. Concrete casks have the same inner disposition that a metallic cask has. Spent fuel assemblies are distributed in internal baskets inserted into these containers. But structural strength and radiological shielding are provided by reinforced regular or high density concrete. Concrete is neutrons and gamma radiation shielding. Generally, concrete casks are heavier than the metallic ones since their wall thickness is greater and are less expensive than the metallic ones. Concrete casks that rely on conductive heat transfer have more thermal limitations than those using natural convection air passages.
Some of the cask designs can be used for both storage and transportation. These dual purpose casks were developed and they allow storage and transport to and from a storage facility without rehandling of fuel assemblies. The fuel containers of some storage systems may be used for transport and/or final disposal (multipurpose casks). Spent fuel is transferred underwater from spent fuel pools to these casks using primarily fuel handling equipment already available at the reactor site. These casks are then drained, filled with inert gas, and sealed. The external surface of the cask has trunnions which allow the cask to be lifted and displaced. Shock absorbers of the cask installed at the bottom and the cover assure transport stability.
During interim storage the lid system consisting of the two barriers is permanently being monitored for leak-tightness. The radiation emitted by the radioactive inventory is safely shielded by the cask body. For neutron moderation, axial boreholes are drilled into the monolithic body and filled with polyethylene moderator rods.
The operations often occur in the same space and use the same equipment meaning the vast majority of steps must be completed sequentially and dry storage casks cannot be loaded in parallel. Sometimes, dry storage casks use the same lifting equipment (polar crane) as another outage activities and it extends plant outage. The main steps to load a cask are performed as follows:
- Preparation of a cask for fuel loading
- Transfer the cask into spent fuel pool
- Load fuel into the
- Remove the loaded cask from the spent fuel pool
- Decontaminate cask exterior
- Drain small amount of water from cask cavity, then weld/bolt and inspect inner lid (vacuum or forced helium drying system)
- Transfer the cask from plant to storage facility
- Store cask
Safety of Spent Fuel Casks
Safety of spent fuel casks stands on various criteria. These criteria may be grouped according to following aspects:
- Subcriticality. Fulfillment of this criterion is based on:
- the design of the spent fuel cask,
- requirements on materials of the spent fuel casks ( e.g. adding neutron absorbing materials (typically boron) in storage racks baskets),
- limiting of stored fuel (e.g. fuel enrichment, assembly burnup)
- Cooling. Fulfillment of this criterion is based on:
- the design of the spent fuel cask (e.g. shape and orientation of cooling fins),
- requirements on inert gas pressure,
- Radiation Shielding. Fulfillment of this criterion is based on:
- the design of the spent fuel cask (e.g. wall thichness),
- neutron shielding (polyethylene moderator rods)
- Integrity. Fulfillment of this criterion is based on:
- the design of the spent fuel cask,
- the design of shock absorbers,
- ensuring periodic inspections
These goals are not difficult to achieve. In dry cask storage there are very few scenarios that can be imagined that could provide the energy needed to break the cask and spread the radioactive material into the surrounding environment. Because of their inherent flexibility, cask systems have proved popular with reactor operators.
Dry Storage Vaults
In a dry storage vault, the spent fuel is stored in a reinforced concrete building, whose exterior structure serves as the radiation barrier, and whose interior has large numbers of cavities suitable for spent fuel storage units. These cavities contain metallic cylinders in which externally air is insufflated or sometimes the air circulation is natural. Spent fuel is received (either dry or wet) at a vault facility using transfer or transportation casks. The fuel is typically stored in sealed metal storage tubes or storage cylinders, which may hold one or several fuel assemblies. These cylinders provide containment of the radioactive material in the spent fuel. Shielding is provided by the exterior structure. Heat removal is normally accomplished by forced or natural convection of air or gas over the exterior of the fuel containing units or storage cavities, and subsequently exhausting this air directly to the outside atmosphere or dissipating the heat via a secondary heat removal system.
Thus, vault systems typically also require cranes or fuel-handling machines. Typical features of vaults are their modularity, which facilitates incremental capacity extension, separated shielding and containment functions, capability for containment monitoring, and a vertical fuel loading methodology.
Dry Storage Silos
In s dry storage silo system, the spent nuclear fuel is stored in metallic canisters, inside a concrete cylinder on the floor (silos). Silos are monolithic or modular concrete reinforced structures, which provide the radiation shielding (as the building exterior does in the case of a vault) while the sealed inner metal liner or canister provides containment. The storage position can be vertical or horizontal. Spent fuel is received (either dry or wet) at a vault facility using transfer or transportation casks. Canisters are then loaded into silos. Concrete is the structural material and the radiation shielding (like the building in vault storage), as the canisters provide containment. Heat is removed by air convection through ducts in the concrete cylinders. The metal canisters are also fitted with a double lid closure system, which following welding is tested for leak tightness.