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.