Concrete casks are movable structures with a single storage cavity. They are used for storage and sometimes for transport of spent fuel. Structural strength and radiological shielding are provided by reinforced regular or high-density concrete.

A concrete cask/container system may use sealed metal canisters or metal liners inside the storage cask to contain radioactivity. Some canisters can be removed from the concrete cask and are licensed for transportation in an off-site transportation package. Dissipation of spent fuel decay heat relies more on conductive heat transfer to the surface of the container and external natural convection by air. Concrete casks use single or double lid closure systems. Lids are welded closed and tested for leak tightness. In most cases the canisters or liner are backfilled with an inert gas to prevent corrosion and to improve heat transfer within the canister. Nitrogen or helium gases are most often used. An example of a concrete container is shown in Fig. 15.11. It is a dry storage container (DSC) for

Seal/structural weld

Vent port

Reinforced

high-density

concrete

Steel inner liner

Steel outer hner

Drain port

15.11 Dry storage container (DSC) for CANDU-type fuel (courtesy of Ontario Power Generation).

HWPR CANDU fuel where the fuel bundles are placed horizontally in the spent fuel rack. In the design and licensing of dry storage containers, the key parameters are the maximum burnup of the nuclear fuel (GWd/tHM) and the maximum heat load to the storage unit (kW).

Silos

Silo systems are monolithic or modular concrete-reinforced structures. The concrete provides shielding while containment is provided by either an integral inner metal (liner) vessel, which can be sealed after fuel loading, or by a separate metal canister. In silos, spent fuel may be stored in vertical or horizontal orientation. Fuel loading always takes place at the storage site. The NUHOMS storage system is an example of a horizontal concrete silo system. Fuel is loaded vertically into metal canisters, which are stored in a horizontal orientation inside concrete storage modules as shown in Fig. 15.12.