The storage of spent fuel in dry casks has the same three primary objectives as pool storage (Chapter 3):
Cool the fuel to prevent heat-up to high temperatures from radioactive decay.
Shield workers and the public from the radiation emitted by radioactive decay in the spent fuel and provide a barrier for any releases of radioactivity.
Prevent criticality accidents.
Dry casks are designed to achieve the first two of these objectives without the use of water or mechanical systems. Fuel cooling is passive: that is, it relies upon a combination of heat conduction through solid materials and natural convection or thermal radiation through air to move decay heat from the spent fuel into the ambient environment. Radiation shielding is provided by the cask materials: Typically, concrete, lead, and steel are used to shield gamma radiation, and polyethylene, concrete, and boron-impregnated metals or resins are used to shield neutrons. Criticality control is provided by a lattice structure, referred to as a basket, which holds the spent fuel assemblies within individual compartments in the cask (FIGURE 4.1). These maintain the fuel in a fixed geometry, and the basket may contain boron-doped metals to absorb neutrons.3
Passive cooling and radiation shielding are possible because these casks are designed to store only older spent fuel. This fuel has much lower decay heat than freshly discharged spent fuel as well as smaller inventories of radionuclides.
The industry sometimes refers to these casks using the following terms:
Single-, dual-, and multi-purpose casks.
Bare-fuel and canister-based casks.
The terms in the first bullet indicate the application for which the casks are intended to be used. Single-purpose cask systems are licensed4 only to store spent fuel. Dual-purpose casks are licensed for both storage and transportation. Multi-purpose casks are intended for storage, transportation, and disposal in a geologic repository. No true multi-purpose casks exist in the United States (or in any other country for that matter) because specifications for acceptable containers for geologic disposal have yet to be finalized by the Department of Energy. Current plans for Yucca Mountain do not contemplate the use of multi-purpose casks.
Nevertheless, some cask vendors still refer to their casks as “multi-purpose.” These are at best dual-purpose casks, however, because they have been licensed only for storage and transport. Because true multi-purpose casks do not now exist and are not likely to exist in the future, the committee did not consider them further in this study.