The terms in the second bullet indicate how spent fuel is loaded into the casks. In bare-fuel5 casks, spent fuel assemblies are placed directly into a basket that is integrated into the cask itself (see FIGURE 4.3B), The cask has a bolted lid closure for sealing. In canister-based casks, spent fuel assemblies are loaded into baskets integrated into a thin-wall (typically 1/2-inch [1.3-centimeter] thick) steel cylinder, referred to as a canister (see FIGURE 4.1 and 4.3A), The canister is sealed with a welded lid. The canister can be stored or transported if it is placed within a suitable overpack. This overpack is closed with a bolted lid.
Bare-fuel and canister-based systems are sometimes referred to as “thick-walled” and “thin-walled” casks, respectively, by some cask vendors. This designation is not strictly correct because the overpacks in canister-based systems have thick walls. The only thin-walled component is the canister, which is designed to be stored or transported within the overpack.
The designation of a cask as single- or dual-purpose often has less to do with its design and more to do with licensing decisions. Indeed, bare-fuel and canister-based casks can be licensed for either single or dual purposes. Consequently, one should not expect the performance of a cask in accidents or terrorist attacks to depend on its designation as single- or dual-purpose. Rather, performance will depend on the type of attack and construction of the cask. For the purposes of discussion in this chapter, therefore, the committee uses the designations “bare-fuel” and “canister-based,” rather than single- or dual-purpose, when referring to various cask designs.
All bare-fuel casks in use in the United States are designed to be stored vertically. Most canister-based systems also are designed for vertical storage, but one overpack