Emptying ISFSIs

Used US light water power reactor fuel has been placed in dry storage canisters (DSC) and casks since the mid-1980s, and during that time the canister/cask systems have continuously evolved. Currently, there are more than 1600 dry storage canisters containing roughly 64,000 assemblies or approximately 20,000 MTHM at independent spent fuel installations (ISFSI) in the US [1]. Updated information on the details of dry stored commercial light water reactor fuel is available from recently-published documents including the EPRI Industry Spent Fuel Storage Handbook [2] and the Gap Analysis to Support Extended Storage of Used Nuclear [3].

Status of fuel in dry storage

Most assemblies in dry storage in the US are in welded metal canisters inside vented concrete vertical overpacks or horizontal storage module. For this configuration, the canister with its internal basket, fuel and fuel component contents is the only portion of the storage cask system which is transported. These systems all require a separate transportation cask with a type B containment vessel to overpack the fuel canister (for example, see [4]). The transfer usually requires the use of a transfer cask, except for the NUHOMS transportation casks which can interface directly with the storage module. Some welded metal canisters cannot currently be transported for various design reasons.

There are four categorical descriptions of dry cask storage:

1. Metal canisters in vertical concrete overpacks or horizontal concrete modules

2. Metal canisters in metal overpack/storage/shipping casks

3. Metal canisters in concrete vaults

4. Bare fuel casks that provide both primary containment and shielding for storage and transportation (a number of these casks have never been certified for transport).

Concepts of a consolidated storage facility must be capable of receiving any of these dry storage canister and transportation over-pack configurations. Since the mid 1980’s eight cask vendors have provided 11 cask systems comprised of 30 different canister types (Tables 1-3).

Cask systems for dry fuel storage

Dry storage in the US can be divided into two broad categories, those in which the fuel is stored bare in a fuel basket inside a metal cask and those in which the fuel is in a welded canister inside a vented concrete overpack or inside a metal dual purpose cask.

Bare fuel casks

Light water power reactor transportation casks capable of meeting the 10CFR71 requirements for Normal Conditions of Transport (NCT) and Hypothetical Accident Conditions (HAC) are generally metal casks with bolted closures and containment vessels which meet leak tight requirements of ANSI N14.54. For the case where fuel is placed directly into such a cask and used for long-term storage in that cask, the cask is often referred to as a “bare fuel” cask since no welded canister is used. If the cask also has a licensed transport configuration it is also sometimes referred to as a dual purpose cask.

Bare fuel casks employ bolted closures with the fuel is placed directly in a basket inside the cask cavity. Each of the bare fuel casks listed in Table 1 was designed for transportation cask licensing although few of these casks have an existing transportation license nor are in application for a 10 Code of Federal Regulations 71 licence for transport. Dry storing fuel in a bare fuel cask is most beneficial if the storage times are short and a receipt facility exists that can directly handle and unload fuel from the cask. They also eliminate the need for a transfer cask and/or canister transfer inherent in canister storage.

Canister transport casks

Approximately 84% of commercial fuel in the US is stored in single welded canisters inside individual concrete or steel-encapsulated concrete cylindrical storage overpacks or rectangular horizontal storage modules. All of the storage systems, whether cylindrical vertical overpacks or horizontal storage modules in the US, contain upper and lower vents that allow passive cooling of the internal canister. The canisters for these systems consist of a basket inside a steel shell with an outer diameter ranging from five to six feet in diameter. Cask vendors use different designators on their particular canister system. These include Multi-Purpose Canister (MPC), Dry Shielded Canisters (DSC), and Transportable Storage Canister (TSC).

Documents discussing canister transport casks often refer to the transportation containment vessel as an “overpack”, or “transportation over-pack” since it over-packs the canister during transport. Except in the case of the 12 direct stored HI-STAR canisters, all other canisters in the US require transfer of the canister from the storage over-pack into the transportation over-pack prior to shipment. This operation must be reversed at the consolidated storage facility in order to place the canister in a low-cost vented concrete overpack for long-term storage. The receiving facility must be configured to accommodate any existing transportation over-packs. In no case is a transport cask of one vendor licensed to ship a canister design of another vendor.

Transfer casks are lead and steel casks used for handling of fuel canisters during loading, drying, welding and transfer operations. Transfer casks provide biological (gamma and neutron) shielding during canister closure, drying, welding and transfer but do not provide containment or criticality control features. Unlike canister transport casks, these transfer casks do not meet

10 CFR 71 requirements for shipment of used fuel in commerce. In general, transfer casks are not pressure vessels and do not consist of a pressure boundary. Some transfer casks are designed to ASME Section III Subsection NF or NC, and other aspects of the ASME Boiler & Pressure Vessel code such as welding and weld inspections may apply to their fabrication and inspection. Each transfer cask in use at US ISFSIs is designed to transfer and handle a single canister at a time. Transfer casks are a heavy-lift device designed, fabricated and proof load tested to the requirements of NUREG-0612 and ANSI N14.6 (a withdrawn ANSI standard still cited by the industry). Transfer casks are fabricated predominantly of carbon steel meeting ASTM specification.

Neutron shielding is provided by either water jacket or solid neutron absorber material. Water jackets often contain ethylene glycol or another agent to prevent freezing.

Despite the fact that ISFSI sites are not intended to be permanent stores, fuel stored in them is not ready for immediate load-out; in some cases fuel may need to be repackaged, for example. The minimum lead time for shipment of canisters and casks at each location has been estimated (Figure 1). The lead time includes the time to prepare existing casks for shipment, time to fabricate casks and the time to obtain transportation licenses. It does not include factors such as approval of routing, security requirements, requirements for special rolling stock or the implementation of “smart train” technology, or most importantly the time to make available a repository or interim storage site.

This article is based on extracts from Dry Storage of Used Fuel: Transition to Transport, by D. Leduc, Savannah River National Laboratory, August 2012, FCRD-UFD-2012-000253. Data in Tables 2&3 is provided by kind permission of Ux Consulting Company’s StoreFUEL newsletter.