Dry storage

The spent fuel assemblies in general are amenable for naturally cooled dry storage after a few years of initial cooling in the water pool (of about 5 years for most fuel and about 10 years for high burnup fuel). The minimum required time of initial cooling in pools is mainly related to the burn up and the irradiation history. A review of spent fuel storage facilities implemented during the past several decades shows that the storage in a dry environment is becoming more common. Taking into consideration the extending period for decades or even longer that will required for spent fuel storage, it is obvious that the naturally cooled dry storage facilities are an attractive alternative to water pools, especially for long term storage, in terms of such aspects as economics and safety.

Dry storage methods rely on metal or concrete for shielding radiation from spent fuel assemblies, which continue to emit considerable decay heat that must be dissipated to the atmosphere. Discussion here is limited to only generic technologies, provided primarily to develop a context for the selection process described later. The four key types of dry storage options are:

• Metal cask;

• Concrete cask / module;

• Vault;

• Others.

The technologies are also distinguishable by their major technical characteristics, namely, the predominant heat transfer method, type of shielding, transportability, location with respect to the geological surface; degree of independence of the individual storage units; and the storage structure.

There are several generic types of these technologies available from vendors on the international market. An increasing number of storage facilities are coming into operation for each of these types. There are also a large number of facility designs based on these generic technologies that are now available. These technologies differ largely in terms of materials of

construction, size, modularity, spent fuel configuration, layout of the storage containers (horizontal, vertical etc.) and methods for fuel handling.

A summary of AFR storage facilities in Member States is given in Annex II.

2.2.4.1. Metal cask

As metal casks have long been used for spent fuel transportation in the nuclear industry, there was a natural transition in its role to storage service. These casks can be designed solely for storage or as dual-purpose containers for both storage and transportation. Metal casks hold several spent fuel assemblies within a dry controlled environment. The casks are filled with inert gas and sealed after loading with the spent fuel.

The casks are sometimes stored in the open on a concrete pad or housed in storage buildings, depending on regulatory requirements. Primarily the cask structural material, which may be forged steel, modular cast iron or composite materials, provides shielding. Double-welded closures provide for radionuclide confinement. Heat removal is by conduction through the structural material.

There are a number of designs available from the international market. Metal casks are in general transportable. This is an advantage in the case of future need to move them to a further destination. On the other hand prices are higher due to the requirements for transportation.

Examples of these metal casks are Transnucléaire’s TN-series, GNS’s CASTOR series, and Westinghouse’s MC-10 casks used in the USA, and Nuclear Assurance Corporation’s NAC casks in USA and Spain (see Annex I).

2.2.4.2. Concrete cask / module

A common technical feature of this category of storage systems is its use of concrete shielding which was a development of significant technical and economic implications. Despite the common use of concrete shielding, there is a large variety of system designs offered in the market that can be grouped broadly into two categories, concrete casks and concrete modules.

• Concrete casks

A concrete cask is generally similar to the metal cask by shape. The concrete provides shielding, but a steel liner in the inner cavity of the concrete cask provides containment. The liner is sealed after loading the spent fuel. Concrete casks could be naturally cooled or ventilated. The designs are similar with the exception that the ventilated type is equipped with inlet and outlet airflow ducts. This permits greater dissipation of heat; hence a greater heat load can be accommodated in a cask of this type.

These concrete casks can be stored either horizontally or vertically. They can be stored in the open or inside a building to protect them against the weather. Since concrete casks are generally not transported loaded with the spent fuel, these casks will require facilities for loading (and unloading) of spent fuel from (and to) transportation containers. Concrete casks could also be built as dual-purpose containers for storage and transportation in which case the casks could require special armors or overpacks for meeting transportation regulations.

Examples of concrete casks are Sierra Nuclear’s Ventilated Storage Cask (VSC) and the concrete silo⁶ developed by AECL as a “storage-only” system. Another example of this category is the Dry Storage Container (DSC), developed by Ontario Power generation (OPG), which is a transportable system and the CONSTOR development by GNS.

• Concrete modules

Concrete modules are large monolithic structures, usually with a reinforced concrete wall, which are in general not portable in that they are anchored to the storage pad on the ground.

The concrete module provides the shielding, while containment is provided by placing of spent fuel assemblies into canisters. The canister is sealed after loading the spent fuel. Some refer to these designs as canister-based systems.

The use of internal cooling allows significant amount of heat to be removed from the storage system by natural convection and prevents overheating and degradation of the concrete material in the shield. This is a significant feature, which is advantageous for loading hot fuel in large quantities. Similar to concrete casks, however, ancillary systems would be required for loading (and unloading) of spent fuel from (and to) transportation containers.

A representative example of concrete module is NUHOMS horizontal module of Transnucleaire. NAC adopted the multi-purpose system MPC and UMS that are canister-based systems. Holtec International in the US has also developed and commercialized Hi-Star/Storm system, which is also a canister-based system.

2.2.4.3. Vault

A vault is a reinforced and shielded concrete structure containing an array of storage cells built either above or below ground. Shielding is provided by the surrounding structure.

Commercially available vault systems are located above the ground level and the heat is generally transferred to the atmosphere by natural convection of air over the exterior of the cells. Each storage cell or cavity⁷ can contain one or more spent fuel assemblies stored in metal storage tubes or storage cylinders. Spent fuel is loaded into these tubes either on-site with fuel handling machines in a charge hall or off-site at the reactor pools. The vault itself can be a relatively simple design, but requires additional installation infrastructure for the reception and handling of the spent fuel assemblies.

The storage concept permits modular construction and incremental capacity extension.

Examples of on-site loaded vaults are Magnox dry storage at Wylfa in the UK and the MVDS facility in PAKS in Hungary. Examples of vaults, which receive pre-loaded containers, are CANSTOR/MACSTOR at the Gentilly-2 NPP in Canada (and at Cernavoda site in Romania) [24], CASCAD facility in France, and the Fort St. Vrain MVDS facility in the USA.

2.2.4.4. Subsurface store

There are several other storage concepts which have been in development, mostly based on subsurface application of heat conduction or convection. Even though they have not been used on commercial scale yet, changing circumstance in the spent fuel management area may make alternative concepts attractive provided they become competitive in the new criteria,

6 Also called concrete canister.

7 This cavity is sometimes called ‘pit’.

such as the security issues which have lately become a higher priority throughout the world [25].

The concept of placing nuclear facilities underground originates from the nuclear shelter concept of the cold war period. Not only military facilities, but also some civilian nuclear facilities were built underground, like the Swedish facilities for spent fuel storage (CLAB) and waste disposal (SFR).

• Underground vertical ventilated storage concept

Holtec International has recently applied for a license for an underground version of its current HI-STORM 100U. The design concept of HI-STORM 100U overpack is an underground vertical, ventilated modular dry spent fuel storage system engineered to be fully compatible with their HI-STORM 100 system. Each module stores a single canister/overpack unit and functions independent of any additional units.

The system provides for storage of fuel elements in a vertical configuration inside a subterranean cylindrical cavity entirely below the top-of-the-grade of the storage area. The enclosure container defines the MPC Storage Cavity, consisting of the container shell integrally welded to the Bottom Plate. The storage cavity is ventilated by outside air through the insulated ducts via natural convection [26].

• Drywell

A dry well is a stationary, below ground, lined, individual cavity. Shielding is provided by the surrounding earth and closure shield plug. Primary heat removal is by conduction into the earth⁸.

Each storage cavity may be designed to contain several spent fuel assemblies, the actual number of fuel assemblies to be determined by the fuel characteristics and storage media. The storage medium can be air, nitrogen, carbon dioxide or any of the inert gases helium, argon or neon.

• Twin tunnel storage concept

This is a subsurface storage method which combines the drywell concept with borehole emplacement in a geological disposal facility, with a view to long term storage before disposal or retrieval for reuse.

In a design concept proposed by Colenco Power Engineering Ltd, the spent fuel transported from AR or AFR storage is placed into canisters with double wall sealing to be brought to a pair of horizontal tunnels interconnected with vertical boreholes for the canister emplacement.

The cooling air ventilation flows from the lower drift to the lower tunnel and passes through the borehole to the upper tunnel and drift [27].

8 Drywell had been one of the options considered for storage of spent fuel in the past, but has not yet been licensed anywhere in the world.