Multi-purpose canister storage of spent nuclear fuel in modular vault system

Full text: The original Modular Vault Dry Storage (MVDS) technology was developed in the early 1980s leading on from the experience gained with the Magnox fuel dry storage facilities at the Wylfa power station in Wales (UK). The Wylfa dry stores were commissioned in 1969 and the MVDS can, therefore, rightly claim to be the only dry storage technology that has an operational and technological background of over thirty years. The MVDS is a very flexible design approach and is suitable for the storage of all types of spent nuclear fuel or high level waste streams. The MVDS design has continued to be updated since the United States Nuclear Regulatory Commission granted the Topical Report license approval in 1988. The MVDS Topical Report covers the interim storage of Light Water Reactor fuels, including both PWR and BWR, at any reactor site in the USA. The first MVDS constructed in the United States was built at the Fort St Vrain high temperature gas reactor site in 1990, and the MVDS technology has also been applied to the storage of WWER fuel at the Paks nuclear power plant in Hungary. A MVDS facility is currently being licensed by the United States Nuclear Regulatory Commission for storage of United States Department of Energy owned fuels at the Idaho National Engineering and Environmental Laboratory site in Idaho, USA. The MVDS system was originally designed to store individual fuel assemblies within a Storage Tube or Canister. This system provides maximum flexibility for future off-site transportation as the individual fuel assemblies can be removed from their storage locations and placed into a transportation cask for either road or rail off-site shipment. A vault storage system based on the proven MVDS technology using a large Multi-Purpose Canister (MPC), is being developed to provide cost-effective interim spent fuel storage system. Integrating the MVDS technology with a large MPC and adapting the cooling, shielding and handling system, allows the new vault storage system to provide high storage efficiency in compact storage buildings suitable for a large spent fuel interim storage facility. The MVDS vault passive cooling system is capable of rejecting approximately 450kw of heat from each vault module, before either the bulk air temperature causes the vault concrete temperature or the fuel temperature to reach an acceptable temperature limit. It has been possible to re-configure the vault storage array from individual storage tubes to large diameter canisters, and to re-configure the handling equipment transfer larger canisters. By modifying the design of the MVDS to accept large, multiple assembly, multi-purpose canisters, it has been possible to maintain the technical and operational benefits of the original MVDS, with the additional benefits of multi-purpose canisters. A storage tube is typically 0.2 to 0.4 metre diameter, whereas a multiple assembly canister is typically 1.6 to 1.8 metres outside diameter. In recognizing the difference between the two size of storage vessel, the new MVDS system has been named: Mega-Vault Dry Store. The Mega-Vault storage facility consists of three main systems: 1. The Storage Vault Modules, where canistered spent fuel is stored. 2. The Canister Handling Machine, which raises and transfers canisters from the cask in the Transfer Tunnel to the storage position in the Storage Vault. 3. The Cask Reception Bay and Transfer Tunnel, where canistered spent fuel is received and transferred to a port under the vault where it can collected by the Canister Handling Machine. The Transfer Tunnel and Cask Bay are also used to despatch fuel at the end of storage life. The Mega-Vault MPC has a 1.8m outer diameter and stores up to 76 BWR or 24 PWR fuel assemblies. The MPC is cooled by passive horizontal airflow during storage. The height of the storage vault has been modified to suit the large MPC, structures have been simplified in the storage area, and earthquake protection of canisters has been achieved. The radiation dose at the site boundary is reduced by below ground storage and radiation streaming reduction structures. Also, providing thick concrete shielding minimizes the expected dose of workers. The compact canister handling machine also enables the building size to be minimized. The paper will describe in more detail the technical and operational features of the Mega-Vault Storage system, together with process flow descriptions and concept drawings. (author)