Contingency Options for the Dry Storage of Magnox Spent Fuel in the UK

The UK Nuclear Decommissioning Authority (NDA) is responsible for safe and secure management of spent nuclear fuel. Magnox fuel is held at some Magnox reactor sites and at Sellafield where it is reprocessed using a number of facilities. It is intended that all Magnox fuel will be reprocessed as described in the published Magnox Operating Programme (MOP) [1]. In the event, however, that a failure occurs within the reprocessing plant, the NDA has initiated a programme of activities to explore alternative contingency options for the management of wetted Magnox spent fuel. Magnox fuel comprises metallic uranium bar clad in a magnesium alloy, both of which corrode if exposed to oxygen or water. Consequently, contingency options are required to consider how best to manage the issues associated with the reactivity of the metals. Questions such as whether Magnox spent fuel needs to be dried, how it might be conditioned, how it might be packaged and held in temporary storage until a disposal facility becomes available, all require attention. During storage in the presence of water, the corrosion of Magnox fuel produces hydrogen (H₂) gas, which requires careful management. When uranium reacts with hydrogen in a reducing environment, the formation of uranium hydride (UH₃) may occur, which under some circumstances can be pyrophoric, and might create hazards which may affect subsequent retrieval and/or repackaging (e.g. for disposal). Other factors that may affect the choice of a viable contingency option include criticality safety, environmental impacts, security and Safeguards and economic considerations. Magnox fuel has been successfully dry-stored as intact fuel elements in CO_2- and air-filled primary and secondary cells at Wylfa Power Station, UK. Storage of some fuel elements in the Wylfa secondary cells has been carried out successfully for over 25 years. Other relevant experience includes the French UNGG (Uranium Naturel Graphite Gaz) and U.S. Hanford N-Reactor spent fuels, both of which have been retrieved and dried after decades of wet storage. The dried fuels are respectively stored in sealed canisters in modular vault stores at Cadarache (CASCAD) and Hanford (Canister Storage Building). The applicability of these and other potential store designs, such as concrete and metal casks and silos, to the storage of Magnox spent fuel is discussed.