Knowledge Status on Uranium Hydride
Current industry knowledge on uranium hydride behaviour and the potential consequences of this behaviour for geological disposal facility (GDF) safety cases.
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This document was prepared by Galson Sciences Limited (GSL) and Orchid partner companies for Radioactive Waste Management Limited (RWM). Authors: Hugh Godfrey (National Nuclear Laboratory) and Robin Strange (Galson Sciences Limited)
This report presents the results of a programme of work to review and collate the current industry knowledge on uranium hydride behaviour and to assess the potential consequences of this behaviour for the geological disposal facility (GDF) safety cases, with particular emphasis on the formation, persistence, and pyrophoric behaviour of uranium hydride.
The UK has a significant inventory of fuels that were manufactured from natural or low-enriched uranium metal:
- Magnox fuel, consisting of a metallic uranium element clad in a magnesiumaluminium-beryllium alloy called Magnox, comprises the majority of this inventory
- there is a smaller inventory of legacy / research fuels
Historically, spent Magnox fuel and uranic material associated with wastes generated at reactor stations and reprocessing operations at Sellafield was stored, clad and unclad in ponds and/or silos under a range of different chemical environments. Under some of these conditions, formation of uranium hydride is feasible and some of these materials are due to be disposed of in a GDF.
Uranium hydride is frequently cited as a hazard during the storage and handling of uranium metal owing to its potentially pyrophoric behaviour and the associated risk of fires. Ignition of wastes containing uranium as a result of uranium hydride formation during storage has been implicated in a number of events. These observations, and confusion about where and when uranium hydride could form and how it could react, have historically led to a highly conservative approach to the design of treatment, storage and handling processes for metallic uranium fuel and associated wastes.
However, significant progress has been made in understanding the formation and properties of uranium hydride under fuel and waste storage conditions in recent years.
This report focuses on establishing whether uranium hydride formation within waste packages could significantly affect any conclusions of Radioactive Waste Management Limited’s (RWM’s) 2016 generic Disposal System Safety Case (gDSSC) for a GDF.
Further, it also identifies from the perspective of disposability whether there are significant gaps in current understanding that require research. The assessment approach was based on a comprehensive literature review and a workshop with a range of UK experts.
In this report, the results of a literature review are presented followed by an overview of the chemistry and behaviour of uranium hydride, including experience of industry stakeholders. Subsequently, the key claims and arguments of the gDSSC are reviewed with respect to the potential impact of uranium hydride behaviour, and knowledge gaps are identified.