Pyrophoricity of uranium and uranium compounds: Mechanisms, knowledge gaps, and implications for nuclear safety

Depleted uranium materials pose a significant challenge with respect to their safe, long-term storage, not only from a radiological standpoint, but also with regards to fire safety owing to the reactivity of finely-divided uranium material with air. The risk of pyrophoric ignition is known to be strongly dependent on the material's specific surface area, however, other factors are known to likely play a role in either increasing or decreasing such a risk. In this article, the research to date on the pyrophoric behaviour of uranium compounds is compiled and examined with a view to understanding the factors controlling pyrophoric ignition and determining the gaps in current knowledge. Although some influencing factors, such as specific surface area, dominate ignition behaviour, several other factors can disrupt expected behaviour, demonstrating that pyrophoric ignition is a complex, dynamic interplay of different factors.