The formation and persistence of α-UH3 in uranium hydride under conditions relevant to metallic fuel and nuclear waste storage

The formation of uranium hydride is recognised as a hazard during the storage of uranium metal owing to its potentially pyrophoric properties. Uranium hydride exists mainly in the form of cubic trihydride with two crystal structures. Denoted as α-UH₃ and β-UH₃, the two forms of hydride have different chemical properties, with α-UH₃ being the more chemically reactive phase. The formation and persistence of α-UH₃ under conditions relevant to waste storage will likely have a significant bearing on the reactivity of the residual uranium hydride in waste stored in legacy storage facilities. This study has assessed the formation and persistence of α-UH₃ at a range of temperatures. The work has shown that the fraction of α-UH₃ in uranium hydride gradually increases at decreasing formation temperatures. This means that it could potentially be the dominant phase formed under typical waste storage conditions, which, for the purposes of this paper, can be broadly considered to be typically about 25 °C, with uranium containing wastes either submerged under water, dry or drying with air access or damp in sealed containers. Therefore, in some cases, waste may be stored under conditions where a hydrogen atmosphere may be present to some extent. The work has further shown that in the absence of appreciable oxidant, α-UH₃ is stable at 30 °C and 50 °C for at least 100 d and for over 300 d at ambient temperature and pressure. Given the high α-UH₃ fraction at low formation temperatures and the stability of α-UH₃ at low storage temperatures, it is feasible that α-UH₃ could be the dominant phase in any uranium hydride remaining in uranium metal bearing waste. Hence, the properties of α-UH₃ could have a significant bearing on the behaviour of the waste during the planned retrieval and packaging of legacy uranium bearing wastes. These results add significantly to the understanding of uranium hydride behaviour in the context of storage, retrieval and packaging of uranium metal bearing wastes.