The oxidation of uranium hydride during its instantaneous or gradual exposure to oxygen

The oxidation behaviour of uranium hydride when exposed to oxygen gas either gradually or instantaneously at an ambient temperature of approximately 37 °C has been investigated. The proportion of the sample converted to oxide and the mean hydride mound temperature rise accompanying the oxidation process were recorded. Pressure changes in the reaction cell were used to deduce the nature of the oxide generated during the oxidation process and the nature of the gaseous reaction products. For the gradual oxidation of the hydride, typically only about 15% of the hydride mass was converted to oxide during the initial period of oxidation defined by an observed temperature excursion lasting a finite time; during this oxidation period it is concluded that, despite the only moderate recoded mean temperature rises, the generated oxide is U₃O₈ rather than UO₂. Typically also for such gradual exposure experiments, hydrogen was the principal gaseous reaction product with negligible water being produced. For the instantaneous exposure of the hydride to oxygen, much higher hydride mound mean temperature rises were recorded but the proportion of hydride converted to oxide was quite variable and apparently dependent on several experimental variables; for these experiments, although hydrogen was a major reaction product typically some water was also produced. The boundary between pyrophoric and non-pyrophoric oxidation of uranium hydride for given oxidising conditions is discussed in terms of a calculated particle "thermal runaway temperature". - Highlights: • Only about 15% of UH₃ is oxidised on its gradual exposure to oxygen. • During the gradual exposure of UH₃ to oxygen U₃O₈ rather than UO₂ is generated. • For such gradual exposure, predominantly H₂ rather than H₂O was generated. • For the instantaneous exposure of UH₃ to O₂ the amount oxidised was variable. • For these experiments, H₂ + some H₂O was generally produced. • Pyrophoric or non-pyrophoric behaviour was related to "thermal runaway temperature"