The effect of thermal pre-treatments on the nucleation of uranium hydride

• First report of effect of cooling on uranium hydriding site nucleation.
• Unexpected nucleation behaviour observed.
• Two hypotheses based on differing mechanisms proposed to account for observations.
• Observations have implications for current hydride site nucleation models.

Uranium hydriding experiments are reported in which samples experienced a single thermal cycle within the range of 35 °C to 75 °C, before undergoing reaction with hydrogen at a pressure of 110 mbar. Under equivalent reaction conditions, elevated nucleation kinetics were observed for samples that had been thermally cycled prior to hydrogen exposure, as compared to samples that had not been cycled. These observations have been rationalised in terms of two alternative hypotheses based on i) the anisotropic thermal expansion of uranium leading to disruption of the surface uranium oxide overlayer during cooling of the experimental samples or ii) differing levels of water present on the oxide overlayer following pre-treatment.