The influence of hydrogen pressure and reaction temperature on the initiation of uranium hydride sites

The influence of hydrogen pressure and reaction temperature on the initiation of uranium hydride sites has been determined in the ranges 60–1000 mbar and 75-130 °C, respectively. In order to study the influence of these reaction conditions independently of other possible experimental influences, it was necessary to develop a standard sample preparation (wet abrasion) and a drying pre-treatment (16 h, 75 °C, in-vacuo) methodology. It is shown that, under constant pressure and temperature conditions, the number of hydride sites varies as a power-like function of time. A power-like function may also be used to describe the relationship between the inverse of the time for the nth hydride site to appear (for n = 1, 5, 10, 20 and 50) and the applied hydrogen pressure, with an exponent value of 1.3 and an associated activation energy of 57 kJ mol− 1. These observations are broadly consistent with existing hydride initiation models in which the uranium surface oxide layer is considered to act as a barrier to hydrogen diffusion to the oxide–metal interface.