Kinetics and mechanism of the U-H reaction

The kinetics of the hydrogenation reaction of uranium were studied over a wide range of pressures (10²-10⁷ Pa) and temperatures (0-500 °C). The results are discussed in terms of two possible models: model 1, hydrogen diffusion through a protective product hydride layer; model 2, hydride growth at the hydride-metal interface. A two-dimensional (i.e. pressure-temperature) grid of the experimental data was successfully fitted to the pressure-temperature rate relations derived from model 2. This model relates the microscopic distribution of hydrogen atoms in the metal to the phase transformation probability, from which the pressure-temperature dependence of the hydride growth rate is obtained. The model also qualitatively accounts for the negative apparent activation energy reported in the literature for the hydrogenation of the powdered metal.