A model for the initiation of reaction sites during the uranium–hydrogen reaction assuming enhanced hydrogen transport through linear oxide discontinuities

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A model for the initiation of hydride sites on uranium metal is described for hydride attack in the region of linear discontinuities in the surface oxide film. The model considers the effect of variations in hydrogen permeation through such discontinuities due to intrinsic oxide and metal parameters and operational variables. Expressions are derived for the time dependence of the hydrogen concentration in the metal and the maximum attained hydrogen concentration in terms of these parameters. The derived expressions therefore relate hydride precipitation time in the metal underlying any oxide discontinuity to the physical width of the discontinuity, its length, the hydrogen diffusion coefficient down the oxide discontinuity, the hydrogen diffusion coefficient in the underlying metal lattice and the hydrogen concentration in the oxide discontinuity at the gas-oxide interface. The model can therefore account for how changes in operational conditions such as hydrogen pressure may change hydride attack times in the region of such oxide discontinuities. The model can also explain why, after any given time, hydride attack may occur at some but not all of the oxide discontinuities surrounding any individual metal grain. The model also considers how hydride attack times may, or may not, be modified by the intersection of linear discontinuities in the surface oxide film. Finally, the model is able to explain the reported experimental observation that oxide discontinuities having only a short rather than a long length are less preferred sites of hydride attack.