The influence of oxide thickness on the early stages of the massive uranium–hydrogen reaction

The effects of metal surface geometry on the formation of uranium hydride

“…Bearing in mind that the metal at the tip of the prism is in tension and the overlying oxide is in compression, the UD3 nucleating at the prism tips would be expected to rupture the oxide rapidly to expose the fresh UD3 beneath, as shown in Figure 3. The subsequent growth rate of further UD3 at these sites would then be expected to increase as UD3 is reportedly a significantly better diffusion medium for D than the oxide [15]. Matured nucleation sites at these locations in comparison to the nucleation sites existing on the flat surface may be direct evidence for this.…”

“…As mentioned in the introduction, the integrity and thickness of the SPL largely contributes to the length of the induction period prior to the onset of hydride formation. Like uranium hydride, uranium oxides form at the metal oxide interface via an anionic diffusion mechanism and due to the geometry, thicknesses of oxide are probably greater at the tips of the prisms [15,25]. In controlled conditions on a smooth surface, oxide formation begins and grows at an initially parabolic and then linear rate whilst the oxide layer increases in thickness.…”

“…Depending on the conditions of growth, compressional stresses from the uranium on the new voluminous oxide at the metal-oxide interface will enhance strain in the overlying growing oxide layer. This is relieved by fracturing and spallation of the old oxide at the oxide-gas interface or part way through the oxide, termed 'break away' behaviour [15]. The magnitude of this effect is dependent on the oxidation rate and purity of the oxide and uranium [15,26,27].…”

“…In practice, a surface passivation layer (SPL) consisting of oxides, hydroxides and water, ubiquitously covers the surface of the uranium metal [10]. This acts as a barrier for hydrogen diffusion to the metal [15], but at points where the SPL is weakest e.g. oxide fractures, a direct and preferred route for hydrogen migration, dissociation and chemisorption at the metal surface is provided [16].…”

Nuclear waste viewed in a new light; a synchrotron study of uranium encapsulated in grout

“…Oxide formation on bare uranium metal, no matter what the gas species (air or water vapour), initially grows very rapidly to form a layer approximately 20-30 nm thick [18][19][20]. After this, the growth rate slows parabolically eventually resulting in a stable linear reaction rate, as the transport of oxygen species through the oxide layer is limited by the thickness of the oxide layer.…”