Hydride prediction during late-stage oxidation of uranium in a water vapour environment

We present a reaction-advection-diffusion (RAD) model for (low temperature) uranium oxidation in a water-vapour environment, where both OH− and H• are diffusing. In this model an intermediate UH3 phase sits between the bulk U metal and a protective surface UO2 layer. This surface oxide layer only remains adhered up to a maximum depth Δadh∗ before spallation occurs leading to significantly increased diffusive transport across the spalled layer. Under these conditions, this mechanistic model is shown to support both a parabolic (∝t) oxide growth up to the point of spallation, before smoothly transitioning to a linear (∝t) oxidation solution at later times. In the late-stage linear regime, a UO2−UH3 interface propagates into the bulk metal at a constant velocity ofD13∗C∗2Δadh∗N2∗; D13∗ being the diffusion coefficient of OH− in UO2 and C∗/N2∗ the peak relative concentration of OH− to U. This model predicts that the intermediate hydride layer approaches a constant thickness in the linear regime, with a UH3−U interface propagating into the bulk metal at the same velocity. The length scale of this emergent hydride layer is shown to be most sensitive to the diffusivity of OH− in UH3 and the corresponding reaction rate constant. Plausible parameter values are shown to lead to hydride layers <10 nm for room temperature oxidation in a vapour pressure of 20 Torr (Δadh∗=50 nm) consistent with recent atom-probe tomography results.