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

Abstract

The influence of oxygen-grown UO 2+ x films on the early (induction, acceleration) stages of the massive uranium–hydrogen reaction has been studied in a non-visual, constant-volume cell. The potential for (i) adsorbed small molecules (e.g., H 2O) impeding the reaction and (ii) unintended oxide fracture has been reduced. Induction times were determined by pressure fall, while reaction site nucleation rates were determined by post-reaction visual analysis. Oxidation data from this work suggests that under these conditions the reaction exhibits break-away behaviour at ∼6 μg O 2 cm −2 (calculated mean oxide thickness ∼500 Å). Hydriding results indicate that the induction time is related to the mean oxide thickness and increases with increase of oxygen consumption up to at least 25 μg O 2 cm −2 (calculated mean oxide thickness ∼2000 Å). However, the hydriding induction time, unexpectedly, does not appear to plateau or reduce as the consumption exceeds 6 μg O 2 cm −2. Results also indicate that, for any given oxide film, the nucleation rate, as calculated from post-reaction visual analysis, increases with exposure time to hydrogen. Also in general, for a fixed consumption of hydrogen per unit area, the thinnest oxide films result in more reaction sites and a higher mean nucleation rate. Conversely, the thickest oxide films result in fewer reaction sites and a lower mean nucleation rate. In a similar manner to the induction time results, the reaction site nucleation rate is influenced by the degree of prior oxidation, both pre- and post-break-away.