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

The influence of oxygen-grown UO2+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., H2O) 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 O2 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 O2 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 O2 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.