Corrosion of uranium in liquid water under vacuum contained conditions. Part 1: The initial binary U + H2O(l) system

•Bulk-UH3 forms at the metal-oxide interface, on the majority of the samples.•Lower temperature reaction conditions yielded higher UH3 proportions in the products.•Absolute UH3 quantities are higher at the higher temperature reacting samples.•A ‘threshold’ headspace pressure ∼0.5 bar exists, over which UH3 formation is facilitated. The reaction of unirradiated-U with liquid water was investigated under initial vacuum-contained conditions. Ten samples were examined at varying temperatures and reaction times. Post-corrosion examination of the surfaces was conducted using FIB, SIMS and XRD. Measurements of the reacting water pH and degassing of the samples was conducted after the reaction was stopped. From the analyses, it was concluded that bulk-UH3 formation occurred on the majority of the samples. It is suggested that UH3 formation was facilitated by a critical ‘threshold’ headspace pressure ∼0.5bar over which pressure deficiency is observed in the free headspace.