Oxidation of uranium in steam

The steam oxidation rates of U-metal foil and bar stock have been measured thermogravimetrically between 473 and 823 K. Both materials exhibited Arrhenius-type behaviour in their steam oxidation rates below 573-623 K, with activation energies of 46.9±3.3 kJ mol-1 for U foil below 623 K and 58.0±3.8 kJ mol-1 for U bar stock below 573 K. However, a significant rate reduction was observed at higher temperatures. In all cases, the oxidation products were uraninite-structured UO2-x compounds. Non-adherent oxide layers were formed on bar samples below similar 573 K, becoming increasingly adherent at higher temperatures. Similarly, the oxide layers produced on foil samples below similar 623 K had highly porous cellular structures, whereas the layers formed at higher temperatures were significantly more dense. For both materials, the fall in oxidation rate above 573-623 K was attributed to reduced steam ingress to the underlying metal caused by the increasingly protective surface oxide. ((orig.))