The oxidation of uranium dioxides

<jats:title>Abstract</jats:title><jats:p>The oxidation of three types of uranium dioxide (UO<jats:sub>2</jats:sub>) material has been investigated in the 120—360° range.</jats:p><jats:p>In UO<jats:sub>2</jats:sub> powders, of surface area greater than about 2 m.<jats:sup>2</jats:sup>/g., oxidation was diffusion‐controlled up to U<jats:sub>3</jats:sub>O<jats:sub>7</jats:sub> which oxidised to U<jats:sub>3</jats:sub>O<jats:sub>8</jats:sub> above about 240°. The activation energies were in the range 20—27 kcal./mole.</jats:p><jats:p>For material with very low surface‐area, both electrolytic UO<jats:sub>2</jats:sub> powder (0.0405 m.<jats:sup>2</jats:sup>/g.) and sintered pellets (5 cm.<jats:sup>2</jats:sup>g.) were oxidised by nucleation and growth controlled processes. The growth activation energies for the two materials were 32.19 and 26.38 kcal./mole respectively. Considerable differences occurred in the oxidation rate of pellets from batch to batch.</jats:p><jats:p>Material with intermediate surface‐area comprising sintered powder (0.135 m.<jats:sup>2</jats:sup>/g.) and crushed sintered pellets (0.095 m.<jats:sup>2</jats:sup>/g.) gave oxidation curves which were hybrid between the other two types above. At a high enough temperature (∼ 420°), the 0.095 m.<jats:sup>2</jats:sup>/g. material showed a completely sigmoid ‘nucleation’ type of curve.</jats:p><jats:p><jats:italic>X</jats:italic>‐Ray diffraction studies were performed at intermediary stages in the oxidations to trace the presence of the γ‐UO<jats:sub>2</jats:sub> phase, and suggestions are put forward to explain the above differences in behaviour.</jats:p>