The oxidation of uranium dioxides

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Abstract
The oxidation of three types of uranium dioxide (UO2) material has been investigated in the 120—360° range.In UO2 powders, of surface area greater than about 2 m.2/g., oxidation was diffusion‐controlled up to U3O7 which oxidised to U3O8 above about 240°. The activation energies were in the range 20—27 kcal./mole.For material with very low surface‐area, both electrolytic UO2 powder (0.0405 m.2/g.) and sintered pellets (5 cm.2g.) 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.Material with intermediate surface‐area comprising sintered powder (0.135 m.2/g.) and crushed sintered pellets (0.095 m.2/g.) gave oxidation curves which were hybrid between the other two types above. At a high enough temperature (∼ 420°), the 0.095 m.2/g. material showed a completely sigmoid ‘nucleation’ type of curve.X‐Ray diffraction studies were performed at intermediary stages in the oxidations to trace the presence of the γ‐UO2 phase, and suggestions are put forward to explain the above differences in behaviour.

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