The influence of nitrogen dioxide on the oxidation of UO 2 in air at temperatures below 275°C

Abstract

Specimens of polycrystalline, sintered UO 2 were cut from unirradiated CANDU fuel, and heated in air or air+NO 2 mixtures (up to 1 vol.% NO 2), to investigate the effects of nitrogen oxides on the rates of oxidation to U 3O 7 and U 3O 8. Oxidized specimens were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy to determine the extent of surface oxidation. The kinetic expression for the formation of U 3O 7 was initially parabolic, but switched to linear at long reaction times. The first, parabolic stage of oxidation to U 3O 7 was accelerated slightly by the addition of small quantities of NO 2 to air, probably because NO 2 increases the oxygen potential of the outermost surface of the UO 2 by forming a thin film of UO 3. The rate of oxidation in the region of linear kinetics is approximately the same for all atmospheres up to 1.0% NO 2. The effect of NO 2 on the rate of U 3O 8 formation is difficult to ascertain because of the close similarity between the XRD patterns of α-UO 3 and U 3O 8. However, it appears that there is a substantial increase in the rate of formation of higher oxides (U 3O 8 and possibly also UO 3) in the presence of NO 2. These results indicate that the effect of nitrogen oxides (which are formed by radiolysis of air) should be included in detailed models of air oxidation of irradiated fuel.