The influence of nitrogen dioxide on the oxidation of UO2 in air at temperatures below 275oC

Specimens of polycrystalline, sintered UO2 were cut from unirradiated CANDU fuel, and heated in air or air + NO2 mixtures (up to 1 vol.% NO2), to investigate the effects of nitrogen oxides on the rates of oxidation to U3O7 and U3O8. 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 U3O7 was initially parabolic, but switched to linear at long reaction times. The first, parabolic stage of oxidation to U3O7 was accelerated slightly by the addition of small quantities of NO2 to air, probably because NO2 increases the oxygen potential of the outermost surface of the UO2 by forming a thin film of UO3. The rate of oxidation in the region of linear kinetics is approximately the same for all atmospheres up to 1.0% NO2. The effect of NO2 on the rate of U3O8 formation is difficult to ascertain because of the close similarity between the XRD patterns of α-UO3 and U3O8. However, it appears that there is a substantial increase in the rate of formation of higher oxides (U3O8 and possibly also UO3) in the presence of NO2. 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. (author)