Rj. Mceachern et al., THE INFLUENCE OF NITROGEN-DIOXIDE ON THE OXIDATION OF UO2 IN AIR AT TEMPERATURES BELOW 275-DEGREES-C, Journal of nuclear materials, 255(2-3), 1998, pp. 234-242
Citations number
29
Categorie Soggetti
Nuclear Sciences & Tecnology","Mining & Mineral Processing","Material Science
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 oxida
tion to U3O7 and U3O8. Oxidized specimens were analyzed by X-ray diffr
action (XRD) and X-ray photoelectron spectroscopy to determine the ext
ent 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 r
egion 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 di
fficult to ascertain because of the close similarity between the XRD p
atterns of alpha-UO3 and U3O8. However, it appears that there is a sub
stantial 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. (C) 1998 Elsevier Science B.V. Ail rights reserved.