The kinetics of the initial stages of the reaction of uranium with oxygen

Abstract

Abstract The initial stages of the oxidation of uranium at temperatures below 100 °C have been shown to obey the inverse logarithmic equation with an experimental activation energy of 27 kcal mol−1. At higher temperatures, the initial stages of the oxidation are parabolic and the final stages are linear. There may be either a smooth transition or an acceleration in rate between the parabolic and linear rates. For the cases where a smooth transition from a parabolic to a linear rate has been reported, the kinetics have been fitted to a model in which the rate of reaction is determined both by the rate of diffusion of oxygen ions through an adherent oxide layer and by the rate of conversion of the outer surface of this oxide to an unprotective form. The activation energy of the parabolic rate is 20 kcal mol−1. The observed activation energies for the initial stages of uranium oxidation are in good agreement with the activation energy for the diffusion of oxygen ions in hyperstoichiometric uranium dioxide (22 kcal mol−1). An improved method of fitting experimental data to the inverse logarithmic equation and of determining activation energies for reactions obeying this equation is described.