DETERMINATION OF THE ACTIVATION-ENERGY FOR THE FORMATION OF U3O8 ON UO2

Published data for the activation energy of U3O8 formation on UO2 are critically reviewed. The range in reported activation energies is very wide (48-194 kJ mol(-1)) because of improper deconvolution of the fir st and second stages of UO2 oxidation and because of the complex tempe rature dependence of the reaction kinetics, A general method is then p resented for quantitative analysis of nucleation-and-growth kinetics b y using X-ray diffraction to measure the rate of product formation on a flat surface. This quantitative method is then used to analyze the k inetic data for the formation of U3O8 on the surface of UO2 disks over the range 168-300 degrees C. The resulting Arrhenius plot is linear, and the activation energy for U3O8 formation was found to be 146 +/- 1 0 kJ mol(-1) (90% confidence interval). The quantitative procedure dev eloped herein can be used to estimate the rate of U3O8 formation at lo w (< 175 degrees C) temperatures, and thus makes an important contribu tion to the safe dry air storage of used nuclear fuel. (C) 1997 Elsevi er Science B.V.