Fission Product Release from Spent UO₂ Fuel Under Uranium-Saturated Oxic Conditions

Observations on the mechanism of oxidation of UO₂ in air and results from X-ray photoelectron spectroscopy surface analyses of UO₂ electrodes exposed to aqueous solutions show that the dissolving solid under oxic conditions is essentially U₃O₇ formed by oxygen diffusion on the UO₂ surface. Saturation effects with respect to U₃O₇ can be of importance for the overall reaction rate if oxygen transport to the dissolving surface is limited. The release of soluble radionuclides in solid solution with the UO₂ matrix appears to be limited by the mass transfer rates for the conversion of U₃O₇ to alteration products such as schoepite. The rates of ⁹⁰Sr and ¹³⁷Cs release decrease with the square root of time under uranium-saturated conditions. This time dependence may be explained by either grainboundary diffusion or by oxygen diffusion through the alteration product phase.