A key concern in the use of nuclear energy is the disposal and long term storage of spent fuel. The aqueous chemistry of the uranyl(VI) ion under strongly alkaline conditions, such as those found in many above-ground waste storage tanks, remains poorly understood. To this effect, the oxygen atom exchange between the uranyl oxygen atoms and bulk solvent water has been studied by 17O NMR spectroscopy under acidic and alkaline conditions[2a??4] using line-broadening by Clark et al. and, recently, saturation-transfer methods by Szab , Grenthe et al., but no determination of activation volumes has been attempted.