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Direct disposal of spent nuclear fuel in underground repositories in stable geological formations is an alternative option for the reprocessing of spent fuel in Japan. Upon breakdown of the barriers between waste storage containers and the environment in a deep geological repository, release of radioactive nuclei from spent nuclear fuel into the environment will occur via groundwater. The release of radioactive nuclei will be dependent on the dissolution rate at the spent fuel/groundwater interface. The surface of UO 2  in the spent fuel will be in the oxidized form UO 2+x , and the extent of hyper-stoichiometry has implications for U dissolution. A key component of typical groundwater is bicarbonate which can range in concentration depending on repository location. Therefore, to understand the effect of bicarbonate on the UO 2+x  surface, UO 2  pellets were immersed in bicarbonate solution for 298 days, and the UO 2+x  oxide surface was analysed by Raman spectroscopy. Detailed peak analysis of the Raman spectra showed increased defects after immersion attributed to the preferential dissolution of U V/VI  from UO 2+x . Differences in the defect concentration with and without bicarbonate were attributed to a change in U speciation (U(OH) 4  to UO 2 (CO 3 ) 34- ) as determined by speciation calculations. Raman depth profiles showed a constant value of x through the oxide surface.

A Raman spectroscopy study of bicarbonate effects on UO 2+x