A snapshot review on uranyl secondary phases formation in aqueous systems

Abstract

AbstractIrradiated fuel based on UO2 is expected to be disposed of in an underground repository according to a “once-through” fuel cycle policy. In addition, those countries that have chosen reprocessing their irradiated fuel will also need this kind of repository. There is an international consensus on this option as the best to dispose properly high-level waste. In the multiple-barrier concept of repository, the fuel pellet is the first barrier to the release of fission products (FP) to the environment. Nevertheless, after storage times of several thousand years, it is foreseen that groundwater will penetrate and infiltrate in the emplacement, and will eventually interact with the spent fuel. Upon contact with groundwater both, the matrix and FP might be released. Because of water radiolysis, uranium is oxidized from its tetravalent state to the hexavalent uranyl ion, being known to be far more soluble in water than uranium (IV). Then, during this oxidative-dissolution and depending on the surface/volume ratio, secondary phases (containing uranyl ion, UO22+) might precipitate at the whole pH range. These secondary phases play an essential role on the radionuclide release in the final disposal environment due to its capacity to seize trace radioelements, and therefore, to reduce radionuclides mobility. Some important radionuclides can precipitate into its inner solid structure. Here, we examine the state-of-the-knowledge and advances on uranyl secondary phases potentially formed under repository relevant conditions. An overview of likely uranyl compounds that can be formed under repository conditions, as well as the progress made concerning experimental data on the field is presented. Graphical abstract