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Geological disposal of radioactive wastes currently stored at Earth's surface is now the favoured management pathway for these materials. Typically, intermediate level wastes (ILW) are grouted and emplaced in a geological disposal facility (GDF) which will be backfilled, possibly with cementitious materials. Post-closure leaching of the cementitious materials in a GDF is expected to create hyperalkaline conditions in and around the repository, resulting in mineral alteration and crystallisation, both within the engineered barrier and host rock; creating a persistent chemically disturbed zone (CDZ). Iron derived from within the host rock as a result of alkaline breakdown of Fe-bearing silicate minerals (e.g. biotite, chlorite); corrosion products formed within the repository; or iron contained within the waste; will form secondary iron (oxyhydr)oxide minerals. The formation and re-crystallisation of these reactive mineral phases may sequester radionuclides through a host of processes: surface-mediated reduction to less soluble forms; adsorption onto, and/or incorporation into stable secondary or tertiary iron oxide phases. Therefore iron (oxyhydr)oxides will be key to the fate of radionuclides potentially released from within radioactive wastes disposed of in a GDF.In this study, the fate of U(VI) and Tc(VII) was considered during crystallisation of ferrihydrite to more stable iron oxide phases (e.g. hematite and magnetite) and, in three synthetic cement leachates (pH 13.1, 12.5, 10.5) designed to reflect the early-, middle- and late-stage evolution of the CDZ. XRD and SEM/TEM have been used to characterise the mineralogy during crystallisation. Partitioning of U(VI) and Tc(VII) between the solid and solution has been followed throughout, with chemical extractions used to determine the distribution of the radionuclides adsorbed to, and incorporated within the solid. Synchrotron-based XAS techniques have been utilised to probe the oxidation state and molecular scale bonding environment of the radionuclides associated with the solids. The data suggest that: U(VI) is incorporated into the hematite structure in place of Fe(III), in a distorted octahedral environment with elongation of the uranyl bond; Tc(VII) is reduced to Tc(IV) and incorporated into the octahedral site within the magnetite structure in place of Fe(III), and is retained in the same environment even after extensive oxidation of the magnetite to maghemite; and that U(VI) may also be incorporated as U(V) or U(VI) into the magnetite structure, with similar recalcitrant behaviour during oxidation. These results highlight the importance of mineral reactions within the CDZ as potentially significant pathways for immobilising radionuclides released from a GDF. |
