Geochemical immobilization of uranium in a spent fuel repository in the Canadian Shield: Evidence from natural analogue investigations

摘要

At Cigar Lake in northern Saskatchewan, the Pocos de Caldas site in Brazil, and at Palmottu in Finland, groundwaters are sufficiently reducing to prevent significant oxidation of U(+4) to the more soluble U(+6) oxidation state. Despite being one of the richest uranium deposits in the world, uranium concentrations in the groundwater around the Cigar Lake ore is less than 5 x 10 to the -8th power. Studies at 'negative analgoue' sites indicate that where the host rocks contain low concentrations of reductants such as iron sulfides, strongly reducing conditions are not established and high concentrations of dissolved uranium can result, even in areas where uranium ore deposits are not known to occur. The release rate of radionuclides from a spent fuel repository is strongly dependent on the redox conditions that are established following resaturation of the repository. Accordingly, other shield rocks richer in reductants, such as greenstone belts, should be considered as potential host rocks for a repository or the repository should be constructed at depths closer to 1000 m in granitic rock where more reducing conditions are likely to prevail. Addition of reductants to the waste containers may also maintain reducing conditions around the fuel bundles. (Atomindex citation 28:004734)