Reduction of uranium in disposal conditions of spent nuclear fuel

This literature study is a summary of publications, in which the reduction of uranium by iron has been investigated in anaerobic groundwater conditions or in aqueous solution in general. The basics of the reduction phenomena and the oxidation states, complexes and solubilities of uranium and iron in groundwaters are discussed as an introduction to the subject, as well as, the Finnish disposal concept of spent nuclear fuel. The spent fuel itself mainly (~96 %) consists of a sparingly soluble uranium(IV) dioxide, UO2(s), which is stable phase in the anticipated reducing disposal conditions. If spent fuel gets in contact with groundwater, oxidizing conditions might be induced by the radiolysis of water, or by the intrusion of oxidizing glacial melting water. Under these conditions, the oxidation and dissolution of uranium dioxide to more soluble U(VI) species could occur. This could lead to the mobilization of uranium and other components of spent fuel matrix including fission products and transuranium elements. The reduction of uranium back to oxidation state U(IV) can be considered as a favourable immobilization mechanism in a long-term, leading to precipitation due to the low solubility of U(IV) species. The cast iron insert of the disposal canister and its anaerobic corrosion products are the most important reductants under disposal conditions, but dissolved ferrous iron may also function as reductant. Other iron sources in the buffer or near-field rock, are also considered as possible reductants. The reduction of uranium is a very challenging phenomenon to investigate. The experimental studies need e.g. well-controlled anoxic conditions and measurements of oxidation states. Reduction and other simultaneous phenomena are difficult to distinghuish. The groundwater conditions (pH, Eh and ions) influence on the prevailing complexes of U and Fe and on forming corrosion products of iron and, thus they determine also the redox chemistry. The partial reduction of sorbed uranium by metallic iron or by its corrosion products (magnetite, green rusts) has been observed in many studies performed under anaerobic solution conditions. A longer reaction time, several months, was needed to observe UO2 crystals. The pyrite in the buffer or pyrite or micas in the near-field rock may reduce uranium to some extent, whereas, hematite, can function as a catalytic surface in the U(VI) reduction by aqueous Fe2+. The surface catalytic reaction seem to outcompete the direct enzymatic U(VI) reduction by bacteria. Some studies suggested the reduction of U(VI) to occur also by aqueous Fe2+ in solution.