SORPTION AND REDUCTION OF URANIUM(VI) ON IRON CORROSION PRODUCTS UNDER REDUCING SALINE CONDITIONS

Pertinent to the behaviour of carbon steel based nuclear waste package s in saline geological environments, sorption and reduction of U(VI) o n real container corrosion products has been studied in an Ar/8%H-2 fl ushed glove box. A carbon steel was corroded in MgCl2-brine at elevate d temperatures. U(VI) was added and the redox states both of uranium a nd iron and their distribution among solid and liquid phases were inve stigated. The corrosion products initially consisted to more than 97% of hydrous Fe(II) oxides containing Cl- and Mg2+, but always some magn etite was present. In the course of the sorption step, the amount of m agnetite increased. Reaction products buffer pH values of the system i n a very narrow neutral range. Less than 1% of iron or uranium were fo und in colloidal state and already after one day, more than 98% of ini tially dissolved uranium was found associated with the immobile phases . A reciprocal relationship of solution concentrations of uranium with the nominal mass of magnetite was found. Behaviour of uranium species was rationalized in terms of Eh/pH diagrams. Reduction of hexavalent to tetravalent uranium was observed but to a much less pronounced exte nt than expected from thermodynamical considerations.