79 Se: Geochemical and Crystallo-Chemical Retardation Mechanisms
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"Se: GEOCHEMICAL AND CRYSTALLO-CHEMICAL RETARDATION MECHANISMS Fanrong Chen "',Peter C. Burns **,Rodney C. Ewing `2 Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, MI 48109-2104 "Department of Civil Engineering and Geological Sciences University of Notre Dame, Notre Dame, IN 46556 ABSTRACT "Se is a long-lived (1.1×106 years) fission product which is chemically and radiologically toxic. Under Eh-pH conditions typical of oxidative alteration of spent nuclear fuel, selenite or selenate are the dominant aqueous species of selenium. Because of the high solubility of metalselenites and metal-selenates and the low adsorption of selenite and selenate aqueous species under alkaline conditions, selenium may be highly mobile. However, 79Se released from altered fuel may be immobilized by incorporation into secondary uranyl phases as low concentration impurities, and this may significantly reduce the mobility of selenium. Analysis and comparison of the known structures of uranyl phases indicate that (Se0 3 ) may substitute for (SiOOH) in structures with the uranophane anion-topology (o.-uranophane, sklodowskite, boltwoodite) which are expected to be the dominant alteration phases of UO, in Si-rich groundwater. The structural similarity of guillemninite, Ba[(UO 2)3(Se0 3)2024(H 20) 3, to phurcalite, [(U02) 3(P0 4 y)021(H 2 0)X, suggests that the substitution (SeO) "-*(PO4 ) may occur in phurcalite. The close similarity between the sheets in the structures of rutherfordine and [(UO2)(SeOA] implies that the substitution (Se0 3)
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