79Se:: geochemical and crystallo-chemical retardation mechanisms

Se-79 is a long-lived (1.1x10(6) yrs) fission product which is chemically and radiologically toxic. Under Eh-pH conditions typical of oxidative alteration of spent nuclear fuel, selenite, seO(3)(2-) or HSeO3-; or selenate, SeO42-, are the dominant aqueous species of selenium. Because of the high solubility of metal-selenites and metal-selenates and the low adsorption of selenite and selenate aqueous species by geological materials under alkaline conditions, selenium may be highly mobile. However, Se-79 released from altered fuel may become 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 (SeO3) may substitute for (SiO3OH) in structures of alpha-uranophane and boltwoodite that are expected to be the dominant alteration products of UO2 in Si-rich groundwater. The substitutions (SeO3)<-> (SiO3OH) in sklodowskite, Mg[(UO2)(SiO3OH)](2)(H2O)(6) and (SeO3) tt (PO4) in phurcalite, Ca-2[(UO2)(3)(PO4)(2)O-2](H2O)(7), may occur with the eliminated apical anion being substituted for by an H2O group, but experimental investigation is required. The close similarity between the sheets in the structures of rutherfordine, [(UO2)(CO3)] and [(UO2)(SeO3)] implies that the substitution (SeO3) tt (CO3) can occur in rutherfordine, and possibly other uranyl carbonates. However, the substitutions: (SeO3) <-> (SiO4) in soddyite and (SeO3) <-> (PO4) in phosphuranylite may disrupt their structural connectivity and are, therefore, unlikely. (C) 1999 Elsevier Science B.V. All rights reserved.