{sup 79}Se: Geochemical and crystallo-chemical retardation mechanisms
{sup 79}Se is a long-lived (1.1x10{sup 6} 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 metal-selenites and metal-selenates and the low adsorption of selenite and selenate aqueous species under alkaline conditions, selenium may be highly mobile. However, {sup 79}Se 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 (SeO{sub 3}) may substitute for (SiO{sub 3}OH) in structures with the uranophane anion-topology ({alpha}-uranophane, sklodowskite, boltwoodite) which are expected to be the dominant alteration phases of UO{sub 2} in Si-rich groundwater. The structural similarity of guilleminite, Ba[(UO{sub 2}){sub 3}(SeO{sub 3}){sub 2}O{sub 2}](H{sub 2}O){sub 3}, to phurcalite, [(UO{sub 2}){sub 3}(PO{sub 4}){sub 2}](H{sub 2}O){sub 7}, suggests that the substitution (SeO{sub 3}) {leftrightarrow} (PO{sub 4}) may occur in phurcalite. The close similarity between the sheets in the structures of rutherfordine and [(UO{sub 2})(SeO{sub 3})] implies that the substitution (SeO{sub 3}) {leftrightarrow} (Co{sub 3}) can occur in rutherfordine. However, the substitution: (SeO{sub 3}) {leftrightarrow} (SiO{sub 3}OH) in soddyite and (SeO{sub 3}) {leftrightarrow} (PO{sub 4}) in phosphuranylite may disrupt their structural connectivity and are unlikely to occur. 50 refs., 3 figs., 1 tab.
- Research Organization:
- Univ. of Michigan, Ann Arbor, MI (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG07-97ER14816; FG07-97ER14820
- OSTI ID:
- 20015871
- Resource Relation:
- Conference: 1998 Materials Research Society Fall Meeting, Boston, MA (US), 11/30/1998--12/04/1998; Other Information: Single article reprints are available from University Microfilms Inc., 300 North Zeeb Road, Ann Arbor, Michigan 48106; PBD: 1999; Related Information: In: Scientific basis for nuclear waste management XXII. Materials Research Society symposium proceedings: Volume 556, by Wronkiewicz, D.J.; Lee, J.H. [eds.], 1355 pages.
- Country of Publication:
- United States
- Language:
- English
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