THE STRUCTURAL CHEMISTRY OF MOLYBDENUM IN MODEL HIGH LEVEL NUCLEAR WASTE GLASSES, INVESTIGATED BY MO K-EDGE X-RAY ABSORPTION SPECTROSCOPY

The immobilisation of molybdenum in model UK high level nuclear waste glasses was investigated by X-ray Absorption Spectroscopy (XAS). Molybdenum K-edge XAS data were acquired from several inactive simulant high level nuclear waste glasses, including specimens of Magnox and Magnox / UO2 waste glasses prepared during commissioning of the BNFL Sellafield Waste Vitrification Plant. These data demonstrate that molybdenum is immobilised in the Mo (VI) oxidation state as the tetrahedral MoO42- species, with a Mo-O contact distance of ca 1.76 Å. The MoO42- species are not immobilised within the polymeric borosilicate network, instead they are likely to be located in extra-framework cavities, together with network modifier cations. Molybdenum K-edge XAS data acquired from “yellow phase ” material, removed from a sample of simulant Magnox waste glass, show that this substance incorporates Mo (VI), also present as tetrahedral MoO42- species. The presence of isolated MoO42- tetrahedra within the glass matrix provides an explanation for the initial rapid release of molybdenum from simulant HLW waste glasses in static dissolution experiments and the strong correlation of the initial Mo leach rate with that of Na and B