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Matthew A. Douglas, Sue B. Clark, Judah I. Friese, Bruce W. Arey, Edgar C. Buck, Brady D. Hanson, Satoshi Utsunomiya, Rodney C. Ewing

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V)

The uranium(VI) silicate phases uranophane,
Ca[(UO₂)(SiO₃OH)]₂·5H₂O,
and sodium boltwoodite,
Na[(UO₂)(SiO₃OH)]·1.5H₂O, were
synthesized in the presence of small, variable quantities (0.5–2.0 mol
% relative to U) of pentavalent neptunium (Np(V), as NpO₂⁺), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780–15800 μg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78–97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 93, 05/2005
Pages: 265 - 272

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