U(IV) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of otherwise immobile U(IV). This is particularly relevant in decommissioning and clean-up of nuclear facilities such as legacy ponds and silos at the Sellafield nuclear facility, U.K., and in long-term radioactive waste disposal. Previous work has indicated that silicate can stabilise U(IV) colloids at alkaline to neutral pH, potentially making these colloids significant environmental transport vectors for uranium under anoxic conditions. This study utilises a combination of laboratory and synchrotron based investigations to probe the size, structure and stability of U(IV)-silicate colloids formed under conditions relevant to spent nuclear fuel storage and disposal.<br/><br/>U(IV)-silicate colloids produced under varying silicate concentrations (0-4 mM) and pH (9-12) were fully characterised using extended x-ray absorption fine structure (EXAFS) spectroscopy, small angle x-ray scattering (SAXS) and pair distribution function analysis (PDF) alongside scanning transmission electron microscopy (STEM), ultrafiltration and dynamic light scattering. SAXS investigations indicate that the colloids existed as primary particles between 5 and 10 nm in size, with aggregates also present under low silicate conditions. In terms of stability, SAXS data showed that higher silicate content correlated to higher primary particle stability. PDF/EXAFS analysis indicated the U(IV)-silicate colloids have a core-shell structure with a crystalline UO<sub>2</sub>-like core and U(IV)-silicate shell. This structure would be expected to stabilise colloidal U(IV)-silicate particles due to silicate enrichment on the particle surface. Overall, these data provide new insights into the fundamental structure and stability of U(IV)-silicate colloids and confirms their likely relevance during decommissioning of nuclear facilities and long-term storage and disposal of uranium-containing radioactive waste.

Stability and core-shell structure of U(IV)-silicate colloids

255th National Meeting (2018)

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