U(IV)-Silicate Colloids and their Interactions with Sr in Natural and Engineered Alkaline Environments

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Abstract

Colloids have the potential to enhance the mobility of radionuclides in a range of scenarios, including legacy spent nuclear fuel storage facilities at sites such as Sellafield, UK. Some of the spent nuclear fuel stored in these facilities has corroded, resulting in the formation of corroded sludge. A greater understanding of the radionuclide speciation in these legacy facilities is required to predict radionuclide behaviour during sludge retrieval operations and decommissioning activities. Of particular interest is the highly abundant U and 90Sr, a high yield fission product with high specific activity. Colloid formation may increase radionuclide migration within effluent plants, including the site ion exchange effluent plant (SIXEP). In this study the formation, stability and structure of U(IV) silicate colloids, and the interactions of Sr with UO2 and U(IV)-silicate phases was investigated. Particle size and colloidal stability were examined using ultrafiltration, SAXS and DLS. Colloids were stable at pH 6-10.5 when silicate concentrations exceeded those of U, and formed particles <10 nm. The colloids were more stable and less prone to aggregation under higher silicate concentrations. A combination EXAFS and PDF suggested a core-shell structure for these colloidal particles with a crystalline, 1.5 nm UO2 core coated by a poorly ordered, silicate rich, U(IV)-silicate shell. Additionally, UO2 and U(IV)-silicate were both shown to interact strongly with Sr, with UO2 showing higher Sr sorption than U(IV)-silicate. EXAFS, TEM and desorption studies indicated Sr incorporated into the surface of UO2 at pH >10 and formed Sr-silicates at pH >10 in the presence of U(IV)-silicate. The effect of CO2 gassing on U(IV)-silicate with sorbed Sr was also investigated as CO2 gassing is an important step in SIXEP that reduces the pH of effluents from ~11.5 to 7. CO2 gassing resulted in immediate desorption of the majority of Sr from the U(IV)-silicate but also mobilisation of U(IV)-silicate colloid and increased dissolved U(IV) over 8 days, indicating that carbonation is successful in desorbing Sr but may mobilise U(IV). These results indicate that it is important to consider U(IV)-silicate colloids and UO2 when assessing radionuclide mobility, particularly for 90Sr. The improved understanding of U(IV)-silicate colloid particle structure and formation, and previously unknown Sr-U(IV) interactions are crucial to predicting radionuclide speciation and behaviour during nuclear decommissioning.

Keyword(s)

Sellafield; colloids; nuclear decommissioning; silicate; strontium; uranium

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