Different Pathways of Secondary Phase Formation Induced by Colloidal and Dissolved Silica During the Dissolution of UO₂ Nuclear Fuel in Leaching Tests

We investigated the different dissolution behaviour of UO₂ nuclear fuel material in waters containing silica in two different physical and chemical forms (dissolved ions and as SiO₂ colloids, respectively) at elevated temperatures (95 °C in autoclaves). It was investigated if SiO₂ colloids can act as carrier material for U ions during a interface geochemical dissolution process, a process that might possibly enhance the mobilization of uranium. Herefore, leaching / dissolution tests were conducted in batch reactors, using both dissolved Si (sodium metasilicate solution), as well as synthetic SiO₂ colloids (100 nm diameter). Solid materials were examined with scanning electron microscopy (SEM-EDX) after the tests and ICP-OES was used for analysis of concentrations of U and Si in solutions. Thermodynamic calculations were applied for modelling the surface charges of the solid materials. Results show that a treatment with colloidal SiO₂ has different effects on the surfaces than a leaching in dissolved silicate solutions. In the presence of colloids, well-crystallized secondary phases containing U and Si (most obviously uranyl silicates) were found on the surfaces, which were attacked by the treatment. This was not the case when dissolved Si was used. SiO₂ colloids were partly found to remain on the surfaces after 1000 h at 95 °C. Dissolved U concentrations decreased with increasing Si content in the systems, especially so when colloidal Si was used. Ultrafiltration showed that the greatest part of the dissolved U was associated with Si colloids. A surface charge model suggests that the different effects are due to the development of electrostatic interactions between the UO₂ and SiO₂ surfaces.