Silicate stabilisation of colloidal UO2 produced by uranium metal corrosion

• Published in: Journal of nuclear materials Vol. 526; no. C; p. 151751
• Main Authors: Neill, Thomas S., Morris, Katherine, Pearce, Carolyn I., Abrahamsen-Mills, Liam, Kovarik, Libor, Kellet, Simon, Rigby, Bruce, Vitova, Tonya, Schacherl, Bianca, Shaw, Samuel
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.12.2019; Elsevier BV; Elsevier
• Subjects: Anaerobic conditions; Coating; Coatings; Coffinite; Colloid; Colloids; Corrosion; Corrosion products; Crystal structure; Crystallinity; Environmental cleanup; Fine structure; MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; Nanoparticles; Nuclear engineering; Nuclear facilities; Nuclear fuels; Radioactive wastes; Silica; Silicate; Silicates; Silicon dioxide; Spectroscopy; Ultrafiltration; Ultrastructure; Uranium; Uranium dioxide; X ray absorption; Coffinite; Uranium; Silicate; Corrosion; Colloid
• DOI: 10.1016/j.jnucmat.2019.151751
• ISSN: 0022-3115

U(IV) mobility can be significantly enhanced by colloids in both engineered and natural environments. This is particularly relevant in decommissioning and clean-up of nuclear facilities, such as legacy fuel ponds and silos at the Sellafield site, UK, and in long-term radioactive waste geodisposal. In this study, the product of metallic uranium (U) corrosion under anaerobic, alkaline conditions was characterised, and the interaction of this product with silicate solutions was investigated. The U metal corrosion product consisted of crystalline UO2 nanoparticles (5–10 nm) that aggregated to form clusters larger than 20 nm. Sequential ultrafiltration indicated that a small fraction of the U metal corrosion product was colloidal. When the uranium corrosion product was reacted with silicate solutions under anaerobic conditions, ultrafiltration indicated a stable colloidal uranium fraction was formed. Extended X-ray absorption fine structure (EXAFS) spectroscopy and high resolution TEM confirmed that the majority of U was still present as UO2 after several months of exposure to silicate solutions, but an amorphous silica coating was present on the UO2 surface. This silica coating is believed to be responsible for formation of the UO2 colloid fraction. Atomic-resolution scanning TEM (STEM) indicated some migration of U into the silica-coating of the UO2 particles as non-crystalline U(IV)-silicate, suggesting alteration of UO2 at the UO2-silica interface had occurred. This alteration at the UO2-silica interface is a potential pathway to the formation of U-silicates (e.g. coffinite, USiO4). [Display omitted] •Product of anaerobically corroded U metal characterised as nano-UO2 (5–10 nm).•UO2 forms colloids in silicate solutions, no colloids in silicate free systems.•Particles have SiO2 coating but maintain UO2 core in silicate solution.•SiO2 coating has U atoms present suggesting UO2 alteration at the UO2-SiO2 interface.