Sorption of Strontium to Uraninite and Uranium(IV)-Silicate Nanoparticles

Spent nuclear fuel contains both uranium (U) and high yield fission products, including strontium-90 (⁹⁰Sr), a key radioactive contaminant at nuclear facilities. Both U and ⁹⁰Sr will be present where spent nuclear fuel has been processed, including in storage ponds and tanks. However, the interactions between Sr and U phases under ambient conditions are not well understood. Over a pH range of 4-14, we investigate Sr sorption behavior in contact with two nuclear fuel cycle relevant U(IV) phases: nano-uraninite (UO₂) and U(IV)-silicate nanoparticles. Nano-UO₂ is a product of the anaerobic corrosion of metallic uranium fuel, and UO₂ is also the predominant form of U in ceramic fuels. U(IV)-silicates form stable colloids under the neutral to alkaline pH conditions highly relevant to nuclear fuel storage ponds and geodisposal scenarios. In sorption experiments, Sr had the highest affinity for UO₂, although significant Sr sorption also occurred to U(IV)-silicate phases at pH ≥ 6. Extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy, and desorption data for the UO₂ system suggested that Sr interacted with UO₂ via a near surface, highly coordinated complex at pH ≥ 10. EXAFS measurements for the U(IV)-silicate samples showed outer-sphere Sr sorption dominated at acidic and near-neutral pH with intrinsic Sr-silicates forming at pH ≥ 12. These complex interactions of Sr with important U(IV) phases highlight a largely unrecognized control on ⁹⁰Sr mobility in environments of relevance to spent nuclear fuel management and storage.