Nevada System of Higher Education
The purpose and scope of this report is to present an overview of the experiments, methods, results, and conclusions from research performed for the project “Groundwater Characterization at Yucca Mountain Task 2: Surface Complexation and Solid Phase Dissolution”. The impact of surface complexation, alteration phase formation, and solution competition with metal ions on the solubility and speciation of actinide elements (U, Pu, Np) will be examined. In particular the role of iron (as Fe2+ and Fe3+) and silicate (as SO3 2-) concentrations on speciation, solubility, sorption, and secondary phase formation of actinides will be investigated. While a large body of literature exists on the interaction of actinides with iron and iron oxide phases, relatively little has been explored regarding the impact of silicates on actinide speciation. Therefore the role of silicates will be the main focus of the report, as it is the primary factor which meaningfully contributes to the enhanced understanding of actinide environmental speciation. The described topics are examined through two main studies areas: formation of precipitates from solution phase species and sorption of dissolved species to solids. The main actinide ion species of interests are UO2 2+, NpO2 +, and Pu4+. These species were selected based on their importance as components of spent nuclear fuel and their potential to form soluble species. The main component of spent nuclear fuel is uranium; neptunium is expected to have a high solubility due to its pentavalent oxidation state, and plutonium may form colloidal species . The aqueous phase pH, Fe concentration, and SiO3 2- concentration are varied. The role of Fe is of importance since canister corrosion may elevate aqueous levels of iron. Since the groundwater near the Yucca Mountain site is approaching saturation in silicate concentration, evaluation of this anion is deemed crucial. Furthermore, geochemical research at the Nevada Test Site has identified goethite and silicates as important geominerals, offering further motive for the investigation of iron oxides and silicates . The role of pH is fundamental in dictating actinide and iron hydrolysis  and is evaluated to ascertain its importance in speciation in the presence of the other solution constituents. The project results will elucidate the relative importance of Fe and silicates in actinide speciation, in particular the formation of precipitates and subsequently sorbed species. The main focus will be on the role of silicate.
Actinide elements – Solubility; Groundwater; Iron ions; Nevada – Yucca Mountain; Radioactive waste canisters – Corrosion; Silicates; Water chemistry
Chemistry | Environmental Chemistry | Hydrology | Radiochemistry
Smiecinski, A. J.
Groundwater characterization at Yucca Mountain task 2: Surface complexation and solid phase sorption.
Available at: https://digitalscholarship.unlv.edu/yucca_mtn_pubs/60