Status and future needs in actinide chemistry for performance assessment of nuclear waste disposal

Actinides are important components of nuclear wastes. Many actinide isotopes have such long half-lives that they are expected to persist in nuclear wastes over periods extending to hundreds of thousands to millions of years. Because the environmental behavior of actinides is highly dependent on many factors including chemical, physical, and mineralogical properties of waste-form/geologic media and these properties are not expected to stay constant over millions of years, fundamental approaches need to be developed to predict actinide behavior over long periods. Before these approaches can be developed, thermodynamic data pertaining to dominant aqueous species and solubility of environmentally important solid phases are required. These data are generally used to set upper limits on soluble actinide concentrations under specific environmental conditions. The reliability of the predicted concentrations will depend upon the quality and extensiveness of the thermodynamic data that is obtained. This paper provides only a cursory discussion of the quality and extensiveness of currently available data for actinide reactions with environmentally important ligands. Areas where further fundamental actinide data are required include mixed and complex systems, elevated temperatures, solubility-controlling solid phases in waste forms, solid solutions, complexing/binding agents, and kinetic/equilibrium redox reactions. This paper provides examples that illustrate the importance of these various aspects and the challenges we face in acquiring the necessary data. (author)