Upscaling laboratory and mesoscale experiments to understand radioisotope subsurface migration at the DOE Savannah River Site

Abstract:

The migration of trace elements in the environment is dependent on the chemical species which dominate under given geochemical conditions. The mobility can be enhanced or retarded by altering the oxidation state or forming soluble organic ligand-metal ion complexes. This work examines three case studies to evaluate the impacts of these changes in chemical speciation on the transport of uranium, neptunium, and plutonium at the United States Department of Energy Savannah River Site (SRS). Our approach seeks to characterize the time and length scales over which non-equilibrium states are maintained by rate-limiting (or rate-enhancing) reactions between radionuclides and co-reactants due to interactions between physical mass-transfer processes (i.e., flow, advection, diffusion) and (biogeo)chemical reactions.

Neptunium transport in field lysimeter studies has demonstrated enhanced mobilization of Np due to oxidation of Np(IV) to Np(V). Solid NpO₂(s) waste form samples were deployed in field lysimeters for up to 2 years. The effluent concentrations of Np were continually monitored and after retrieval from the field, the lysimeters were destructively sampled. The retrieved solid phases indicated surficial oxidation of NpO₂(s) along grain boundaries which is controlled by redox gradients between the soil pore water and the waste forms. This suggests microstructural features such as grain boundaries are key factors affecting dissolution and fate of neptunium in the environment.

Complexation with organic ligands can also alter radionuclide mobility. This work examined the influence of plant root exudates and nutrient availability on preferential water flow through field lysimeters and dissolution of uranyl phosphate minerals through formation of soluble U(VI)-ligand complexes. Results indicate that less soluble phosphate sources lead to enhanced plant exudate production which in turn enhances uranium solubility and mobility. These results are used to develop a conceptual model to describe overall transport of uranium through the Tims Branch Wetland at the SRS. Comparable experiments examining the influence of organic matter on Pu sorption indicate similar processes control Pu migration in field lysimeter experiments and in benthic sediments of a former reactor cooling water basin.