Conceptual Model of Potential Radionuclide Transport in the Saturated Zone at Yucca Mountain, Nevada

The saturated zone is a likely groundwater pathway for the potential release of radionuclides from beneath the proposed repository to the accessible environment at Yucca Mountain. The saturated zone constitutes a component of the Lower Natural Barrier because it delays the release of radionuclides and lowers the released mass due to radioactive decay. The conceptual model of radionuclide transport, and the evaluation of uncertainties in associated processes and features in the saturated zone, forms a basis for numerical simulations used in repository performance assessment analyses. Aspects of the conceptual model of particular importance include: 1) matrix diffusion in fractured volcanic rocks, 2) aqueous radionuclide chemistry, 3) sorption, and 4) colloid-facilitated transport. The degree to which diffusion into the rock matrix can retard the migration of radionuclides in the saturated zone is a function of uncertain factors such as the spacing between fractures with flowing groundwater, the matrix diffusion coefficient, and the flow porosity. In addition, matrix diffusion provides access to the sorptive capacity of the matrix for sorbing radionuclides. Groundwater chemistry can affect the migration of some radionuclides through aqueous complexation and redox conditions. Although sorption can be affected by variations in surface reactions and reaction kinetics, a simple linear sorption relationship is appropriate for simulating radionuclide transport at the time and length scales relevant to repository performance assessment. The conceptual model includes two modes of colloid-facilitated radionuclide transport. In the first mode, equilibrium exists between the aqueous phase, sorption onto colloids, and sorption onto the aquifer material. In the second mode, some radionuclides are irreversibly attached to colloids generated within the repository and these colloids are subject to retardation in the groundwater.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.