Program Brief

Statement of Problem: The client needed a model of far-field radionuclide transport that incorporated dispersive and sorptive processes characteristic of the saturated fractured tuff aquifer and the alluvial aquifer near Yucca Mountain, Nevada. The model was used to establish the technical foundations for a new dose-based regulation for Yucca Mountain and provide a model abstraction for use in a Total-System Performance Assessment code.

Approach and Accomplishments: Using background information derived from a regional-scale flow modeling study of the Nevada Test Site conducted by the U.S. Geological Survey, scientists at CNWRA^® developed 2D site-scale flow and transport models for a region extending from Yucca Mountain to an area of Amargosa Valley where farmland is irrigated with groundwater. The steady-state flow model, constructed with the MAGNUM-2D finite-element simulator, was calibrated to 146 measured values of hydraulic head using an automated inverse procedure. Radioactive decay and advective-dispersive transport of sorbing and nonsorbing radionuclides were modeled using the CHAINT transport simulator, which also accounts for ingrowth of radioactive decay daughter products. Transport simulations were conducted to estimate unretarded travel times from the potential repository to a compliance boundary located 20 km to the south and to assess the degree of dose reduction realized by natural attenuation during transport.

Client Benefits: Results from simulations conducted with the transport model for Yucca Mountain were used by the U.S. Nuclear Regulatory Commission to evaluate various dose-based regulatory criteria for the proposed repository. A simplified 1D streamtube network model was abstracted from the saturated zone transport model for use in the Total-system Performance Assessment computer code.