Earth Sciences Division (ESD) Department of Energy (DOE) Lawrence Berkeley National Laboratory (LBNL)

The Yucca Mountain Project: Numerical Modeling: Ambient Unsaturated Zone Flow and Radionuclide Transport Models

Simulated Percolation Fluxes at the Repository Horizon  under a Present-Day Infiltration Case The unsaturated zone flow models integrate geological, hydrological, and geochemical information on a three-dimensional domain of about 22 cubic kilometers encompassing Yucca Mountain. This domain extends from the ground surface to the water table with an average thickness of about 560 meters over a footprint of nearly 40 square kilometers. The repository footprint covers about 6 square kilometers and lies approximately midway between the surface and the water table. The entire model domain is represented using nearly a quarter of a million numerical gridblocks.

The unsaturated zone at Yucca Mountain consists of gently dipping pyroclastic flow and fall deposits that resulted in alternating layers of welded and nonwelded fractured tuffs. Detailed geospatial information on the thickness and continuity of these layers as affected by depositional processes, post-depositional alteration, and faulting were incorporated into the three-dimensional model. Spatially variable rock properties were incorporated using 32 distinct model stratigraphic units and 13 fault zones.

Simulated Cumulative, Normalized Mass Arrival Contours of a Conservative  Tracer at the Water Table after 1,000 Years, Indicating Potential  Breakthrough Locations at the Time, Using a Present- Day Infiltration Scenario Because water can flow through both the rock fractures and the much smaller pores of the rock matrix, flow and transport processes were represented through the dual-permeability modeling approach. This modeling method can represent distinct fracture and matrix hydrologic behavior within a continuum flow approach. This modeling method also accounts for local disequilibrium between fracture and matrix hydrologic and chemical state and the associated flow and transport processes that are driven by such disequilibria.

The unsaturated zone flow and transport model was calibrated using a variety of hydrologic, pneumatic, geochemical, and temperature measurements. Laboratory measurements of hydrologic and transport properties were conducted to provide model parameter values. Inverse modeling techniques were used to calibrate and up-scale hydrologic properties using water saturation, water potential, and pneumatic pressure data. Pore-water chloride and temperature data were used to calibrate net infiltration rates and uncertainty ranges. Model validation comparisons were carried out using independent hydrologic, pneumatic, and geochemical measurements of the natural system and field tests of flow and transport processes.

The unsaturated zone flow model was used to predict percolation rates above waste emplacement drifts that are needed by drift seepage models to predict seepage rates into the drifts. The unsaturated zone flow model was also used for predicting the flow distribution between the repository and the water table. This was needed to predict radionuclide transport after engineered barriers within the waste emplacement drifts have breeched. Both of these unsaturated zone flow model results were essential inputs to the total-system performance assessment model for calculating repository performance. The unsaturated zone radionuclide transport model was used to validate the radionuclide transport abstraction model used directly in total-system performance assessment calculations. Unsaturated zone flow and transport model results were also instrumental in the evaluation and screening of features, events, and processes important for repository performance at Yucca Mountain.

References

  • Wu, Y-S.; Lu, G.; Zhang, K.; and Bodvarsson, G.S. 2004. “A Mountain-Scale Model for Characterizing Unsaturated Flow and Transport in Fractured Tuffs of Yucca Mountain.” Vadose Zone Journal, 3, 796-805. Madison, Wisconsin: Soil Science Society of America. TIC: 257371.
  • Wu, Y-S.; Pan, L.; Zhang, W.; and Bodvarsson, G.S. 2002. “Characterization of Flow and Transport Processes within the Unsaturated Zone of Yucca Mountain, Nevada, Under Current and Future Climates.” Journal of Contaminant Hydrology, 54, (3-4), 215-247. New York, New York: Elsevier. TIC: 253316.
For more information, please contact:

Jim Houseworth
Yucca Mountain Project Lead
Earth Sciences Division
Phone: 702-295-4833
Fax: 702-295-7742
Email: JEHouseworth@lbl.gov