Earth scientists' 3-D computer model puts Yucca Mountain to the test

March 15, 1996

By Lynn Yarris, [email protected]


A computer model developed by researchers in the Earth Sciences Division is helping the federal government move closer to selecting Nevada's Yucca Mountain as the site of a permanent underground repository for high-level radioactive waste.

The three-dimensional site-scale model is designed to characterize hydrogeologic conditions inside the mountain under a wide range of different scenarios. It has been successfully calibrated with early field tests and is showing the site to be a good choice.

"Based on field data and the predictions of our model, the conceptual ideas that led to the selection of Yucca Mountain as the first proposed burial site are real," says hydrogeologist Gudmundur "Bo" Bodvarsson, who co-leads the project with Yu-shu Wu. "We believe we are close to the end of the site characterization phase of our modeling and are ready to move on to the verification phase."

The 3-D model designed by Bodvarsson and his ESD colleagues is a tool for predicting the flow of moisture, gas, and heat through Yucca Mountain's "unsaturated zone"--the soil between the ground surface and the water table--based on data collected by the U.S. Geological Survey and Los Alamos and Sandia national labs. Not only will the model be used to characterize the site, it will also play a critical role in the design and engineering of the repository.

There are an estimated 24,000 metric tons of high-level radioactive waste in the United States as a result of decades of nuclear reactor operations. Even without the addition of any new nuclear power plants, this waste is expected to exceed 85,000 metric tons early in the next century. Current plans call for the Department of Energy to begin collecting and disposing of high-level nuclear waste in a permanent underground repository by about the year 2011.

Although Yucca Mountain is the site proposed for the first repository, decision-makers need to be certain that once the waste is buried, little if any will ever escape into the environment. This means that long-term hydrogeologic conditions at the repository site must be thoroughly understood. Geologic barriers must be able to isolate the waste for at least 10,000 years--a time span ranging from before construction of the Egyptian pyramids until the year 5000 AD.

"USGS approached us (Berkeley Lab) to develop the unsaturated zone model on the basis of our expertise in hydrological characterization of fractured rocks and numerical modeling," Bodvarsson says.

The unsaturated zone inside Yucca Mountain encompasses some 40 square kilometers and is bounded by major faults to the north, east and west. The 3-D model that Bodvarsson and colleagues designed had to account for flow patterns through the entire zone, even though the repository itself will be encased within an area of only about eight square kilometers.

Among the many issues, they had to prove they could accurately predict how much rainfall percolates through this zone and what its potential for picking up radionuclides would be. They also had to show how long it would take for escaped gas to reach the ground surface. This gas, which might be hot air or vaporized water, could potentially contain radionuclides and pose a substantial threat. Still another issue was predicting the effects of minor changes in atmospheric pressure on the flow of moisture and gas hundreds of meters below the mountain's surface.

"When storms passed over the site, we could see flow pattern changes deep inside the mountain, like the pressure of someone touching it with their hand," Bodvarsson says. "Our model had to be able to characterize these effects to provide confidence in its ability to model aqueous radionuclide transport."

Further complicating the challenge was the fact that, in addition to dealing with the effects of natural thermal conditions, Bodvarsson and colleagues had to allow for the enormous heat that will be introduced into the site when nuclear waste is finally buried.

"The buried waste will be creating an artificial geothermal zone with a temperature of around 200-300 degrees Celsius," he says. "We have to know what the hydrogeologic response will be to this change."

As many as 100 boreholes have now been drilled and sampled for moisture and air content. There is also a two-mile long tunnel through the center of the mountain that has been tested. Results of these tests have been in good agreement with predictions made using the model, particularly regarding gas flow. Yucca Mountain was the first choice as a site because it is in an arid climate and has a large unsaturated zone. It was thought there would be little moisture and gas flowing through this zone, and the predictions of the ESD model bear this out.

Bodvarsson says that more field testing will soon be under way to verify key elements of the model. If this phase of testing, which is expected to take place over the next four to five years, confirms that the predictions of the Berkeley Lab model (as well as other Yucca Mountain models) are reliable, he feels that DOE will be in a good position to make Yucca Mountain its final site selection. Once the repository is completed and begins accepting nuclear waste, all of the models could then be used to help monitor the site.

The 3-D site-scale model was developed in cooperation with USGS. Berkeley Lab researchers who worked with Bodvarsson on its development include Rick Ahlers, Mark Bandurraga, Gang Chen, and Charles Haukwa.