Earth scientists' 3-D computer model puts Yucca Mountain to the test
March 15, 1996
By Lynn Yarris, LCYarris@LBL.gov
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.
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.