The Challenges of Modeling
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Deforestation adds one to two billion metric tons of carbon to the atmosphere each year, contributing to the rise in atmospheric levels of carbon dioxide, a greenhouse gas that promotes global warming. |
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ESD's nuclear waste program is headed by hydrogeologist Gudmundur Bodvarsson, who also leads the development of its predictive models. He says the challenges in modeling flow and transport at the Yucca Mountain site are daunting.
"The concept behind the repository is that of multiple barriers to contain the waste. First the waste is packaged inside canisters engineered to last about 50,000 years with no loss of radioactive material. The canisters are then contained in the geological barrier of the mountain itself, which 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."
The 3-D site-scale model of Yucca Mountain that Bodvarsson and his colleagues developed has been successfully calibrated to various data sets, including those from field tests conducted by an "ambient testing group" within ESD's nuclear waste program under the leadership of Joseph Wang. Focusing on the movement of water through fractured rock, these various tests are proving that the 3-D site-scale model of Yucca Mountain is performing "quite well," Bodvarsson says. In turn, the computer model is validating the conceptual ideas behind the selection of Yucca Mountain as an underground nuclear repository site.
Bodvarsson and his colleagues developed their 3-D site-scale model using the computational code from a program called tough. Developed in the early 1980s at Berkeley Lab, tough stands for Transport Of Unsaturated Groundwater and Heat. ESD hydrogeologist Karsten Pruess created the original version to track the flow of water and vapor in geothermal reservoirs. Since then, tough has been modified to the point where today it can be used to model a wide variety of complex subsurface processes.
Working off data collected by the U.S. Geological Survey (USGS), Los Alamos National Laboratory, and others, the 3-D site-scale model of Yucca Mountain can predict the flow of moisture, gas, and heat through the "unsaturated zone"-the rocks between the ground surface and the water table. The model can also be used in the design and engineering of the repository, and for monitoring the site once it begins accepting nuclear waste.
The unsaturated zone (UZ) inside Yucca Mountain encompasses some 40 square kilometers and is bounded by major faults to the north, east and west. The 3-D site-scale model must account for flow patterns throughout the UZ even though the repository itself will be encased within an area of about only eight square kilometers. Predicted flow patterns include how much rainfall percolates through the UZ and what are the chances this percolation will pick up radionuclides. Also predicted is the time it would take for gas escaping from the UZ (another opportunity for the release of radionuclides) to reach the ground surface. The model is even required to predict the effects of atmospheric pressure change on the flow of moisture and gas hundreds of meters below the mountain's surface.
"When storms pass over the site, we can measure minute pressure changes deep inside the mountain of a magnitude similar to the pressure of someone touching it with their hand," says Bodvarsson. "Our model has to be able to reproduce these effects to provide confidence in its ability to model aqueous radionuclide transport."
Bodvarsson's group, working with Stefan Finsterle and Chin-Fu Tsang, have also developed a hydrological model of Yucca Mountain that works on a scale of tens of meters rather than the original site model's hundreds of thousands of meters. This hydrological model accurately calculates seepage into waste emplacement tunnels (called "drifts") under various hydrogeologic conditions. Any water coming into contact with waste canisters is a concern because it will significantly enhance corrosion and shorten effective containment life-spans.
"Both the site-scale and the drift-scale models are key to the Total System Performance Assessment required for site licensing," says Bodvarsson. "The performance of the potential repository is only as reliable as these two models."
In addition to hydrogeologic conditions, models of the Yucca Mountain site must also consider thermal conditions. This consideration is complicated by the enormous heat that will be emitted at the site once nuclear waste is finally buried there.
"The buried waste will be creating an artificial geothermal zone with temperatures that may reach as high as 200 degrees Celsius (392 degrees Fahrenheit)," says Bodvarsson. "We have to know what the hydrogeologic response will be to this change."
Yvonne Tsang heads the nuclear waste program's thermal testing group. Her group's goal is to determine whether the heat generated by high-level nuclear waste is likely to alter the hydrogeological properties of the surrounding rock in a way that will adversely affect the containment
of radionuclides. Working in the Exploratory Studies Facility, a complex of laboratories and alcoves in a main tunnel eight kilometers (nearly five miles) long and 300 meters (1,000 feet) deep through the center of the mountain, Tsang and her group have been key participants in two major "heater tests," the first of which they have already completed.
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| To determine how the heat from nuclear waste will affect Yucca Mountain's interior rock, researchers have rigged up electrical heaters inside mock storage containers and will be closely monitoring the results for the next few years. |
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The first study, dubbed the "Single Heater Test," involved a 700-cubic-meter block of stone carved out from an underground vault. A four-kilowatt electric heater five meters long was inserted into the center of the block and run for a year. During this time, thermal processes were monitored through a network of sensors installed in instrument bore holes. Periodic tests were also made to measure any changes that might be taking place in the rock as a result of the heat.
Says Tsang, "The Single Heater Test was successful in meeting the stated objective of the Yucca Mountain thermal test program, namely to acquire a more in-depth understanding of the coupled processes-thermal, mechanical, hydrological, and chemical-in fractured, partially saturated rock under the influence of heat. The test results showed that our basic understanding of these coupled processes is sound."
The second study is the "Drift Scale Test." A much larger test which requires much more extensive measurement simulations is now underway and being conducted in collaboration with researchers from Lawrence Livermore and Sandia National laboratories. In this test, a tunnel 47.5 meters long, roughly the same size and shape as one that would store nuclear waste canisters in an actual repository, has been filled with mock canisters containing electrical heaters. These and other heaters mounted along the walls and on the floor of the tunnel have raised the temperatures of the most immediate rock to 100 degrees Celsius (212 degrees Fahrenheit), comparable to the thermal conditions that would exist in a repository. The heated tunnel has been insulated from the rest of the experimental facility and is now being closely monitored by some 4,000 sensors, plus remote video and infrared cameras. There will be four years of heating, followed by four years of cooling. Final results should come in the year 2006.
Carbon Sequestration
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| Sally Benson has been director of Berkeley Lab's Earth Sciences Division since 1993. Her affiliation with the Lab began in 1977 after she obtained her undergraduate degree in geology from Columbia University. Today, she holds a Ph.D. in material science and mineral engineering from U.C. Berkeley, a faculty appointment at Clemson University, and is the co-chair of a national task force on carbon management. |
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While researchers continue to seek creative new means of keeping the energy fires burning, human civilization will continue to rely largely on fossil fuels. Two hundred plus years of industrialization has resulted in the emission of an enormous amount of carbon dioxide (CO2) into the atmosphere. High concentrations of CO2 promote global climate change, and experts predict that atmospheric CO2 concentrations will double from pre-industrial levels by the middle of next century.
Although the effects of doubling atmospheric CO2 levels are not entirely understood, the scientific consensus is that serious environmental consequences are possible if steps are not taken to curb CO2 emissions. Once again, thoughts have turned to the world below.
ESD division director Sally Benson is the co-chair of a national task force commissioned by the U.S. Department of Energy's Offices of Science and Fossil Energy to develop a research roadmap for investigating an approach to CO2 management called "carbon sequestration." The goal of carbon sequestration is to prevent CO2 emissions from reaching the atmosphere by capturing a significant amount, as much as 4 billion tons by the year 2050, and securely storing it. Two very different approaches to sequestration are being investigated. In one approach, CO2 is captured from the stack of a power plant and disposed of underground or in the ocean. The other approach uses natural ecosystem processes to remove CO2 from the atmosphere.
"The natural ecosystems are essentially a huge natural biological scrubber for CO2," says Benson.
Benson and her ESD colleagues have been among the leaders in setting the scientific agenda for studying various strategies for storing captured carbon somewhere safe. Among the possibilities being considered are burying concentrations of CO2 in abandoned oil fields, coal deposits that are too deep to be mined, or in aqueous formations. Another possibility would be to inject pure streams of CO2 directly into the ocean where they would become trapped in sediments or ice-like solids called hydrates.
"Science has also made the case that forests and oceans can sequester large quantities of CO2," says Benson. "But carbon sequestration is an immature field with much that needs to be learned. I see this area as being a major focus of future research for the Earth Sciences Division."end
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