Yucca mountain- Pursuing a license
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Sandia National Laboratories scientists, engineers and technicians are performing critical experiments deep in the volcanic heart of Nevada's Yucca Mountain. They are producing information that will assist the Nuclear Regulatory Commission (NRC) in deciding whether to license the Yucca Mountain Project (YMP) as a permanent repository for high-level radioactive wastes and spent fuel rods from nuclear power plants.
Work began more than 20 years ago at the Yucca Mountain site, located about 100 miles northwest of Las Vegas. It has accelerated in recent months as project managers Bechtel-SAIC, backed up by experimental evidence from Sandia and other participants, push to meet an approaching deadline. Formal application for construction authorization will be made to the NRC late this year.
Sandia held sole responsibility for scientific research at the Waste Isolation Pilot Project (WIPP) transuranic waste repository now receiving waste at a site near Carlsbad, New Mexico. But the Labs are just part of a much larger team at YMP. Sandia is one of six national laboratories working with the U.S. Geological Survey and several private companies.
DOE's Office of Civilian Radioactive Waste Management oversees YMP and its principal regulator is the NRC, rather than the Environmental Protection Agency (EPA), which has oversight of WIPP.
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At WIPP, stable and uniform beds of salt run for miles, with tidy marker beds. The salt beds have remained intact for 225 million years. Marker beds help workers know exactly where they are mining. At YMP, the situation is radically different. The main horizontal tunnel bores through four different geologic zones of volcanic rock called tuff. Some of the rock was fractured by ancient seismic activity, causing faults to develop. Some contains voids caused by trapped gases during the formation of the rock. The tuff also contains zeolites, minerals that can slow the movement of radioactive materials.
10,000 Years
"We have to demonstrate we can make confident predictions for performance over 10,000 years," says Cliff Howard, manager of Sandia's YMP Repository Test and Analysis department, "but generic approaches aren't appropriate here because of the distinctive rock types."
The Experimental Studies Facility at Yucca Mountain consists of support structures, two tunnels and a cluster of experimental alcoves along the tunnels, or drifts.
The main YMP drift is 25-feet in diameter, generally horizontal, and carves a five-mile U-shape through the mountain. A 16-foot diameter cross-drift has also been bored off the main tunnel into the rock horizon where most of the waste would be stored. A large-diameter air duct runs along the top of the tunnel. A conveyor belt, used to carry debris from the boring machine during the three-year drilling effort, runs down one wall. Along the other wall, thick high-voltage cables snake their way into the drift.
Sandia specialists are determining how Yucca Mountain's native volcanic rock responds, mechanically and thermally, to the heat and pressure expected during millennia of repository storage. This information is being combined with the results of other teams working on the project. These data are used in the creation of computer models for the assessment of the repository's long-term performance.
Sandia's is a tricky job, carried out in an unknown geological environment. "Because natural systems are inherently uncertain, the test and analyses we conduct are designed to support a risk-informed decision process," explains Howard. "We ask ourselves, 'What are the chances of a certain scenario occurring?' and then, 'What are the consequences of that?'"
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Howard and his fellow managers meet frequently with other team members to make sure experiments are on track, quality assurance issues are being addressed, and the architecture of having multiple teams feeding data for modeling repository performance and providing design information continues to be robust.
All in a Day's Work
In preparation for one such meeting, Howard gets started before 5 a.m., heading for Yucca Mountain from Las Vegas. As the desert flows past and the sun peeks over a range of mountains to the east, he briefs a visitor on the unique challenges posed by volcanic tuff, a dense, welded ash that showered down 12 million years ago.
At a building just outside the north portal of the Yucca Mountain drift, nearly 100 miners, experimenters and support staff are gathered for the 7 a.m. safety and operational briefing. To get this far -- to the verge of a trip into the tunnel -- visitors must have been trained in general underground and radon safety and the operation of a selfrescue device, to be used in the unlikely event of a fire inside the tunnel. It's clear at the briefing that safety is important in this environment.
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After strapping on a belt with the selfrescuer and a batterypowered lamp that attaches to his hardhat, Howard is ready for a trip inside the mountain. Visitors carry a radon dosimeter and a card that shows they have received safety briefings. Paperwork to ensure accountability for all workers is also a part of the process. Soon in a diesel-powered, open mining train, Howard joins other workers rolling into the five-mile-long drift. Just beyond a cross-drift, the train comes to a stop at Alcove 5, where two key experiments were conducted.
To determine how heat will influence the rock, experimenters placed an electric heater of several thousand watts into a small bore and measured heat flow in the surrounding rock to help calculate its thermal conductivity. Nearby, along the alcove, several technicians sample water and study the temperature and physical response of a large section of rock. Heated from December 1997 to January 2002 with a large cylindrical heater, the rock section will take several years to cool down. Measurements continue from time to time to understand processes that conceivably could cause corrosion of waste packages.
Later in the morning, Howard walks past the bulk of a giant boring machine at the South portal to examine another largescale Sandia test. Inside the portal workers used hydraulic jacks to squeeze sections of rock to measure strength. The results gathered will help in the design of support systems and to help assess the repository's stability if and when seismic events occur.
Before returning to Las Vegas, Howard takes a radio, clears with ranch control, and starts up toward the crest of Yucca Mountain in a government pickup. From the top, the classic basin and range topography spreads in all directions. There is still a lot of work ahead for Sandia on the application to the NRC, he cautions. "There are hard decisions yet to be made."
Moving to a Safer Place
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Twenty percent of the electricity used in homes and businesses across the U.S. is generated by nuclear reactors, a percentage likely to increase as the U.S. grapples with its increasing dependency on imported fossil fuels. When the nuclear fuel is "spent" and no longer able to produce electricity efficiently, it is removed from the reactor and stored in cooling ponds or dry storage facilities. But this waste and some reprocessing residues, collectively known as "high-level radioactive waste," currently have no safe, permanent means of storage. Now they are designated for disposal at Yucca Mountain.
When licensed, the repository at Yucca Mountain will be in an active operating mode for about 75 years, with waste being put into storage before the site is permanently sealed. "We have to be able to answer questions about mechanical stability and worker protection in the mine," explains Cliff Howard. "How many rock bolts will we need to use? What about wire mesh? What other structures and components are needed to operate in the underground environment to emplace radioactive waste packages weighing in excess of 90,000 pounds?
| "In order to build a safety case you'd normally go to civil project experience from the mining of tunnels and other similar work. But mining isn't done in volcanic rocks and there are few civil projects to turn to for analogs. |
"There's almost nothing out there that's comparable, so you can't go to a textbook for clues."
Crushing Rocks the Scientific Way
In order to develop a sound basis for licensing the proposed Yucca Mountain high-level waste repository, Sandia's Ron Price is studying the site one rock at a time.
Price has been breaking rock samples of tuff from the Nevada site for the past two decades. His work is critical to an understanding of the matrix rock properties, providing numerical and descriptive information needed for repository modeling, design and performance assessment.
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The thick layer of tuff at Yucca Mountain is generally divided into three horizons, with the central zone the most dense and homogeneous. As Price explains, "heat and pressure rearranged molecules in the center into a dense hard rock, and this is the zone where most of the data has been collected." By the late 1990s, volumes of valuable data had been amassed on the central zone.
But then, after crews began mining the main drift, the design concept changed, and more data were needed on the upper and lower parts of the three zones -- zones rich in lithophysae, which are voids in the rock that affect the material's mechanical strength. These formed during deposition of the rock, when pockets of gas were trapped by ashfalls with the consistency of molasses.
Subsequently the Albuquerque lab run by Price and co-worker Dave Bronowski was stocked with a large collection of specimens of red tuff, ranging in size from one-inch-diameter to one-foot-diameter cylinders. Samples were carefully mounted in a one-million pound rock press, instrumented, squeezed and the results recorded.
| Price analyzes the results, calculating stress and strain at failure, and sends the information along to Bechtel-SAIC and Sandia scientists for incorporation into numerical models. Part of the analysis is a physical description of how the samples failed. "The rock is so ultra-fine-grained, it behaves more like glass than a classic igneous rock," Price comments. |
"We want to know how the rock changes under stress. As the rock is squeezed, for example, it decreases in length and broadens. These measurements give us the necessary elastic properties. We continue to test until it fails, then we note the stress, strain and other information and feed that into our models." Sometimes the samples come out fairly intact. "Other times they come out like this," he says, holding a plastic bag filled with rubble.
Though a major part of the experimental work for the license application is now complete, Price and Bronowski are not likely to soon be out of work. "We'll continue to gather data for performance confirmation as operations begin," he says. "We want to know if it's going to perform the way we said it would."