Slicing Yucca

In a small laboratory at UNLV, geologist Nick Wilson and graduate student Sarah Lundberg sit among sheaves of colorful computerized printouts under a bank of five computer screens that are attached to one of the most modern electron probes in the world.

From this vantage point, they are trying to peer into an invisible world inside Yucca Mountain, 90 miles northwest of Las Vegas.

"It's detective work," Wilson said, as he sat next to the JX A-8900R microprobe housed in a one-story white stucco building next to the Lily Fong Geoscience Building.

Like detectives, Wilson and Lundberg are hunting for evidence of water that at one time invaded Yucca Mountain, the only site the federal government is studying to become the world's first high-level nuclear waste repository.

Working with UNLV Geology Professor Jean Cline, the researchers are trying to answer a question that could be crucial to the suitability of the planned repository: How long ago did the water work its way into the volcanic rock of the site, 900 feet beneath the mountain's top?

If those liquid and gas bubbles left behind are younger than 1 million years old, it could predict a danger of water leaking into the site again during the repository's planned 10,000-year lifespan. Water could corrode containers holding highly radioactive materials and cause a leak among the 77,000 tons of commercial and defense waste.

The DOE and some U.S. Geological Survey scientists believe the bubbles, called fluid inclusions, formed when nearby volcanos exploded more than 12 million years ago, forming the ash layers of Yucca's tuff. But independent research cast doubt on that assumption.

To find the answer, the researchers are following the chemical traces left in the rocks -- traces that are not visible to the human eye, even under a microscope.

Up until a month ago, when the probe became operational, the pair combined their microscopes with a Polaroid camera to try to capture the scientific nuances of Yucca's rock. The photos from the microscope allowed the researchers to study the crystal structure, but the chemical makeup of the samples eluded them.

They could not use traditional methods of analyzing the chemicals, because such methods require destroying the delicate samples through crushing, dissolving and vaporizing in a complex process of elimination.

The probe, on the other hand, sends a beam of electrons onto a sample that bounces off the rock in a distinct pattern, in effect fingerprinting chemicals that lurk in the minerals. The five monitoring screens show Wilson and Lundberg what the sample looks like up close and what it contains, elements such as magnesium or arsenic.

To the left of the screens, two cylinders that analyze the fingerprints extend at an angle from the main, 4-foot-tall vertical tube that holds the electron beam. Inside a fourth metal cylinder, the probe's motor runs, protected from dust.

The soft whir of the motors can barely be heard by Wilson or Lundberg. So to keep track of the motor's twirl, they have put red and green paper cocktail umbrellas into pins outside the cylinder that rotate when the probe goes into motion.

The computer screens show them colorful maps of the sample's surface, which they print out onto letter-size paper. Other printouts contain a specific chemical analysis.

Wilson and Lundberg face months of work to scan and analyze each puzzling slice of Yucca before the final report is due to the Department of Energy, which is funding the research, in April 2001.

Wilson spent most of last spring inside the 5-mile-long exploratory tunnel bored into the side of Yucca by the DOE. There he gathered the chunks of rock. Then he and Cline hand-carried them to a Denver laboratory, where experts sliced, polished and mounted them on the slides for future study.

The thin slices of rock have unveiled a complex texture of layers, forming Yucca over millions of years.

"It's all discovery because no one else has ever seen this before," Wilson said. "It's one of the last places you can be an explorer, doing geology."

First, using the probe they will determine the major and minor elements, figuring out what chemicals the water carried into the calcite. Then they will search for trace amounts of other chemicals.

They aren't sure that they will be able to provide a conclusive answer. Science doesn't always work that way. Wilson notes that the more than 150 samples of rock removed from Yucca so far have raised more questions than answers.

But the work is valuable no matter what it finds, the researchers believe. The complexity of the rocks at Yucca could reveal other geological finds, a boon for the science, Wilson said.

"In geology, you win either way," he said.

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