Volcanoes wild card in Yucca equation

By SCOTT R. BURNELL, UPI Science News   |   April 16, 2002 at 9:59 AM

WASHINGTON, April 16 (UPI) -- (Editor's note: This is the first article in a four-part series from United Press International examining some of the scientific issues related to using Nevada's Yucca Mountain as a nuclear waste repository site. Congress has started a 90-legislative-business-day period where it must vote to override the state's objections to continue the project. The House Energy and Commerce Committee is to hold a hearing on the project on April 18.)


A new study has refocused attention on possible volcanic activity near the proposed Yucca Mountain nuclear waste storage site -- though the most heated arguments still revolve around water, the factor most likely to bring radioactive material into the surrounding environment.

The lead article in the current issue of GSA Today, the Geological Society of America's monthly journal, suggests the need for revising upward the Department of Energy's calculations of ongoing risk for volcanic activity at Yucca. The work was funded primarily by the Nevada Agency for Nuclear Projects, an avowed opponent of the proposed repository.

Authors Eugene Smith and Deborah Keenan, both with the Geoscience Department at the University of Nevada at Las Vegas, along with Terry Plank of Boston University's Department of Earth Sciences, reviewed earlier studies of eruptions in Nevada and the possible makeup of the crust under the area. Based on a possible pattern of activity and the likelihood of Yucca-area volcanoes being part of a larger system, they said the area's current "quiet period" of several hundred thousand years could end during the repository's expected 10,000-year existence.

Such a possibility makes volcanoes and tectonic activity a double-edged sword for Yucca, posing a threat of releasing nuclear waste that must be considered. Yet, they also are partly responsible for putting the 9,000-foot hill atop the scientific list of suitable sites for long-term waste storage.

Eruptions some 12 million years ago laid down layers of alternating porous and densely welded volcanic ash that today are several thousand feet thick, creating a natural barrier to water passage. Movement of the North American, Pacific and other crustal plates both created the mountain ranges that help keep the surrounding area one of the driest in the world and helped make the ash layers more accessible.

Not all geologists agree with the conclusions contained in the GSA Today article. William Dickinson, professor emeritus of geology at the University of Arizona, who has studied the Southwest's geology for the past 50 years, believes they amount to guesswork.

"Volcanic eruptions don't come on a regular schedule, so it's impossible to be very definite about when (one might happen) or where the next one will be," Dickinson told United Press International. "Most of those eruptions (that occurred near Yucca in recent geologic times) are what geologists call monogenetic, so they're not necessarily going to erupt in the same place. They're not recurrently active volcanoes, but one-shot deals."

Yucca Mountain lies within a seismic belt that has generated several large earthquakes in the past 100 years, said Richard Bennett, who studies the movement and stresses of pieces of the Earth's crust at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The same uncertainties involved with volcanoes apply to earthquakes, he told UPI.

"If a fault zone hasn't produced an earthquake in some time, does that indicate that it has accumulated a large amount of strain that's likely to be recovered in a future earthquake, or does that mean it's dead?" Bennett said. "That's one of the questions we try to answer with (our studies)."

The proposed repository would lie far enough underground to basically be unaffected by seismic waves from an earthquake, both Dickinson and Bennett said. But the shifting of strain from a quake could force groundwater to move more rapidly for a time, according to Bennett.

Volcanic or seismic risks aside, water remains the most troubling thing to consider about long-term storage of nuclear waste. As the most prevalent solvent on Earth, water is capable of degrading even anti-corrosive alloys over thousands of years. As a transport mechanism, it is the best candidate for carrying waste particles from degraded containers into the area's groundwater system.

Yucca is situated close to Death Valley, the driest place in the United States, and the mountain itself gets less than 10 inches of rain per year. Groundwater is capable, however, of traveling the 1,000 feet or so downward to the repository level in about 50 years, said Robert Loux, NANP's executive director.

"We know this because there are isotopes from the Pacific weapons testing programs that show up in the tunnels," Loux told a group of reporters recently.

Those estimates are based on mapped geologic faults and related fracture areas, said Patrick Rowe, an engineer at DOE's Yucca Mountain Project. The figures bear no relation to how the water could affect the storage areas, he said.

"There are no major faults in the repository block," Rowe told UPI in an e-mail. "Water traveling in these faults will tend to stay within in the (path of least resistance offered by) faults and fractures rather than enter into the host rock and (storage tunnels) and be available for radionucleotide transport."

It is reasonable to assume water will stay in whatever small cracks and crevices exist, said Brenda Ekwurzel, a professor of hydrology at the University of Arizona.

"Especially in an arid environment, unless you have a large enough volume of water coalescing to create pressure, it would tend to stay coating the surfaces of the grains and fractures," Ekwurzel told UPI.

Rowe believes geology has handed Yucca another barrier to the spread of any possible contamination. Between the repository level and the water table about 1,000 feet below lies a layer of sediment containing crystals called zeolites. These act as a natural filter for radioactive particles, he said.

Another water-related phenomenon to consider is the heat the waste would generate for decades or even hundreds of years as it sits in its storage casks. The radiated heat would drive the temperature of the surrounding rock well past the boiling point of water; DOE currently is studying how long the rock would retain that heat once the casks cool off, Rowe said.

The thermal zones around storage tunnels would drive off any water already existing in the rock. It remains to be seen how the water would react once it reached cooler areas, Ekwurzel said. In some instances, desert soils are so dry water evaporates upward, she said.

"If the water condenses again, and the tunnels are far enough apart that you have a cool zone between them, you're going to have water from both areas coalescing there," she said. "If (the tunnels) are far enough apart it would just go vertically down below the waste packages. I don't know if you can make a case where the water would necessarily hang around and then suddenly fall back on the waste packages."

The arguments offered by Yucca opponents have focused on areas other than zeolites and heat effects.

What happens to deep groundwater in the Yucca area is also a matter of intense discussion. Drainage at the surface is controlled by encircling high ground. The area has been considered a "closed hydrologic zone" that no groundwater escapes from, Dickinson said. Such zones exist throughout the Southwest, including the one responsible for Utah's Great Salt Lake, he said.

The DOE's models for groundwater flow past the repository show the water would not travel farther than the Armagosa Valley/Death Valley Junction farming areas about 20 miles from Yucca Mountain.

The matter is not closed, however, according to Ekwurzwl. Geologists currently are debating whether any extremely deep aquifers might leave the Yucca region, she said.

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