Controversy simmers at atomic-waste site.

Deep within the parched landscape of southwest Nevada, scientists are analyzing the geologic personality of an unassuming ridge called Yucca Mountain. At issue is whether the bald, elongated promontory has a stable character--steadfast enough to house the highly radioactive waste generated by nuclear power plants across the United States. The range must lock up this hot debris for the next 10,000 years.

During the past 15 years, hundreds of geologists have crawled over Yucca Mountain, making it the best-studied piece of real estate on the planet. Recently, however, a debate has erupted over some curious events discovered in the mountain's past that could signal an underlying restlessness in its constitution.

"The implications of this finding can be very serious for the [planned] repository," says Yuri V. Dublyansky, a Russian geologist studying Yucca mountain under contract with the state of Nevada, which opposes the repository.

Scientists with the U.S. Geologic Survey counter that the unruly behavior was confined to Yucca Mountain's infancy, millions of years ago, and has no bearing on its current character. The Department of Energy (DOE), which oversees the investigation into Yucca Mountain, is now conducting an independent review of Dublyansky's controversial findings, hoping to resolve the scientific wrangling. The two sides discussed their work last week at a meeting of the American Geophysical Union in Boston.

Yucca Mountain is the only site currently under consideration as a repository for spent nuclear fuel. DOE began studying the ridge in the mid-1980s and had intended to open the facility by 1998. Although it has yet to finish assessing the site, DOE last December issued a report concluding that no "showstopper" had emerged in its studies to date.

The plans call for canisters of nuclear waste to reside in the mountain's heart, within rooms cut out of the volcanic rock formations 300 meters below the summit. This would keep the waste hundreds of meters above the water table, preventing the radioactive elements from leaking quickly into the groundwater.

The current debate centers on the history of water within the mountain. Dublyansky, a researcher at the Institute of Mineralogy and Petrology in Novosibirsk, Russia, claims that hot brines have surged upward in geologically recent times, reaching a level that would flood the repository.

His evidence comes from microscopic pockets of water and air, or inclusions, within calcite growing along cracks and cavities. The inclusions serve as tiny thermometers, says Dublyansky. Heating them causes the fluid to expand and the air to dissolve. The point at which the bubbles disappear indicates the temperature of the fluids that formed the crust.

Such tests suggest that the fluid temperatures ranged from 35 [degrees] to 80 [degrees] C, says Dublyansky. The only possible source of such hot water, he says, lies deeper in the crust. Earthquakes or other geologic instability must have forced the warm water upward hundreds of meters, and such events have occurred repeatedly to build up the calcite layers, he claims.

The USGS researchers dispute Dublyansky's conclusions. They have studied the calcite and other crusts by analyzing the ratio of two different oxygen isotopes in the minerals. They have also dated the deposits using the radioactive decay of natural uranium in the minerals.

The isotopic thermometers, they say, record high temperatures only early in the history of the Yucca Mountain rocks, which formed during eruptions more than 10 million years ago. The layers created by such hot fluids "were likely deposited shortly after the cessation of volcanic activity in the region," says James B. Paces of the USGS in Denver.

Paces and his colleagues contend that the crusts grew from rainwater percolating down through the mountain, rather than from upwelling hot fluids. They note that mineral crusts appear only on the bottom face of sloping cracks and not on the upper face or on the roofs of cavities. Below the current water table, mineral growths coat all surfaces of fractures and cavities, Paces says.

"We feel very comfortable with the conclusions that we've drawn," says Joseph F. Whelan of the USGS. "But the potential consequences of Dublyansky's hypotheses are serious enough that it definitely needs to be resolved." Dublyanksy, the USGS, and the University of Nevada are now conducting independent tests on inclusions to assess Yucca Mountain's true nature.