Faults found at Nevada nuclear waste site.

Federal geologists have recently discovered several previously unknown faults slicing through Yucca Mountain in Nevada, the prospective site of the first high-level nuclear waste repository in the United States.

The implications of these findings remain unclear. Yet project opponents say the presence of such faults will hamper plans for storing the nation's most dangerous refuse within Yucca Mountain.

Since 1987, the Department of Energy has been studying this bald ridge 150 kilometers northwest of Las Vegas to determine whether it will make a suitable burial ground for spent radioactive fuel currently accumulating in overcrowded facilities at nuclear power plants around the country. If Yucca Mountain passes the assessment, DOE will then apply to the Nuclear Regulatory Commission for a license to build and operate a repository designed to hold 70,000 tons of waste.

At the earliest, Yucca Mountain would open in 2010. Federal regulations require that the repository keep waste from reaching the environment for 10,000 years.

Geologists have long known that at least one fault cuts through this region, but only in the last 2 years has the U.S. Geological Survey started an extremely fine-scale mapping program.

Last year, Richard W. Spengler with the USGS in Denver and his colleagues reported that the principal fault running through the proposed repository area actually has several parallel strands extending across a width of 215 meters. This feature, called the Ghost Dance fault zone, runs north-south, as do most of the faults around Yucca Mountain.

After working on the Ghost Dance fault, Spengler's group discovered a different zone of parallel faults that trend northwest-southeast through the planned waste site. They described the feature last month in a USGS report titled "The Sundance Fault: A Newly Recognized Shear Zone at Yucca Mountain, Nevada."

In early April, the Nuclear Regulatory Commission received a memo from its Advisory Committee on Nuclear Waste describing the faults. According to the memo, the presence of the Sundance fault could -- under certain conditions -- lead DOE to limit the size of the repository.

In its preliminary designs for Yucca Mountain, the Energy Department has indicated it will avoid placing waste directly within the Ghost Dance fault zone, which is believed to reach the depth of the repository, 300 m below the surface. If the Sundance fault zone also extends to this depth, DOE will have to decide whether to work around these faults as well.

At present, geologists do not know whether the Sundance and Ghost Dance faults have generated earthquakes within the last several million years. But even if these structures are not active, they may still threaten the storage facility. Because fractured rocks fill these faults, they could provide a path for groundwater to reach the repository, potentially speeding up the rate at which radionuclides leak into the environment. Faults could have the opposite effect, however, if they contain natural mineral cement that inhibits water flow.

"The basic question that remains unanswered is whether or not any of the faults are barriers to the transport of fluids and gases or whether they are conduits," says Spengler.

Energy Department crews will begin boring an 8-km-long exploratory tunnel into Yucca Mountain this August at the level of the planned repository. The tunnel, which will take 2 years to drill, will intersect the Ghost Dance and Sundance faults. Findings from this excavation and further work at the surface should answer questions about the significance of these faults, says Mark C. Tynan, a DOE geologist in Las Vegas.

Officials of the state of Nevada, which has opposed the Yucca Mountain project, say the fault discoveries will probably reduce the capacity of the repository. Robert Loux, the executive director of the Nevada Agency for Nuclear Projects, says that DOE would have to reduce the repository's volume by as much as 25 percent to avoid the Sundance and Ghost Dance faults.

Tynan calls such estimates ridiculous. "We don't anticipate that they are going to have a major impact at this time, but testing will continue to determine it," he says.