How safe is safe?

The Yucca Mountain area, he speculates, could be nearing the end of a relatively quiet period, and could see more eruptions in the future. At the least, he says, "you have to consider the entire field" in assessing eruption hazards near Yucca Mountain.

Earthquakes have been another concern. Yucca Mountain is laced with faults, as is the region. In 1992, a magnitude 5.6 quake struck not far from Yucca Mountain. The quake registered more than 2,000 aftershocks. Some observers are concerned about how quakes might open or redirect paths by which water seeps through the repository and into the aquifer below.

This could affect rock chemistry and lead to corrosion in the repository. Although the waste canisters are being designed using corrosion-resistant alloys, experts expect corrosion and leakage of radioactive material. Although the water couldn't become radioactive, researchers say, radioactive-waste particles can attach themselves to other small particles in the water and be transported to the aquifer.

Indeed, the simulations designed to mimic the flow of water through unsaturated rock "do not fit reality at all," says Mary Lou Zoback, of the US Geological Survey in Menlo Park, Calif.

Estimates of seepage rates are also being questioned. "Water has moved through the mountain faster than anticipated," notes Rodney Ewing, professor of nuclear engineering and radiological science at the University of Michigan at Ann Arbor. He cites a study that detected an isotope of chlorine typically attributed to above-ground nuclear tests in water that had seeped through the rock to the level at which the repository would be located. The implication is that surface water may take only tens of years, not hundreds or thousands, to reach waste-storage casks.

He acknowledges that finding a little bit of the isotope doesn't necessarily mean that large amounts of water move quickly through the rock from the surface to the aquifer. But, he continues, "we now have the problem of trying to estimate what this signal ... will mean."

Dr. Ewing says doing that will be difficult. "To model such a complicated system over 10,000 years is quite an undertaking," he says. "The challenge for modeling Yucca Mountain is equal to ... modeling climate change."

To its credit, researchers say, the Energy Department appears to be heeding some of their advice. For example, the department has begun thinking in terms of building the repository in stages, with each new stage contingent on an improved scientific understanding of the repository's likely performance. And it appears to be open to using some of the site's natural assets to the repository's advantage, rather than finding engineering solutions that buck the natural structure and system.

Many observers still see gaping holes in the federal government's scientific foundation for picking Yucca Mountain as a repository. But storing waste in a central underground geological formation remains their preferred approach. Storing spent fuel in storage pools or in dry casks on premises of some 100 nuclear plants around the country instead, is a nonstarter.

"We really need to proceed with this experiment," says Dr. Zoback. To reject Yucca Mountain "means that we think it's safe to leave the waste in above-ground pools for however long at 77 distributed sites" around the country. "No one has done the real analysis of the risk that we're accepting as a society by allowing that condition to exist."

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