How safe is safe?

How well will Nevada's Yucca Mountain perform as a long-term storage site for the nation's most radioactive waste?

The US Department of Energy offered a multivolume answer to that question three years ago. But to a National Research Council team that reviewed the findings, that response was hardly reassuring. The group found shortcomings in the study that rendered it "unlikely" that it described the site's probable long-term behavior.

But the team also noted a more troubling issue for those looking to science for absolute answers on the site's suitability. Assessing Yucca Mountain's performance, they said, "may be beyond the analytical capabilities of any scientific and engineering team." The reason: It's too hard to predict what will happen over tens of thousands of years, given the diverse processes Â- such as volcanic activity, seepage, or changes in rock chemistry Â- at the site.

The prospect of turning Yucca Mountain into a nuclear-waste dump site has long been controversial. But the issue reared up again earlier this month when Nevada Gov. Kenny Guin vetoed President Bush's selection of the site as the nation's high-level radioactive waste repository. The April 8 veto triggered a 90-day period during which Congress must override the veto if the project is to move to its next stage.

In the process, lawmakers will help define the role science plays in shaping public policy. The tension between many politicians' quest for "science-based" decisions versus many scientists' hope that their work will merely inform political decisions, is playing itself out not only in regard to nuclear waste but in other environmental issues such as climate change and drinking-water standards.

In the case of Yucca Mountain, the science and engineering studies are being conducted against the backdrop of political pressure on Congress to act. The pressure is coming from the nuclear industry and the states where nuclear plants are located. They are pressing the federal government to fulfill its decades-old promise to take possession of the waste. Nuclear utilities have paid billions of dollars into a fund to pay for a permanent solution to the problem.

"Politicians say that decisions on Yucca Mountain should be made based on science," says Kevin Crowley, director of the National Research Council's radioactive-waste management board. "I agree that decisions should be informed by science.... But in the end, it's our elected officials who must make a judgment on how much uncertainty is acceptable. What is 'safe' and 'When do we have enough information?' are questions science cannot answer."

If the Yucca Mountain decision were left to scientists, a number of them would say the heap of solidified volcanic ash is not ready for prime time.

"The state of the science is not adequate" for making a decision on placing waste there, says Allison Macfarlane, a professor in the security studies program at the Massachusetts Institute of Technology in Cambridge.

This may seem like an odd statement after nearly 20 years of study at a cost of some $7 billion. But even senior Energy Department advisers on the project have acknowledged that, as time has passed, the site has looked less and less ideal.

As if to underscore the point, a team of three vulcanologists argues that the recurrence rate for eruptions at Crater Flat, a volcano about 10 miles southwest of Yucca, may be understated.

Writing in the current issue of GSA Today, a publication of the Geological Society of America, the trio notes that until now, the Crater Flat area has been treated as a volcanic zone isolated from similar groups of cones that form a rough line from the California state line to the Lunar Crater volcanic field northeast of Yucca Mountain. But "Yucca Mountain is not isolated. It's part of a very large volcanic field" that appears to be on a broad common eruption cycle, says Eugene Smith, the University of Nevada geology professor who headed the team.

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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