Questions multiply over Nevada nuclear-waste site

Discoveries raise fresh concerns about safety at Yucca Mountain, but

New research has raised more questions about whether Yucca Mountain - the controversial site slated to accept much of America's nuclear waste - is safe.

For decades, environmentalists, scientists, and government officials have been at odds over how dangerous it would be to store radioactive material in the Nevada mountain. These worries have already delayed the opening of the site - it was scheduled to begin accepting waste last year. And while these new findings shouldn't kill the program, scientists say, they could delay America's most ambitious waste-disposal project even further.

The new concerns center on three separate discoveries, discussed at a Boston meeting of the American Geophysical Union this week.

*Radioactive material can move through ground water in ways that Yucca site planners have not taken into account.

*Warmth generated by radioactive waste could heat the rock in the repository to the boiling point of water. The heating could affect the rock in a number of ways, and scientists have said they may never be able to fully understand the process.

*A Russian researcher has raised new uncertainty about the possibility of ground water rising from below to saturate the chamber. The chamber must remain dry to prevent waste containers from corroding.

Some of the new information has come from a study of 828 underground nuclear-weapons tests conducted at the nearby Nevada Test Site between 1956 and 1992. Scientists had known that plutonium, for example, could adhere to rock and, to a small extent, dissolve into water. But in a report published in the journal Nature in January, a team from the Lawrence Livermore and Los Alamos National Laboratories noted that microscopic particles called colloids can also carry away plutonium. Clay particles can pick up radioactive matter. Plutonium in the form of plutonium oxide also forms colloidal particles. And the research team found such plutonium in ground water leaving holes at the test site.

Annie Kersting, a team member, noted that the level of contamination is very low. But she says all modes by which radioactivity might migrate should be understood before opening the waste repository. So far, the team has not learned "as much as we need to learn" about this new method, Dr. Kersting said in Boston.

The same is true for understanding what radioactive heat may do to the waste-chamber rock. Paul Witherspoon, a Lawrence Livermore senior scientist, said the effect on the rock is a complex interaction of water, chemical processes, and stresses and movements within the rock. It's so intricate that scientists may have to give up on trying to fully understand the problem and try to establish whether or not enough is known to certify the site as safe, he says. Already, the uncertainty has sparked planning to separate the waste containers more widely than originally planned to reduce the heating.

If the site is used, the waste containers are made of material that will strongly resist corrosion so long as it is kept dry. Researchers have looked to the past to see if the site will remain dry for 10,000 years.

A team of US Geological Survey scientists said the chamber has been dry for millions of years. But Yuri Dublyansky, a Russian scientist, counters that his data "do not support" the USGS conclusions. He says his analysis of deposits of minerals indicates that ground water rose up into the waste repository region in the past.

The Department of Energy and State of Nevada have started a study to verify his data.

Trying to put the present situation in perspective, Allison Macfarlane from Harvard University's Kennedy School of Government said dealing with these technical issues now is complicated, but it "will help us better understand what is going on at Yucca Mountain." She added, "I don't think there's a need to rush."

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