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Radioactive wastes are likely to pose serious exposure risks long after the 10,000 years for which the government now calculates risk standards, a panel of eminent scientists says.

The committee of the National Academy of Sciences concluded this week that "there is no scientific basis for limiting the time period . . . in this way.""At least some potentially important exposures might not occur until after several hundred thousand years," and risks from possible leaks into groundwater probably will be greatest long after the 10,000 year period, the panel said.

It added that risks from contamination from a repository may be greatest 50,000 to 250,000 years into the future, long after man-made containers holding wastes have disintegrated.

Scientists rely on natural geology to limit movement of radioactivity in the later years.

The Energy Department is trying to determine whether the Yucca Mountain location in the Nevada desert is suitable to hold more than 70,000 tons of high-level nuclear wastes, most of it used fuel from 104 commercial nuclear power plants.

The government hopes to have the facility ready by 2010 at the earliest.

The 15-member panel of the Academy's National Research Council was asked by Congress in 1992 to make recommendations to the Environmental Protection Agency on how to establish health standards for the Yucca Mountain site.

The academy's report concluded that adequate scientific risk standards are possible, but they "should be designed to protect individuals in the immediate vicinity of the facility."

As a result, the global population also would be protected "as radiation from the repository would pose a much lesser risk to people distant from the site," said Robert Fri, president of Resources for the Future and chairman of the scientific panel.

In noting the long-standing radioactivity that will be present - hundreds of thousands of years for some radioactive elements - the panel alluded to the complexities faced by government regulators.

For example, the report said, to determine risks faced by individuals near the site, "it will be necessary to define hypothetical persons by making assumptions about lifestyle, locations, eating habits and other factors" hundreds of years in the future.

The panel recommended that health and safety standards be developed that are based on actual risks faced by nearby populations, instead of arbitrarily placing limits on allowable radioactive releases.

Dissenting from the rest of the panel, Dr. Thomas Pigford said in an interview that such an approach "cannot be defended on grounds of public health protection."

Pigford, professor emeritus at the University of California-Berkeley, called the panel's approach to risk assessment "speculative" and said it could allow for increased contamination of groundwater.

Most current radiation exposure standards are not based specifically on what danger they pose to a nearby population, but on the amount of radiation released and what effect that might have on a hypothetical individual facing the maximum possible exposure.