The Environmental Protection Agency (EPA) health standard for the proposed nuclear waste repository at Yucca Mountain should be based on limiting risks to individuals who live and work nearby, rather than limiting risk for one maximally exposed individual, concluded a report released in August by a committee of the National Academy of Sciences' National Research Council.

One scientist on the committee, Thomas H. Pigford, dissented on some of the report's findings. In a recent interview, Pigford said that the methods and assumptions the panel proposed to estimate radiation exposure scenarios for Yucca Mountain were flawed, unscientific, and much less stringent than what has been traditionally used in the United States and other countries.

Pigford says that the committee's approach would greatly underestimate the actual cancer risk in the immediate vicinity of Yucca Mountain. He believes that about 3,000 times more radioactivity would be allowed to be released under the committee's health standard than by the one he advocates.

Pigford, who has served as a professor of nuclear engineering at Berkeley since 1959, said he considers the councils' recommendations "an unjustified departure" from traditional safety standards for repositories.

In his dissent to the report, Pigford emphasized that the subsistence farmer exposure scenario, in which a farmer relies on contaminated groundwater for drinking water and to raise his own crops for food, is the most conservative scenario for estimating long-term dose and risk predictions, and offers the public the highest level of safety.

"For years and years, the U.S. and other countries have used the subsistence farmer approach when we don't know how people will behave. The Waste Isolation Pilot Plant (a repository for low-level defense waste in New Mexico) is using this approach, so are Sweden, Switzerland, England, and Canada. Why did the committee choose to depart from the traditional approach?"

But for Robert Fri, chairman of the Committee on the Technical Bases for Yucca Mountain Standards, said the subsistence farmer scenario was not at issue.

"In my own view, the issue boils down to do you assume that this person (farmer) is going to drill a well into the area where the radioactive nuclide component is highest? The majority view of the committee said no."

Fri said there is currently much debate in the National Academy of Sciences about whether a worst-case scenario is reasonable considering the scenarios that sometimes play out. "The trouble that everybody is dealing with is that for the long term you can come up with scenarios that either pass or fail the project."

"The Environmental Protection Agency said there were concerns about using the theoretical upper bound (the worst-case scenario). Many of the group felt this scenario was more reasonable than the worst-case approach," Fri said in a phone interview.

Pigford says the panel turned away from the subsistence farmer scenario after early documentation by Yucca Mountain contractors applying the traditional subsistence farmer method showed that much higher radiation doses would be released from the proposed repository than are now acceptable, a dose of about 1 rem/year.

"All of them showed doses much, much higher than would normally by acceptable (10-30 millirems per year)," said Pigford.

By Pigford's calculations, the probability of cancer based on the 1 rem/year dose would amount to "one chance out of 30 for a person living in the area to develop cancer during a lifetime."

Once those kinds of dose rates were shown, Pigford said, discussion on the panel turned to what effect the dose standard would have on the Yucca Mountain project.

In the report, the committee acknowledged that there was no scientific basis for predicting where future people will live or how they will act. Nevertheless, after hearing from a contractor with the Electric Power Research Institute (EPRI), the committee decided to adopt the probabilistic approach. For the purposes of illustration, the EPRI contractor assumed that most residents would live only part-time in the vicinity of Yucca Mountain, would rely manly on outside food sources, that contaminated water would seldom be used for farming, and that residents would seldom dig wells into the contaminated groundwater.

"There's no scientific basis for these assumptions," says Pigford, "but the panel adopted the EPRI probabilistic concept."

Although the committee did not define the critical group that would be affected by releases, leaving that to the EPA as a policy making decision, it did lay out a calculation method to determine who the critically affected group would be. Despite the fact that subsistence farmers now live in the Amargosa Valley, the committee's stipulations offer no assurances that this maximally dosed individual would even be included in the critically affected group.

You can't do that with people who will live 10,000 to 100,000 years from now," Pigford said. "You don't know what they will do. This is an unscientific method under the guise of policy."

I've written 15 National Academy of Sciences reports, and I've never before written a dissent. But on this one I was so shocked, I had to do it."

If the EPRI assumptions are adopted, Yucca Mountain will easily meet the health standard, said Pigford, even though actual cancer risk will be much higher than what the standard depicts.

The committee's report does not describe the early data and dose rates that resulted, nor does it state why the subsistence farmer approach was not used, but does make the following statement:

"Although not a purely scientific issue, we believe that a reasonable and practicable objective is to protect the vast majority of members of the public while also ensuring that the decision on the acceptability of a repository is not prejudiced by the risks imposed on a very small number of individuals with unusual habits or sensitivities."