Japan accident causes concern about Yucca

Wednesday, Oct. 6, 1999 | noon

Nuclear scientists no longer believe radioactive waste buried in Yucca Mountain would explode, but if the waste produced a chain reaction similar to the one that caused a nuclear accident in Japan last week, experts aren't sure if or when radiation would escape.

As three Japanese nuclear plant workers remained hospitalized from massive radiation exposure, the Department of Energy and the Nuclear Regulatory Commission met in Las Vegas Tuesday to talk about whether broken, corroded or crushed waste containers in a proposed waste repository at Yucca Mountain could lead to such a chain reaction.

A nuclear chain reaction begins when loose neutrons that occur naturally in uranium hit other particles in the uranium, causing atoms to split. That releases huge amounts of energy called a criticality.

"Does criticality impact the long-term performance of the repository? That is what the commission wants to know," Nuclear Regulatory Commission expert Meraj Rahimi said during a break in the daylong technical session.

The DOE must convince the commission that its repository plan can withstand the test of time during licensing hearings that are as formal as a courtroom trial. At the moment Yucca Mountain, 90 miles northwest of Las Vegas, has not been recommended as a suitable nuclear waste site and licensing hearings are not expected to begin until after 2003.

About 30 scientists from both agencies weighed the different kinds of wastes that could go into Yucca Mountain, as well as the plans to protect buried containers and whether materials such as boron mixed with the waste could slow or stop a nuclear reaction.

By the end of the day the NRC's questions remained unanswered.

The Department of Energy, in light of the Japanese accident, wants to reassure the public that it will do everything it can to rule out the possibility of a nuclear chain reaction, DOE's leader on the subject, Paige Russell, said.

"It is a complex question," she said.

In the DOE's current planning for the proposed repository, the agency is not considering the possibility of a volcano or earthquake at the site, DOE official Peter Gottlieb. DOE officials say the possibility is too slight to worry about.

But the NRC -- given the new concern about a nuclear chain reaction -- now wants the Energy Department to calculate what would happen in the unlikely event that an earthquake or a volcano shook the repository 1,000 feet beneath the surface of the mountain.

One NRC scenario shows that such a major disruption of the site could break open a waste container and allow the nuclear material to pool in a pit on the floor of the repository. If that were followed by ground water flooding the area, a nuclear chain reaction could begin. Water allows the nuclear materials to begin reacting with one another.

A scenario for the proposed Yucca Mountain repository in 1995 theorized that such an accident would create a nuclear explosion. The scenario, developed by physicists Charles Bowman and Francesco Venneri of the DOE's Los Alamos National Laboratory in Albuquerque, N.M., has since been abandoned as unlikely.

But new work at the University of California, Berkeley, has captured the DOE's attention.

Ten papers have been published by a team of Berkeley scientists about nuclear wastes concentrated in fractures that could produce a nuclear chain reaction as ground water flows through the mountain.

After analyzing the chemistry of Yucca Mountain's rocks, however, DOE scientists believe that not enough plutonium could concentrate in the cracks to cause a chain reaction, Gottlieb said. Both uranium and plutonium would be buried in the mountain.

In fact the nuclear wastes from commercial reactors cannot support a chain reaction in a Yucca Mountain repository, Gottlieb said.

However, other DOE waste and Defense Department reactor wastes that would go into the repository -- which are still kept secret -- need to be considered before the DOE can close the issue.