Debate has broken out among federal scientists over whether the planned underground dump for the nation's high-level atomic wastes in Nevada might erupt in a nuclear explosion, scattering radioactivity to the winds or into ground water or both.
The debate, set off by scientists at the Los Alamos National LTC Laboratory in New Mexico, is the latest blow to the planned repository deep below Yucca Mountain in the desert about 100 miles northwest of Las Vegas.
Opponents of nuclear power and Nevada officials have long assailed the project as ill-conceived and ill-managed, and it has encountered numerous delays.
Even if scientists can debunk the new argument that buried waste at Yucca Mountain might eventually explode, the existence of so serious a dispute so late in the planning process might cripple the plan or even kill it.
Planning for the repository began eight years ago, and studies of its feasibility have so far cost more than $1.7 billion. The federal government wants to open the repository in 2010 as a permanent solution to the problem of disposing of wastes from nuclear power plants and from the production of nuclear warheads.
The possibility that buried wastes might detonate in a nuclear explosion was raised privately last year by Dr. Charles D. Bowman and Dr. Francesco Venneri, both physicists at Los Alamos, the birthplace of the atomic bomb. In response, lab managers formed three teams with a total of 30 scientists to investigate the idea and, if possible, disprove it.
While uncovering many problems with the thesis, the teams were unable to lay it to rest, lab officials say. So the lab is now making the dispute public in scientific papers and is considering having it aired at large scientific meetings as well.
"If we knew how to put the stake through its heart, we'd do it," Dr. John C. Browne, head of energy research at the lab, said in an interview. Going further, some panel members said they felt that the new thesis had been refuted.
Mr. Bowman, the idea's chief advocate, said the internal debate had changed some elements of the thesis but over all had left it honed and strengthened.
"We think there's a generic problem with putting fissile materials underground," he said, referring to substances that fission, or split apart, in a nuclear chain reaction.
The few scientists outside the laboratory who have become aware of the debate say the thesis is provocative and probably wrong. Nonetheless, they say, the idea cannot be swept under )) the rug.
Highly radioactive wastes are the main orphan of the nuclear era, having found no permanent home over the decades. In theory, if the Yucca plan wins approval after a careful study of the area's geology, a labyrinth of bunkers carved beneath the mountain would hold thousands of steel canisters for 10,000 years, until radioactive decay rendered the wastes less hazardous.
The spent fuel from nuclear reactors is permeated with plutonium, which is a main ingredient used in making nuclear bombs.
Since plutonium 239 has a half-life of 24,360 years, significant amounts of it would remain active for more than 50,000 years, long after the steel canisters that once held the radioactive material had dissolved. (A radioactive substance's half-life is the period required for the disintegration of half of its atoms.)
With the end of the Cold War, the Nevada site has increasingly been studied for a possible added role as a repository for the plutonium from scrapped nuclear arms.
In January 1994, the National Academy of Sciences, which advises the federal government, suggested that the plutonium be mixed with highly radioactive wastes and buried, or burned in reactors and then buried. In either case, some plutonium would end up going underground.
On Wednesday, President Clinton, trying to win a permanent global ban on the spread of nuclear arms, ordered substantial cuts in U.S. stockpiles of weapons plutonium but did not say what would become of the deadly substance. Officials said it would remain in temporary storage above ground until a decision was made on its ultimate disposition.
The scientist leading the charge against the burial of fissile materials, Dr. Bowman, has an alternative plan in which particle accelerators would, by a kind of nuclear alchemy, transmute radioactive wastes, as well as plutonium, into more benign elements before they were buried. Dr. Bowman is the head of the planning effort for the proposed project.
Last summer and fall, Dr. Bowman began talking of the dangers of underground storage and was urged to set them down in an internal Los Alamos report, which he did by November.
The crux of his argument was that serious dangers would arise thousands of years from now after the steel canisters dissolved and plutonium slowly began to disperse into surrounding rock.
The rocky material, he said, could aid the start of a chain reaction by slowing down speeding subatomic particles known as neutrons that fly out of plutonium atoms undergoing spontaneous decay. Neutrons of a certain speed can act like bullets to split atoms in two in a burst of nuclear energy.
Under some circumstances, Dr. Bowman theorized, the slowing of the neutrons could make an individual pile of plutonium explode in a nuclear blast equal in force to about a thousand tons of high explosive, setting off other blasts throughout the vast repository.
The team assembled to review the thesis concluded that it held serious flaws, said Dr. Browne of Los Alamos. For one, dispersal of plutonium, if it happened at all, would take much longer than envisioned -- so long that the plutonium would have mostly decayed. The review team also felt that if a plutonium pile did begin to heat up, the reaction would automatically slow down and stop as the heat made the pile expand.
"The burden of proof rests on Charlie," said Dr. Browne, referring to Dr. Bowman. "He's hypothesized some scenarios that, if correct, are clearly very important. In spite of the fact that there is a sizable amount of opposition to Charlie's paper, our feeling is that the subject is so important that it deserves additional peer review outside the laboratory."