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Building a nuclear bomb takes so much less plutonium or uranium than generally believed that new safeguards must be adopted as part of a global tightening of defenses against the criminal diversion of atomic materials, private experts argue in a new proposal.

For plutonium, the experts say the official threshold of danger should be lowered from 8 kilograms to 1 kilogram, or from 17.6 pounds to 2.2 pounds. They also propose eightfold reductions for uranium, the other main fuel of atomic bombs.The experts, from the Natural Resources Defense Council, a private group known for its nuclear expertise, wrote the federal government and the United Nations last week to urge such downward revisions.

At a news conference Monday in Washington, the group is to make those letters public along with a report arguing for the proposed changes.

If adopted, the plan would result in more stringent safeguards meant to curb the spread of bombs.

The new proposals cast a harsh light on the recent seizures in Germany of atomic materials that are believed to have been smuggled out of Russia, making the amounts look quite serious. One deal broken up by German authorities was reportedly to have ultimately involved 4 kilograms of weapon-usable plutonium in exchange for $250 million.

Thomas B. Cochran, a senior scientist at the Natural Resources Defense Council and a co-author of the report, said, "The criteria now in use are out of date, technically erroneous and clearly dangerous in light of the recent seizures."

Resistance to the proposals is likely, however, because enhanced safeguards could be costly to enforce and might hamper or cripple some use of plutonium for the production of nuclear power overseas.

The plan is also controversial because the minimal amounts of material needed to make a bomb have long been classified top-secret.

Even so, arms experts said they welcomed the airing of the issue by the council, which favors strict limits on nuclear arms and materials. Many experts said the old rules were dangerously out of date, even if the new proposals were perhaps too restrictive.

"It's important to have this debate," Energy Secretary Hazel O'Leary said in an interview. "Any number of people have the impression that the smaller the size under control, the better we are in the long term." Her agency oversees the nation's nuclear arsenal and plays a central role in limiting the spread of bombs.

Richard L. Garwin, a physicist who has long advised the federal government on nuclear-arms matters, suggested that revisions should fall somewhere between the old rules and the new proposals. "Clearly," he said, "the significant quantity of plutonium should be lowered, at least to 4 kilograms and perhaps somewhat less, if it is to represent the amount that is hazardous."

William J. Quirk, a nuclear-weapon expert at the Lawrence Livermore National Laboratory in California, said that for the moment the exact figure was less important than a public discussion.

"Whether the right number is four or one or 2 kilograms, it probably makes sense to lower the current number if it's feasible economically," he said. "I think it's the right thing to start the debate and then people can decide what they want to do."

Building a nuclear bomb is well known to get progressively difficult as smaller amounts of fissionable material are used, the smallest requiring great expertise and special gear thought to belong only to advanced nuclear states. In addition, the size of the resulting blast decreases.

Nevertheless, 50 years of experimentation have allowed the United States and other nuclear nations to put powerful bombs in remarkably small packages.

In the early 1960s, the United States stockpiled a bazooka-type weapon known as the Davy Crockett whose miniature atomic warhead weighed 51 pounds and had an explosive force equal to 22 tons of high explosive. At its core, the plutonium probably weighed several pounds.

In the 1950s and 1960s, amid a push for nuclear power and peaceful atomic industries, the United States championed an international system whereby safeguards would be applied to the handling and shipment of all kinds of nuclear materials to guard against diversions.

The safeguards were enforced by an arm of the United Nations known as the International Atomic Energy Agency, or IAEA, based in Vienna.

These safeguards in retrospect seem weak. Weapon experts say the rules were probably skewed by disinformation aimed at convincing aspiring nuclear states and terrorists that bomb-building was difficult, and by a desire to keep international controls loose enough to allow the rise of a robust nuclear industry.

The IAEA says the approximate amounts of fissionable material needed for a single nuclear weapon are 8 kilograms of plutonium, 8 kilograms of uranium 233 or 25 kilograms for uranium highly enriched in the 235 isotope.

These figures, known as threshold amounts or significant quantities, are used to establish a wide range of industrial safeguards meant to deter and detect the diversion of materials from peaceful purposes to the making of nuclear warheads.

Thus the Natural Resources Defense Council is proposing the eightfold reduction in these categories.

Cochran, who is a physicist, said in an interview that his organization's proposed revision of the traditional figures was based on his personal calculations, a close reading of government documents and statements, and discussions with weapon experts, who spoke on the condition of anonymity.

Based on his research and calculations, Cochran said, 1 kilogram of plutonium can be fashioned by a skilled designer into a bomb with a blast equal to 1,000 tons of high explosive.

"Detonated in or above a city center, one such `small' weapon would be sufficient to cause severe blast damage over roughly a 40-block area, and many thousands more would likely die from the ensuing fire and radiation effects," said Christopher E. Paine, a co-author of the council's report.

In comparison, the bomb that the United States dropped on Hiroshima in 1945 had a blast equal to about 15,000 tons of high explosive.