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January 25, 2000

Scientists Find Plutonium Has Aging Problem


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    By WILLIAM J. BROAD

    Russian scientists have discovered that weapons-grade plutonium is far more unstable in form than previously suspected, a finding that could have implications for the aging and reliability of America's arsenal of 10,000 or so nuclear warheads.

    American scientists who recently learned of the Russian discovery say they did so in the course of an exchange program. But such teamwork has been curtailed by recent espionage fears, which American scientists fault as exaggerating the risks of cooperation and ignoring benefits like the plutonium insight.

    The Russian disclosure has prompted a number of new American studies of plutonium storage and aging meant to pinpoint the potential hazards.

    "We're changing our experimental and theoretical approach to take this information into account," Dr. Siegfried S. Hecker, a leader in Russian cooperation at the Los Alamos National Laboratory in New Mexico, said in an interview. "We're doing things today that we had not planned to do one year ago."

    Dr. Edward F. Hammel, a plutonium pioneer at Los Alamos, told a laboratory forum last month that the Russian insight could shake up the way nuclear weapons were made, handled, repaired and stored.

    "The implications for the stockpile are immense," Dr. Hammel said, adding that the exchange visits with foreign scientists had produced "tremendous benefits."

    Plutonium is one of the most complex and mysterious elements known to science. Only minute traces of it exist in nature and most has been made artificially, first in 1940, so plutonium studies span little more than half a century.

    The silvery element yellows when exposed to the air and is warm to the touch because of energy released through alpha-particle decay, a kind of radiation. Its complexity stems in part from its large number of stable forms, or allotropes. Carbon has three -- diamond, graphite and amorphous. But plutonium exists in six structural forms, all with differing densities and volumes, that vary according to temperature. They range from jumbled crystals typical of minerals to the highly ordered ones of metals, the latter allotrope being the form used in weapons. But keeping it in the metallic state can be tricky.

    "The ease with which plutonium transforms itself from one crystallographic phase to another is exasperating," Dr. Hecker and Dr. Joseph C. Martz, his Los Alamos colleague, wrote recently. "The resultant phases have such radically different properties."

    Added to such fickleness, plutonium is highly reactive with other metals and materials, especially oxygen and hydrogen. That can make it difficult to cast and manipulate.

    During the Manhattan Project, scientists at Los Alamos found that plutonium behaved more reliably if alloyed with elements like aluminum and gallium. During cooling, these helped the element stay in its most metallic allotrope, known as delta phase. Such alloys of plutonium behaved like normal metals, being relatively plastic, compressible and resistant to corrosion. Most important for making weapons, they were easy to cast and work into special shapes because they had little or no brittleness.

    For decades, the mandarins of American science assumed that the delta phase of plutonium alloyed with gallium was rock stable, a kind of Gibraltar on which the American nuclear arsenal could be erected and expected to weather centuries of storms and service.

    Hints of Russian disagreement first emerged in 1975. While the Americans held that the delta phase of plutonium mixed with gallium was in a stable equilibrium at room temperature, the Russians found that it was poised to decompose to the alpha form, which is more brittle than ductile. And it is far denser, with a smaller volume, raising the threat of ruined mechanical assemblies and perhaps changes in the critical mass needed to start a nuclear chain reaction.

    "The Russian work was not accepted in the West because insufficient detail was presented about the precise nature of the experiments," Dr. Hecker and Dr. Martz wrote in a manuscript for a forthcoming book.

    But that changed last year at an international conference in Oxford, England. The basis for the disclosure, Dr. Hecker said, was years of exchanges after the cold war between American and Russian nuclear arms designers, with the contacts becoming increasingly cordial. At the meeting, Dr. Lydia T. Timofeeva, a Russian metallurgist, laid out the experimental basis for the plutonium insight for the first time, dazzling American scientists with its care and precision.

    Dr. Matthew G. McKinzie, a former Los Alamos researcher now at the Natural Resources Defense Council, a group in Washington that tracks nuclear arms, said Russia's stockpiles of plutonium were older and larger than those of the United States, aiding such studies.

    Plutonium instability, he said, could sharply cut the lifetime of weapon cores, in theory reducing them from perhaps 70 years to as little as 20 years. Rapid aging, he added, could also mean the United States would have to begin remanufacturing its nuclear arms sooner than expected, increasing the effort's scope and cost.

    "The rate of aging has a tremendous impact on things like the size of the production facility and its annual throughput," Dr. McKinzie said.

    Scientists at Los Alamos now believe the Russians are right, and are doing experiments to seek confirmation. Specifically, they are taking delta phase plutonium and raising its temperature slightly, and then monitoring it closely for signs of decomposition to the alpha form.

    "We never would have done this before," said Dr. Hecker, a former director of Los Alamos. "In the Russian view, when you increase the temperature, you increase the speed at which things may decompose."

    "Now," he added, "it's up to us to apply that to our stockpile."

    The repercussions, Dr. Hecker said, could affect how plutonium for warheads is made and stored, perhaps changing things like the spacing between plutonium cores. "If you have such a transformation," he said, "shape changes, and there's distortion, and you'd have to account for that."

    "It doesn't mean it makes it more dangerous," Dr. Hecker added. "But you have to think about those kind of things ahead of time."

    The cooperative basis for such insights is under fire in Washington after Congress last year asserted that that Chinese spies had made off with top American nuclear secrets.

    In November, a federal moratorium went into effect that bars new hiring of scientists at Los Alamos from potentially unfriendly countries like Russia and China. Foreign exchange programs and visits are also down, said Dr. Hecker, who lamented the trend.

    "We're doing what no foreign nation could to do to us," he rued. "We're crippling ourselves."




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