A Secretive Country Gives Experts Few Clues to Judge Its Nuclear Program

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As scientists and world leaders scrambled Tuesday to judge the importance of North Korea's claim that it had detonated a third nuclear bomb, the main thing that quickly became evident is how little is known about the country's increasingly advanced atomic and missile programs.

Even the best news about the test -- that it was small by world standards -- could have a dangerous downside if the North's statement that it is learning to miniaturize bombs is true. That technology, which is extremely difficult to master, is crucial to being able to load a weapon atop a long-range missile that might one day reach as far as the United States mainland.

"We don't know enough to nail it, but we can't rule out that they've done something dangerous" Ray E. Kidder, a scientist who pioneered early nuclear warhead designs at the Livermore weapons lab in California, said of the underground test.

As is usual with tests by the secretive North, it was not even clear if the underground test was nuclear, rather than conventional bomb blasts meant to mimic an underground nuclear test. Experts assume it was nuclear partly from the shape of its seismic signal and because the blast was at the same mountainous site as two earlier nuclear tests.

It also remains unclear whether the North used plutonium or enriched uranium to fuel the bomb. American officials believed that the country's last two nuclear tests, in 2006 and 2009, used plutonium, and they fear a switch to uranium will allow the country a faster and harder-to-detect path to a bigger arsenal. While scientists are actively hunting for the airborne markers of a uranium test, it is not certain that gases needed to make that judgment escaped the test site.

Scientists said the relatively small size of Tuesday's blast calmed, at least temporarily, their worst fears: that the North's recent references to more powerful hydrogen bombs indicated the possibility that it might have at least enough technology to try to test one. Those bombs, nicknamed city-busters, are roughly 1,000 times stronger than atom bombs, and if the North were to get them it would represent an enormous leap in its known abilities. The first American hydrogen bomb to be tested caused the Pacific island of Elugelab to vanish.

What emerged most clearly Tuesday from sensitive global networks that measure faint rumbles in the earth was that the underground blast was most likely larger than North Korea's past explosions. In Vienna, the preparatory commission for the Comprehensive Nuclear Test Ban Treaty Organization, which runs a global seismic network, said the blast measured 5.0 in seismic magnitude. The United States Geological Survey put its own estimate at 5.1 in magnitude.

Nuclear experts said the magnitude of the blast equaled an atomic blast of about 6,000 tons of high explosive, or six kilotons. The first test by Pyongyang is thought to have packed less than a kiloton of power and was considered a partial failure by the West. The state's 2009 blast was judged by American intelligence officials to have a power of two kilotons, though some experts outside the government say it might have been as large as this week's test.

In any case, said Paul Richards, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., it was "a serious explosion."

Still, even the largest estimates are small by world standards. The first three nuclear tests of China, for instance, were measured at 22 kilotons, 35 kilotons and 250 kilotons.

North Korea's tests "are limited in explosive power compared with most previous ones," said Robert S. Norris, a senior fellow at the Federation of American Scientists in Washington, and the author of "Racing for the Bomb," a biography of Gen. Leslie R. Groves, the Manhattan Project's military leader. Determining whether the test was fueled by plutonium or uranium is critical because North Korea in 2007 shut down its reactor that made plutonium, prompting analysts to conclude that its supplies of the rare element are now running low. Intelligence officials estimated it had enough fuel for 6 to 10 bombs.

But in 2010, the state revealed what appears to be a fairly advanced program to enrich uranium, which in theory could fuel many bombs since experts believe that North Korea has rich uranium deposits.

Siegfried S. Hecker, a former director of the Los Alamos weapons lab in New Mexico who has repeatedly visited North Korea and learned details of Pyongyang's nuclear program, has suggested that North Korea may be ready to switch to a pure uranium approach, in part because it might have a blueprint for a miniaturized uranium warhead.

He said the North's leadership might have obtained the blueprint from A. Q. Khan, the rogue Pakistani nuclear expert, a blueprint of the type he gave Libya for a uranium bomb. It is well known that North Korea obtained its centrifuge design for uranium enrichment from Dr. Khan, and many experts say the Pakistani expert may have thrown in the warhead blueprint as a sweetener.

Analysts say the uranium approach may also offer North Korea the allure of a new secrecy. Centrifuge plants are much easier to hide than reactors.

Finding out whether the bomb was fueled by plutonium, uranium or a mix of the two materials could take some time or might never happen, analysts say.

Not all underground tests leak their explosive residues into the atmosphere or surrounding waters, and some say tests of the size of Tuesday's blast are probably strong enough to seal any cracks in the rocks.

"If we get samples, I'm sure we'll learn a lot about it," Jay C. Davis, a nuclear scientist who helped found a federal effort to improve such analyses, said in an interview.

But if no bomb residue leaks, he added, the nature of the fuel that North Korea used for its third blast may remain a mystery.

world

This article originally appeared in The New York Times.


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