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What next for North Korea's nukes?

The May 25 test provided the world with a host of data about North Korea's bomb-making abilities. But only Pyongyang knows if the test was a success.

By Staff writers of The Christian Science Monitor, Peter N. SpottsStaff writers of The Christian Science Monitor / May 29, 2009

South Korean soldiers walk near the demilitarised zone separating the two Koreas on May 27, 2009. North Korea on Wednesday threatened a military strike against the South.

Kim Kyung-Hoon/Reuters

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Washington and Boston

Where does North Korea's nuclear program go from here? That's a key technical question in the wake of Pyongyang's underground test of a nuclear device on May 25.

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Proliferation experts will want to determine, in particular: the nature of that nuclear device, whether it went off as expected, and whether the North Koreans now intend to produce more fissile material for a potential weapons stockpile.

"There's a tremendous amount we do not know about North Korea," said Richard Bush III, a former US National Intelligence Officer for East Asia, at a Brookings Institution seminar on the nuclear crisis May 27.

On one point, however, there is wide agreement: North Korea's scientists did better this time than in 2006, when they made their first attempt at an underground nuclear explosion.

That one was a fizzle, relatively speaking. This one wasn't.

Scientists at the Lamon-Doherty Earth Observatory in Palisades, N.Y., place the yield of the May 25 blast at between 2.2 and 4 kilotons of TNT – significantly larger than North Korea's 2006 experiment.

The 2006 test still "would have been a disaster if it had gone off in a metropolitan area," says Won-Young Kim, a seismologist at the observatory who specializes in detecting underground nuclear tests. "[But] this one was bigger."

Mr. Kim and his colleagues base their estimate on seismograph readings from several sites in the Pacific and central Asian regions, as well as one in China that was only about 230 miles from the blast zone. The China site is run by a group of international universities and the US Geological Survey.

They are reasonably certain the shaking of those seismographs was caused by fissile material reaching critical mass. The blast carried the unique signature of an underground explosion, rather than that of an earthquake.

It is possible to fake a nuclear blast with conventional explosives, but the Lamon-Doherty team considers that explanation to be implausible in this case. There is some uncertainty to their yield estimate because no one outside of North Korea knows exactly how deep the device was planted.

The question of whether the explosion was successful, however, is more difficult to answer. Clearly, it was not a flop. But no one outside of a tight coterie in North Korea has any idea what kind of explosive yield their device was designed to produce.

If it was supposed to produce about a 4 kiloton explosion, that is one thing. But it could have been intended to produce 20 or 30 kilotons. The Nagasaki bomb of World War II produced an explosion equal to about 21 kilotons of TNT, for instance.

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