A Japanese official silhouetted against seismic waves from the 9 October test.

Katsumi Kasahara/AP PHOTO

North Korean "Bomb" May Be Bust for Science

With a muffled explosion deep inside a mountain, North Korea set the world on edge yesterday with a claim that it had detonated a nuclear bomb. But scientists poring over seismic signals from the blast are pondering why the detonation appears to have been so small. Some wonder whether the test was a failure--or even an elaborate hoax. Meanwhile, researchers are bracing for North Korean science to become further isolated from the rest of the world.

About the only thing known for certain is that at 10:39 a.m. local time on 9 October, a small tremor shook North Korea's North Hamgyong Province. The U.S. Geological Survey measured the event at magnitude 4.2 on the Richter scale, whereas South Korean estimates put it in the range of 3.5 to 3.7. The seismic signature--a sharp and large "P" wave relative to the "S" wave--"argues strongly that the event was an explosion," says geologist Jeffrey Park, a seismology expert at Yale University.

But the yield is unclear. That's because analysts have only a sketchy idea of the test site's geology. Assuming a tight coupling between the shock waves and surrounding rock, South Korean and Western analysts peg the blast at the equivalent of several hundred tons of TNT. (For comparison, the "Fat Man" plutonium bomb dropped on Nagasaki in 1945 was 21 kilotons.) Loose rock would dampen the shock waves, suggesting a yield of 1 to 2 kilotons, says Geoffrey Forden, a physicist and weapons expert at the Massachusetts Institute of Technology in Cambridge. That's still considerably less potent than an estimate from Russia's defense minister Sergei Ivanov, who asserted yesterday that the blast equaled 5 to 15 kilotons of TNT.

If the test was indeed nuclear, analysts presume it was a plutonium bomb, as North Korea claims it has separated plutonium from irradiated nuclear reactor fuel. Some experts say the tremor has the hallmarks of a subcritical explosion that failed to achieve a sustained fission reaction, or perhaps only a fraction of the plutonium fissioned. Others have not ruled out the possibility of a faux nuclear test staged with conventional explosives. The only way to know for sure is if surveillance planes or aerial towers in Japan sniff out radioactive fission products in gases venting from the test shaft. If radionuclides are not detected, Park argues, "the chance of a faked test is quite high."

Bomb or no, North Korea will likely face further sanctions. The Bush Administration is considering whether to classify North Korea like Cuba and bar U.S. citizens from spending money there--a de facto travel ban--except in special circumstances. And "you can forget about North Koreans getting visas to come to the United States anytime soon," predicts Matthew Bunn, a nonproliferation expert at Harvard University's Kennedy School of Government.

The test is likely to scuttle follow-up meetings next month in China on university education and chemistry, says Park Chan-mo, president of Pohang University of Science and Technology in South Korea, who helped organize a groundbreaking South-North science conference in Pyongyang last April. "I'm very disappointed," he says. Whether authentic, dud, or outright fake, North Korea's bomb is sure to contaminate efforts to reach out to its scientists.

For further coverage, please stay tuned for the 13 October issue of Science.