A dud, a fake or something else? Experts are dubious about the success of North Korea's underground nuclear test Monday.
Clearly, there was an explosion, a magnitude-4.2 blast from a near-surface location 40 miles north of Kimchaek, according to the U.S. Geological Survey. But an estimate of the blast's strength, equivalent to 500-2,000 tons of TNT, makes it a pipsqueak as nuclear explosions go, says physicist Terry Wallace of the Los Alamos National Laboratory in New Mexico.

Even so, the test had officials in other countries on edge. A magnitude-5.8 earthquake shook northern Japan today, and the Associated Press, quoting unnamed sources in the Japanese Foreign Ministry, reported that Japan suspected North Korea had conducted a second nuclear test. South Korean and U.S. monitors said they had detected no new seismic activity in North Korea, and Japanese Prime Minister Shinzo Abe told Parliament he had no confirmation of a new blast.

Monday's test had the abrupt beginning of a nuclear explosion, not the slow, rumbling start of an earthquake or the drawn-out TNT blasts seen in mine explosions, says seismologist Paul Richards of Columbia University. International monitors are waiting for confirmation in the form of measurements of airborne radioactivity, which usually spike after a test, he says.

"Most likely they tried to get a larger yield and things went wrong," says physicist Wolfgang Panofsky, a veteran of the Manhattan Project, which developed the first atomic bombs six decades ago.

In a plutonium atomic bomb, which North Korea is suspected of building, the radioactive material is compressed by precisely timed TNT blasts. A nuclear chain reaction produces an instantaneous explosion, Panofsky says. Mistiming or other imperfections can cause the chain reaction to fizzle.

"It could even be a fake — conventional explosives masquerading as a nuke," says nuclear proliferation reporter Richard Stone of Science magazine, by e-mail. However, Richards says that timing 500 or more tons of TNT to ignite simultaneously — and getting the explosives to the test site without being detected by satellites — seems "not very credible."

Some analysts, such as David Albright of the Institute for Science and International Security in Washington, D.C., suggest that the North Koreans may have miniaturized a bomb to arm missile warheads.

Panofsky says that even the smallest possible plutonium bomb should produce an explosion the size of that at Hiroshima in Japan in World War II. That blast had an explosive force equivalent to about 15,000 tons of TNT.

"But the main thing is the cat is out of the bag," Richards says. "Even if this is quite small by nuclear standards, something like this set off in a city would be an enormous disaster."


By Dan Vergano, USA TODAY