U.S. Waits for Firm Information On Nature and Success of Device

"You could have something that is very old and off the shelf here, as well, in which case they've dusted off something that is old and dormant," he said.

North Korea's arsenal is estimated by U.S. intelligence to have grown substantially during Bush's presidency.

At the end of George H.W. Bush's time in office in January 1993, North Korea was presumed to have enough plutonium for one to two nuclear devices. But in 2002, Pyongyang announced that it had begun to reprocess additional plutonium for weapons. It could now have plutonium for as many as a dozen devices, depending on their size and sophistication.

Nuclear experts said there was little possibility that the explosion could have been the result of a chemical blast or a radioactive "dirty" bomb masked as a nuclear explosion.

"It would be much more difficult to mimic the radioactive isotopes you get from a nuclear blast" than to conduct an actual nuclear test, said Charles D. Ferguson, a nuclear expert at the Council on Foreign Relations. "A dirty bomb uses one type of radioactive isotope, whereas a nuclear explosion would give off dozens of different ones," he said.

Ferguson agreed with government nuclear scientists that the most likely reason North Korea's blast was relatively small was that only a fraction of the plutonium detonated during the test.

Officials believe the low yield probably resulted from the poor design of the device. To create the kind of plutonium-based blast that North Korea claims, it would have needed to simultaneously set off a series of conventional explosives around a plutonium core. The force of the simultaneous blast produces a shock wave that causes the material to compress into the center and implode.

If any of those steps is imperfect and only part of the plutonium is imploded, the result is a low yield, such as the one produced by the North Korean test Monday. A low yield, deep underground, is more difficult to detect.

A government scientist who was not authorized to speak publicly said that in addition to radiation in the air, ground sensors may be able to pick up seepage that emerges through the soil, sometimes months after a test.

Michael Green, who was the senior director for Asia at the National Security Council during President Bush's first term, said the North Koreans have always made good on nuclear and missile plans they announced ahead of time, leaving him confident that they had in fact conducted a nuclear test.

"They have always telegraphed what they were up to on the plutonium side," he said.

Staff researcher Julie Tate contributed to this report.