Korean test seen as only partial blast

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Four days after North Korea tried to set off its first nuclear bomb, U.S. intelligence agencies think the blast detected by seismic sensors was a plutonium-fueled device that did not fully explode.

"The working assumption is that what happened, more likely than not, was an attempted nuclear test that fell far short of being successful," said one U.S. official familiar with the latest intelligence assessment.

There is still no confirmation that North Korea succeeded in creating a nuclear explosion, and so far no radioactive particles that would confirm a successful nuclear test have been detected. The Washington Times first reported Tuesday that U.S. officials were having doubts, based on preliminary data, about North Korea's boasts about having successfully tested its first nuclear device.

The latest intelligence estimates of Monday's test at a nuclear test site near Kilju, in northeastern North Korea, put the size of the blast at 0.2 kilotons, or the equivalent of 200 tons of TNT. A plutonium-fueled nuclear device normally creates a much larger blast, in the range of 5 kilotons to 20 kilotons. A kiloton is the equivalent of 1,000 tons of TNT.

The detected explosion likely was produced by the conventional high-explosives used to split the plutonium atoms and produce a nuclear explosion, one official said. A second official said, "There was a yield that was in the several hundred ton range, but it at least partially failed."

Complete analysis of the data could take weeks, the officials said.

Plutonium-fueled bombs use a core of plutonium that is surrounded by conventional high explosives. High-speed electronic triggers are used to set off the high explosives, which apply pressure to the plutonium and produce the nuclear blast.

According to officials, the North Koreans informed China's government that the test they planned to carry out would produce a 4-kiloton explosion. That size indicates the North Koreans were trying to test a small warhead that likely would fit on a missile. The North Koreans have several types of ballistic missiles, including a long-range Taepodong-2, medium-range Nodong and shorter-range Scuds.

Still, the top U.S. military official was noncommittal when asked yesterday by reporters whether North Korea's detected explosion was nuclear or merely conventional.

"It is not yet determined -- with any degree of assurance -- what exactly they tested," said Marine Gen. Peter Pace, the chairman of the Joint Chiefs of Staff.

Other explanations for the relatively low size of the detected blast include that North Korea did create a small nuclear explosion, but that seismic waves from it -- it registered as a 4.2 on the earthquake scale -- were muffled by an underground cavern, the officials said.

A third theory that intelligence agencies are examining is that the detected explosion was only a conventional explosion that North Korea is trying to fool the world into believing was a nuclear test, the officials said.

So far, aircraft and other sensors nearby the site of the explosion have not detected any radioactive debris. But while the detection of minute amounts of such materials would confirm a successful nuclear test, the officials warned that even if no sensors detect radioactive particles, a partially successful test would remain a possibility.