U.S. intelligence agencies say, based on preliminary indications, that North Korea did not produce its first nuclear blast yesterday.
U.S. officials, speaking on the condition of anonymity, said that seismic readings show that the conventional high explosives used to create a chain reaction in a plutonium-based device went off, but that the blast's readings were shy of a typical nuclear detonation.
"We're still evaluating the data, and as more data comes in, we hope to develop a clearer picture," said one official familiar with intelligence reports.
"There was a seismic event that registered about 4 on the Richter scale, but it still isn't clear if it was a nuclear test. You can get that kind of seismic reading from high explosives."
The underground explosion, which Pyongyang dubbed a historic nuclear test, is thought to have been the equivalent of several hundred tons of TNT, far short of the several thousand tons of TNT, or kilotons, that are signs of a nuclear blast, the official said.
The official said that so far, "it appears there was more fizz than pop."
A successful nuclear detonation requires a properly timed and triggered conventional blast that splits atoms, setting off the nuclear chain reaction that produces the massive explosions associated with atomic bombs.
Nuclear bombs make big waves, with clear signatures that make them fairly easy to detect, analyze and confirm that they were caused by splitting atoms. But smaller blasts -- as North Korea's appears to have been -- are trickier to break down, scientists told the Associated Press.