http://www.nytimes.com/2006/10/10/wo...gewanted=print



Blast May Be Only a Partial Success, Experts Say
By WILLIAM J. BROAD and MARK MAZZETTI
The North Korean test appears to have been a nuclear detonation but was fairly small by traditional standards, and possibly a failure or a partial success, federal and private analysts said yesterday.
Throughout history, the first detonations of aspiring nuclear powers have tended to pack the destructive power of 10,000 to 60,000 tons — 10 to 60 kilotons — of conventional high explosives.
But the strength of the North Korean test appears to have been a small fraction of that: around a kiloton or less, according to scientists monitoring the global arrays of seismometers that detect faint trembles in the earth from distant blasts.
“It’s pretty remarkable that such a small explosion was promptly apparent on seismometers all over the world,” said Paul Richards, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. “The detection of this was really good. You can’t hide these kinds of things, even very small tests.”
A senior Bush administration official said he had learned through Asian contacts that the North Koreans had expected the detonation to have a force of about four kilotons. Because classified information was involved and there was lingering uncertainty, he would not let his name be used.
Philip E. Coyle III, a former director of weapons testing at the Pentagon and former director of nuclear testing for the Lawrence Livermore National Laboratory, a weapons design center in California, said the small size of the test signaled the possibility of what might be described as a partial success or a partial failure.
“As first tests go, this is smaller and less successful than those of the other nuclear powers,” he said.
Perhaps the North Koreans wanted to keep it small, he added. “But if it turns out to be a kiloton or less,” Dr. Coyle said, “that would suggest that they hoped for more than that and didn’t get it.”
The United States Geological Survey said it had detected a tremor on the Korean Peninsula of 4.2 magnitude, which translates into an explosive force of roughly 1,000 tons. The agency listed 20 seismic stations as having initially picked up the blast’s shockwave, including nearby ones in China, Japan and South Korea as well as distant ones in Ukraine, Australia, Nevada and Wyoming.
The Russian defense minister, Sergei B. Ivanov, told the Itar-Tass news agency that the Russian government believed the strength of the blast to have been 5 to 15 kilotons. The basis for his claim was not immediately clear.
In Washington, intelligence officials said they were still in the early stages of evaluating the North Korean blast. But one said analysts had estimated its force at less than a kiloton.
“We have assessed that the explosion in North Korea was a sub-kiloton explosion,” said the intelligence official, who, like others, spoke on condition of anonymity because they were discussing classified information.
It will probably take several days to determine with confidence if the explosion was in fact nuclear, the official said. He added that so far, sensors had not detected radiation leaking from the blast site. But federal and private experts said it seemed unlikely that the North Koreans had faked an underground nuclear blast with a large pile of conventional high explosives.
First, they said, the political risks involved would seem disproportionate. More important, federal and private analysts said, the United States has long had spy satellites observing the North Korean site and appears to have found no signs of chemical explosives being unloaded.
“It’s difficult to fake it when you know people are looking down on you,” said Dr. Richards of Columbia. “The execution of a chemical explosion would be hard to hide.”
Dr. Coyle, the former director of nuclear testing at Livermore, said small tests were more likely to leak radioactivity than large ones, because the intense heat and gigantic shock waves of bigger blasts tended to melt and pulverize nearby rock into impregnable barriers.
Experts said the United States, Japan and the Comprehensive Nuclear Test Ban Treaty Organization had equipment that could sniff the air and water around North Korea for signs of radioactivity.
The South Koreans estimated the seismic magnitude of the blast at roughly 3.6. William Leith, the coordinator of seismic monitoring at the United States Geological Survey, said the South Korean method was different from and not comparable with the American technique. He added that the American estimate of 4.2 was unlikely to change.
In Vienna, the Comprehensive Nuclear Test Ban Treaty Organization estimated the blast’s magnitude at 4.0, which is within the error margin of the American estimate.
An American intelligence official said the blast had shaken a North Korean site that the nation’s spy agencies had long monitored — the flank of a mountain not far from the northeastern coast.
The small size of the explosion, he said, “tells you they have a lot of work to do in terms of weaponizing what they’ve got.” He added, “If the lower-yield estimates are valid, then it’s not a militarized system, but also not something a terrorist would reject.”
David E. Sanger contributed reporting.