Monday, October 09, 2006

Weak blast hints at partial failure

Oct. 10, 2006. 01:00 AM

NEW YORK—The North Korean test appears to have been a nuclear detonation but was fairly small by conventional 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 official in the administration of George W. Bush said he had learned through Asian contacts that the North Koreans had expected the detonation to have a force of about 4 kilotons. Because classified information was involved and there was lingering uncertainty, he would not let his name be used.

Philip Coyle, a former director of weapons testing at the Pentagon and former director of nuclear testing for the Lawrence Livermore National Laboratory, a weapons design centre in California, said the small size of the test signalled 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," Coyle said, "that would suggest that they hoped for more than that and didn't get it."

Russian Defence Minister Sergei Ivanov told the Itar-Tass news agency Moscow 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.

It will probably take several days to determine with confidence if the explosion was in fact nuclear, one official said. He added sensors had not yet detected radiation leaking from the blast site, which could prove its atomic origin.

Federal and private experts said it seemed unlikely 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 has apparently found no signs of chemical explosives being unloaded.

"It's difficult to fake it when you know people are looking down on you," said Richards, the Columbia seismologist. "The execution of a chemical explosion would be hard to hide."

Xavier Clement of France's Atomic Energy Commission said the natural sound of the Earth, with its constant seismic activity of tectonic plates grinding together, complicates the task of trying to determine whether a smaller blast was caused by conventional explosives or a nuclear device.

He likened the problem to trying to "detect the violins or a flute in a symphony orchestra when you are playing the cymbals."

His agency estimated the North Korean blast at around 1 kiloton or less. For a nuclear device, that would be so weak that the French defence minister suggested "there could have been a failure" with the North Korean reported test.

The Comprehensive Nuclear Test Ban Treaty Organization, or CTBTO, has about 200 stations worldwide designed for monitoring nuclear tests as part of what it hopes will become the world's most reliable source for such tests. But until the treaty comes into force, the data are not made public, only released to governments and vetted partners.

Seismic data comes in almost immediately, and is usually passed to governments within an hour or so. Their scientists must decide what the numbers and graphs mean.

"People have different way of cross cutting the data and interpreting them," said Lassina Zerbo, director of the International Data Centre at the CTBTO, based in Vienna, Austria.

While the North Korean explosion was small, potentially complicating monitoring efforts, sensors in South Korea were likely close enough to categorize it as nuclear, if that is what is was, said Friedrich Steinhaeusler, professor of physics at Salzburg University.

A nuclear blast also gives off a clear signature — a clear graph of peaks and curves — that differentiates it from other kinds of shocks, he added.

"We'll have the confirmation soon," he said.


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