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Low Yield Of Blast Surprises Analysts

By Dafna Linzer
Washington Post Staff Writer
Tuesday, October 10, 2006

The explosion set off by North Korea yesterday appears to have been extremely small for a nuclear blast, complicating U.S. intelligence efforts to determine whether the country's first such test was successful or signaled that Pyongyang's capabilities are less advanced than expected, several senior U.S. and foreign government officials and analysts said.

A variety of seismic readings around the world yesterday appear to have resulted from no more than a half-kiloton explosion, three officials said -- equivalent to 500 tons of TNT and far smaller than the 21- to 23-kiloton plutonium bomb the U.S. military dropped on Nagasaki, Japan, in August 1945.

A senior intelligence official called it a "sub-kiloton" explosion detonated inside a horizontal mountain tunnel and said its low yield caught analysts by surprise. "For an initial test, a yield of several kilotons has been historically observed," the official said.

A U.S. government official said the North Koreans, in a call to the Chinese shortly before the test was conducted, said it would be four kilotons. The official said it is possible the explosive yield was as low as 200 tons. France and South Korea both issued sub-kiloton estimates, and officials dismissed as inaccurate an early Russian estimate that the blast resulted from a five-to-15-kiloton explosion.

President Bush said early yesterday that U.S. experts were "working to confirm North Korea's claim." By the end of the day, intelligence officials were still piecing together data and waiting to review intercepted communications that might shed light on what exactly the North Koreans set out to accomplish in the test and how it was conducted.

If confirmed, North Korea's ascension into a growing nuclear club -- joining the United States, Britain, France, Russia, China, India and Pakistan -- would likely have a major impact on efforts to halt the spread of nuclear weapons. (Israel is widely believed to have nuclear weapons, although it apparently has never conducted a test.) It is the only country to walk away from the nuclear Non-Proliferation Treaty and then violate its central tenet -- a commitment to refrain from building nuclear weapons.

Intelligence and administration officials said yesterday they believed North Korea had managed a nuclear test of some sort, but because of the secrecy of the Pyongyang regime and the lack of scientific data, some observers would not eliminate the possibility that the blast was created by conventional explosives.

The relatively small size of the explosion, along with North Korea's public statement that the test did not produce any radioactive leakage, led some to question how well the test had gone. Small amounts of leakage are normal during nuclear tests, though it can take several days for the ventilation to register. One U.S. official said radiation detectors in the region were being monitored for any signs in the air from the nuclear test.

One analyst, who was not authorized to speak publicly about the topic, said North Korea's statement on leakage may have been political, rather than technical: "It could be their way of showing that they are a member of the nuclear club, as advanced, responsible and capable as other states." Another said the statement may have been intended to lessen environmental concerns in neighboring countries.

Intelligence officials were looking at four possibilities to explain the size of the blast, the most likely of which appeared to be that only a fraction of the device's core exploded. If that were the case, the test would still be considered successful, officials said, because some plutonium was exploded. But it may also lead the North Koreans to conduct additional tests to determine what went wrong.

It is also possible, two analysts said, that Pyongyang used less plutonium because it has less stockpiled than U.S. intelligence believed. Considered more unlikely by experts were theories that the North Koreans had succeeded in manufacturing a smaller, more sophisticated nuclear device or that engineers had set out to test the device's design rather than its yield.

"A low yield can be a failure in design or it can be bad luck," said Michael A. Levi, a nuclear expert at the Council on Foreign Relations. "Anything is possible," he said, including simulating a low-yield nuclear explosion by using large quantities of TNT, as the U.S. military had planned to do last summer.

"But you don't hear anyone who thinks it's a conventional [explosives] test," he said.

One senior government nuclear specialist said it is hard to discern a nuclear detonation from a conventional one just by looking at seismic data. "Explosions are explosions," the analyst said. "They push equally out in all directions and that's what you see at a distant seismic station; you just see the explosion force and you can't tell."

To discriminate between the two, intelligence experts also rely on a string of gas and radiation detectors.

The Bush administration had been expecting a nuclear test for months and was briefly relieved after a set of North Korean ballistic missile tests failed in July. Intelligence analysts in the U.S. government think Pyongyang is still years away from being able to marry a nuclear device to an advanced delivery system.

"I don't think we perceive a great threat that North Korea will attack anyone with nuclear weapons," said Robert J. Einhorn, who was assistant secretary of state for nonproliferation until 2001. "One of the main threats is that in desperation, it might sell its nuclear assets. Another danger is that eventually the government might collapse into chaos, and then we won't know who is in control of those assets."

Staff writer Peter Baker and staff researcher Julie Tate contributed to this report.

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