By Dafna Linzer and Thomas E. Ricks
Washington Post Staff Writers
Wednesday, October 11, 2006
The White House yesterday played down North Korea's nuclear capability as government scientists and intelligence analysts waited for additional data to confirm whether Pyongyang had conducted a successful nuclear test.
Intelligence and administration officials said they were still working under the assumption that North Korea had managed to detonate an atomic device, but they said they needed additional environmental sampling before they could formally rule out other possibilities, such as the blast being caused solely by conventional explosives. Intelligence officials were concerned that North Korea could conduct another test, either to improve upon the first test or to prove its capabilities.
A U.S. military RC-135, an electronic monitoring aircraft, flew around the Sea of Japan yesterday in an effort to detect nuclear radiation, two intelligence sources said. The same aircraft, based in Okinawa, Japan, was used in July after North Korea carried out a set of ballistic missile tests. The sources cautioned that it could take several days before winds push radioactive particles toward an area where they can be clearly detected.
"Over time, whenever the prevailing winds blow out over the Gulf of Japan, it will be more likely that we get some detection," one intelligence official said yesterday, requesting anonymity because the effort involves classified information.
Pentagon spokesman Bryan Whitman said officials would use a variety of means besides seismic data to try to draw a conclusion about the explosion, including some he would not discuss. "There is a possibility that particulate fallout is detectable, and then there's a variety of other intelligence means to determine the veracity of the allegation of the tests that they conducted," he said.
North Korea announced Monday that it had carried out its first nuclear test, and seismic readings suggested a blast inside a mountain in the country's north from the equivalent of 500 tons of explosives.
"We ourselves are operating under the assumption that, yes, in fact it was" a nuclear test, State Department spokesman Sean McCormack said yesterday. "But I can't confirm that."
The aircraft and monitoring stations on the ground are seeking to detect particulate data that would indicate that a nuclear explosion had taken place. But those efforts will not necessarily determine the nature of the blast, a Defense Department official noted, because the explosion was relatively small and the North Korean government said it was contained.
"There are multiple ways" the U.S. government will seek to verify North Korea's claim that it detonated a nuclear device, the official said. But there is no hard information yet, the official said. Intelligence analysts are also reviewing intercepted communications and other data.
The official declined to be quoted by name, saying that the Pentagon is not playing a lead role in the U.S. response and that he wanted to defer to the White House.
White House press secretary Tony Snow said there is a "remote possibility" that U.S. intelligence will be unable to fully determine whether the test succeeded. Several nuclear tests conducted by other countries, including a number of Pakistani tests in 1998, have never been fully understood by U.S. intelligence. Many intelligence analysts believe a 1979 flash in the waters off the southern tip of Africa was caused by a nuclear test carried out by Israel, with South African help. But it has never been confirmed and remains a mystery.
Snow suggested yesterday that it is possible that the test was conducted with an older weapon from before President Bush's time in office.
"You could have something that is very old and off the shelf here, as well, in which case they've dusted off something that is old and dormant," he said.
North Korea's arsenal is estimated by U.S. intelligence to have grown substantially during Bush's presidency.
At the end of George H.W. Bush's time in office in January 1993, North Korea was presumed to have enough plutonium for one to two nuclear devices. But in 2002, Pyongyang announced that it had begun to reprocess additional plutonium for weapons. It could now have plutonium for as many as a dozen devices, depending on their size and sophistication.
Nuclear experts said there was little possibility that the explosion could have been the result of a chemical blast or a radioactive "dirty" bomb masked as a nuclear explosion.
"It would be much more difficult to mimic the radioactive isotopes you get from a nuclear blast" than to conduct an actual nuclear test, said Charles D. Ferguson, a nuclear expert at the Council on Foreign Relations. "A dirty bomb uses one type of radioactive isotope, whereas a nuclear explosion would give off dozens of different ones," he said.
Ferguson agreed with government nuclear scientists that the most likely reason North Korea's blast was relatively small was that only a fraction of the plutonium detonated during the test.
Officials believe the low yield probably resulted from the poor design of the device. To create the kind of plutonium-based blast that North Korea claims, it would have needed to simultaneously set off a series of conventional explosives around a plutonium core. The force of the simultaneous blast produces a shock wave that causes the material to compress into the center and implode.
If any of those steps is imperfect and only part of the plutonium is imploded, the result is a low yield, such as the one produced by the North Korean test Monday. A low yield, deep underground, is more difficult to detect.
A government scientist who was not authorized to speak publicly said that in addition to radiation in the air, ground sensors may be able to pick up seepage that emerges through the soil, sometimes months after a test.
Michael Green, who was the senior director for Asia at the National Security Council during President Bush's first term, said the North Koreans have always made good on nuclear and missile plans they announced ahead of time, leaving him confident that they had in fact conducted a nuclear test.
"They have always telegraphed what they were up to on the plutonium side," he said.
Staff researcher Julie Tate contributed to this report.
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