Pentagon hunting for clues on power, makeup of weapon
WASHINGTON -- US military aircraft and monitoring centers scrambled yesterday to determine the size and type of the weapon that North Korea detonated in what is believed to be the reclusive regime's first test of a nuclear bomb.
Early estimates of the scale of the underground blast in the northeastern mountains ranged widely yesterday. South Korean officials reported a relatively small half-kiloton blast -- equal to roughly 500 tons of TNT -- while the Russian government estimated it was a much more powerful 5- to 15-kiloton detonation on par with the controlled detonations of nuclear powers in the past.
But US officials expressed caution about coming to any firm conclusions as they directed a dragnet of aircraft, ships, seismic listening posts, spy satellites, and other intelligence-gathering systems to glean as much as possible about the size of the detonation and whether it was based on plutonium or uranium. They also sought to determine whether the North Korean design is advanced enough to be mounted atop a missile -- a nightmare scenario for the United States and North Korea's neighbors.
``They say they have done one, and common belief is that something took place," said Major David Smith, a Pentagon spokesman. ``But we're basically in a wait-and-see mode."
Still, private specialists and former government officials said the test, which the US Geological Survey indicated caused a seismic tremor of about 4.2 on the Richter Scale, appeared to be on the smaller end of the spectrum.
``A general rule of thumb is that a seismic magnitude of 4.0 to 4.25 equals about 1 kiloton," said Philip E. Coyle III , who formerly oversaw US nuclear tests for the Department of Energy and is now a senior adviser at the Center for Defense Information in Washington.
The weapon the United States dropped on Hiroshima, Japan, in World War II had an equivalent yield of between 11,000 and 15,000 tons of TNT, according to a fact sheet provided by the Center for Strategic and International Studies.
Added Corey Hinderstein , a nuclear weapons specialist at the nonprofit Nuclear Threat Initiative: ``It would appear from the information coming out that this was likely a relatively small nuclear test. It seems to center around a kiloton."
She said that the makeup of the device cannot be determined by the size of the blast but can be confirmed only by studying air and possibly water particles in the area.
``It is most likely to be a plutonium device, because we do know that North Korea has at least a small stockpile of separated plutonium that could be used in weapons," Hinderstein said. ``If it is a uranium device, then they are much further along than is the conventional wisdom."
Some specialists said the apparently small size of the blast could mark a very crude test or a very advanced design relying on minimal amounts of weapon material.
Others suggested that the North Koreans could have tested only the first part of a two-stage hydrogen bomb, which would first require an implosion caused by conventional high explosives such as TNT or HMX before the nuclear material is compressed to the density necessary for a nuclear explosion. That could explain the detection of a large underground explosion without further evidence of a nuclear blast.
Meanwhile, US defense officials sought to downplay their role yesterday in the test's aftermath. The Pentagon referred reporters to the State Department, saying the test was now a ``diplomatic issue."
Officials at the United States Pacific Command in Honolulu, responsible for military operations in the region, also reflected the Bush administration's emphasis on diplomacy. A military official went out of his way to insist that no military action is being undertaken in response to the test, which took place despite warnings from the United Nations, the United States, China, Russia, Japan, South Korea, and the European Union.
``We are not at a higher alert," offered the official, who asked not be identified because he was not authorized to speak to the media.
Nevertheless, the United States and its allies went into high gear to learn what took place and to keep a watchful eye for any further nuclear tests that might take place in the coming days, officials said on condition of anonymity.
While North Korea said yesterday that no radioactive material had leaked out, the Air Force Technical Applications Center, based in Florida, flew special aircraft downwind from the test site near the Chinese border to try to catch any radioactive material that might have been vented into the atmosphere.
According to the Air Force, the WC-135W ``Constant Phoenix" planes ``collect particulate and gaseous effluents and debris from accessible regions of the atmosphere" to be analyzed by military specialists and the Department of Energy's nuclear weapons laboratories.
These samples would allow the national weapons laboratories such as Lawrence Livermore, Los Alamos, and Sandia to determine whether Pyongyang used plutonium or uranium in the blast.
Meanwhile, the United States has activated a special Air Force detachment located at South Korea's seismic listening post in Wonju , close to the border with North Korea, officials said.
The main objective of the collection effort is to use the data to make a more accurate assessment than ever before about the sophistication of North Korea's outlawed weapons program and to find out how close the nation might be to mounting a nuclear warhead on one of its long-range missiles.
``One major uncertainty is whether North Korea has the technology to build an efficient bomb [or] warhead for a missile," said Anthony Cordesman , a security specialist at the Center for Strategic and International Studies in Washington.
Even if North Korea has a relatively crude nuclear design, nearly all specialists agreed, the test marked a major shift in the balance of power in the region, removing any doubts about the nation's intentions to develop nuclear weapons.
Henry Sokolski, a North Korea specialist at the Nonproliferation Policy Education Center, said that the failure to stop North Korea poses a unique threat to efforts to stop the spread of nuclear weapons.
``It's very serious," Sokolski said. ``They not only signed the Nuclear Nonproliferation Treaty and violated it, but withdrew and tested a bomb. The message is, nothing will happen until you do a test, but that's pretty late in the game. Others will take due note of this, and they will bring their program right up to the edge."
Bender can be reached at bender@globe.com. 