U.S. probes for details of North Korean nuclear test

By Mark Hosenball

WASHINGTON, Feb 12 (Reuters) - U.S. intelligence agencies are scrambling to collect technical data on the type, design and explosive power of the nuclear device North Korea tested on Tuesday, officials said.

A senior U.S. official said it could take a week or two for the United States and others to collect and study data about the test before coming up with a consensus on the device's design and power.

Initial indications are that the underground nuclear explosion was produced by the latest version of a plutonium-based prototype weapon, according to one current and one former U.S. national security official. North Korea tested more primitive plutonium-based devices in October 2006 and May 2009.

North Korea is also believed to be working on a uranium-based nuclear weapons design.

The test, which Pyongyang claimed was of an advanced design, has boosted longstanding concerns over North Korea's nuclear ambitions. If it turns out the test was of a new uranium-based weapon, the stakes would be even higher for the international community.

The U.S. government unit that monitors both peaceful and non-peaceful nuclear activities is a military agency called the Air Force Technical Applications Center.

Headquartered at Patrick Air Force Base in Florida, it operates aircraft known as the WC-135 "Constant Phoenix," U.S. officials said. The mission of these aircraft, known as "sniffers" or "weather birds," is to sample the atmosphere for possible residue of nuclear explosions.

The planes would need to be deployed quickly to detect whether highly enriched uranium rather than plutonium was used because uranium decays to undetectable levels within a matter of days. Plutonium takes much longer to decay.

North Korea's official news agency, KCNA, reported that the nuclear test was carried out "in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously (and) did not pose any negative impact on the surrounding ecological environment."

The statement implied that Pyongyang had made advances in miniaturizing a nuclear device, which puts it a step closer to being able to install a weapon atop a long-range ballistic missile.

U.S. Representative Mike Rogers, chairman of the House Intelligence Committee, said North Korea's nuclear program appears to have advanced "a long way" since the 2009 test. Still, Rogers said in an interview, North Korea remains "two or three steps away" from producing a miniaturized device that could be mated with a longer-range missile that the North Koreans have been developing and most recently test-launched in December.

Experts also interpreted the reference to miniaturization to be at least partial confirmation that the fissile material in the test device was plutonium. Joseph DiTrani, formerly the U.S. intelligence community's top expert on North Korea, said North Korea was believed to have produced enough plutonium to make six to eight explosive devices.

Another reason for initial suspicions that the latest test involved a plutonium-based device is because North Korea had "overproduced" plutonium, one of the U.S. officials said.

North Korea is now believed to have frozen the production of plutonium and instead embarked on a program to produce highly-enriched uranium (HEU), another fissile material which can be used in nuclear bombs. More HEU is needed for an explosive device than plutonium, and there are questions as to whether North Korea has produced enough to build more than one device.

The U.S. Office of the Director of National Intelligence said in a statement that the test had an explosive yield of "approximately several kilotons."

North Korea's first nuclear test in 2006 largely fizzled, Western experts say, while the 2009 test had a yield of about 2 kilotons. A kiloton is an explosive force equal to 1,000 metric tons of TNT.

Rogers said the test could be a manifestation of North Korea's young new leader, Kim Jong Un, flexing his muscles, as much for domestic effect as for international attention. He could be "showing the military that he is in charge," Rogers said.

But others questions whether it was really Kim who decided to conduct the test or whether his advisers decided for him, Rogers said. (Editing by Warren Strobel and Doina Chiacu)