Finding weakens North Korea denials on nukes

WASHINGTON — U.S. scientists have discovered traces of enriched uranium on smelted aluminum tubing provided by North Korea, apparently contradicting the country's denial that it had a clandestine nuclear program, according to U.S. and diplomatic sources.

The United States has long pointed to North Korea's acquisition of thousands of aluminum tubes as evidence of such a program, saying the tubes could be used as the outer casing for centrifuges needed to spin hot uranium gas into the fuel for nuclear weapons. North Korea has denied that contention and, as part of a declaration on its nuclear programs due by the end of the year, recently provided the United States with a small sample to demonstrate the tubes were used for conventional purposes.

The discovery of the uranium traces has been kept quiet by senior U.S. officials concerned that disclosure would expose intelligence methods and complicate the diplomatic process. North Korea has steadfastly refused to open up about its past practices, simply asserting that it is not engaged in inappropriate activities. However, the uranium finding will force U.S. negotiators to demand a detailed explanation from North Korea.

North Korea has made rapid progress on disabling its nuclear reactor in Yongbyon, which produces a different type of fuel: plutonium. But now U.S. officials have encountered resistance from North Korea on the next steps in an agreement reached in February to end North Korea's nuclear arms development. A top State Department official, Sung Kim, is in the capital Pyongyang this week to discuss the declaration with North Korean officials.

In addition to the possibility that the tubes acquired traces of uranium as part of an active enrichment program, sources said the tubing could have been contaminated by exposure to other equipment.

Pakistan, for instance, has acknowledged providing North Korea with a sample centrifuge kit, and so the tubes might have acquired the enriched uranium from the Pakistani equipment.

In 2003, inspectors from the International Atomic Energy Agency detected traces of enriched uranium at an Iranian nuclear facility and ultimately determined that the material came from Pakistani equipment provided by nuclear smugglers.

David Albright, a former U.N. weapons inspector and president of the Institute for Science and International Security, said the equipment did not need to be in the same room but could have picked up the uranium traces from a person who was exposed to both sets of equipment.

U.S. intelligence analysts first concluded in July 2002 that North Korea had embarked on a program to produce highly enriched uranium for use in weapons, with a key piece of evidence being North Korea's purchase of 150 tons of aluminum tubes from Russia in June 2002.

The Bush administration's accusation that North Korea had a clandestine program led to the collapse of a 1994 agreement that had previously frozen the Yongbyon reactor.

Plutonium and highly enriched uranium provide different routes to building nuclear weapons.

North Korean officials have indicated to U.S. officials that any experimentation with uranium enrichment did not work out and so any materials acquired abroad were used instead for conventional purposes. But the North Koreans have refused to explain why the purchases were made in the first place, preferring to show that the materials are not being used in any illicit program, sources said.