U.S. reconnaissance aircraft detect krypton 85 in the atmosphere near the country.

WASHINGTON--U.S. spy planes flying near North Korea have detected traces of a radioactive gas emitted during the reprocessing of spent nuclear fuel rods, which could be a possible sign of a secret nuclear facility, sources here said.

U.S. intelligence analysts are still trying to determine the significance of finding krypton 85, a radioactive isotope that is a byproduct of reprocessing nuclear fuel rods to extract plutonium, in the atmosphere near North Korea last December.

Krypton 85 was also detected in July 2003 when North Korea announced it had completed the reprocessing of spent nuclear fuel rods.

Two months later, the North Korean regime was said to have halted reprocessing operations at its Yongbyon facility about 90 kilometers north of Pyongyang, the capital.

While U.S. WC-135W reconnaissance planes routinely fly over the Sea of Japan as part of a monitoring effort on North Korea's nuclear weapons development program, no krypton had been detected since July 2003.

U.S. officials believe the krypton 85 detected in December likely came from one of two sources, the sources said. One is that North Korea might possess a secret facility unknown to outsiders. The other is that North Korea has encountered problems with its reprocessing operations at the Yongbyon facility.

Only a small number of senior Bush administration officials were informed of the finding, the sources said. Analysts are trying to determine precisely when the gas was emitted.

North Korea claimed it was preparing to reprocess spent nuclear fuel rods. The plutonium extracted from all of its 8,000 fuel rods would be enough to build six to eight nuclear warheads.

In September 2003, then U.S. Secretary of State Colin Powell told a closed session of the U.S. Senate Foreign Relations Committee there was a high possibility that North Korea had halted operations at its Yongbyon facility.

His remarks were based on information provided by spy satellites about the temperature of buildings at Yongbyon and the level of steam emissions from boilers at that facility.

In early 2003, when the North Korean nuclear crisis intensified, Pyongyang resumed operations of a 5,000-kilowatt experimental graphite nuclear reactor and the reprocessing facility at Yongbyon.

Operations of those facilities had been frozen following the signing of the Agreed Framework with the United States in 1994.

In April 2003, at a three-way meeting involving the United States, North Korea and China, North Korean representatives told their American counterparts that Pyongyang had begun reprocessing spent nuclear fuel rods.

In July, North Korean officials told their U.S. counterparts in a New York meeting that reprocessing of the fuel rods had been completed.

U.S. satellites detected spent nuclear fuel rods being taken out of storage at the Yongbyon facility in January 2003 and transported by truck outside. It was never determined where the trucks were headed.(IHT/Asahi: March 3,2005)