The North told Hecker it began construction on the centrifuges in April 2009 and finished only a few days before the scientist's Nov. 12 visit.
"Instead of seeing a few small cascades of centrifuges, which I believed to exist in North Korea, we saw a modern, clean centrifuge plant of more than a thousand centrifuges, all neatly aligned and plumbed below us," Hecker, a Stanford University professor, wrote.
Hecker described the control room as "astonishingly modern," writing that, unlike other North Korean facilities, it "would fit into any modern American processing facility."
The facilities appeared to be primarily for civilian nuclear power, not for North Korea's nuclear arsenal, Hecker said. He saw no evidence of continued plutonium production at Yongbyon. But, he said, the uranium enrichment facilities "could be readily converted to produce highly enriched uranium bomb fuel."
Uranium enrichment would give the North a second way to make atomic bombs, in addition to its known plutonium-based program. At low levels, uranium can be used in power reactors, but at higher levels it can be used in nuclear bombs. Hecker's findings were first reported in The New York Times.
U.S. nuclear envoy Stephen Bosworth's trip to Asia for talks on North Korea comes as new satellite images show construction under way at Yongbyon. That, combined with reports from Hecker and another American expert who recently traveled to the atomic complex, appear to show that Pyongyang is keeping its pledge to build a nuclear power reactor.
North Korea vowed in March to build a light-water reactor using its own nuclear fuel. Hecker, and Jack Pritchard, a former U.S. envoy for negotiations with North Korea, have said that construction has begun.
Yang Moo-jin, a professor at the Seoul-based University of North Korean Studies, said the North's uranium disclosure is meant to force the United States back into nuclear negotiations.
The disclosure, Yang said, also is aimed at a domestic audience during the succession process. "The North wants to muster loyalty among military generals by showing them the North will continue to bolster its nuclear deterrent and uphold its military-first policy," Yang said.
Light-water reactors are ostensibly for civilian energy purposes, but such a power plant would give the North a reason to enrich uranium. While light-water reactors are considered less prone to misuse than heavy-water reactors, once the process of uranium enrichment is mastered, it is relatively easy to enrich further to weapons-grade levels.
North Korea said last year it was in the final stage of enriching uranium, sparking worries that the country may add uranium-based weapons to enlarge its stockpile of atomic bombs made from plutonium. Experts say the North has yielded enough weaponized plutonium for at least a half dozen atomic bombs.
Uranium can be enriched in relatively inconspicuous factories that are better able to evade spy satellite detection, according to U.S. and South Korean experts. Uranium-based bombs may also work without requiring test explosions like the two carried out by North Korea in 2006 and 2009 for plutonium-based weapons.
Hecker said the North Koreans emphasized that the centrifuge facility was operating; although he couldn't verify that statement, he said "it was not inconsistent with what we saw."
"The only hope" for dealing with the North's nuclear program "appears to be engagement," he wrote, calling a military attack "out of the question" and more sanctions "likewise a dead end."
Many questions are still unanswered about North Korea's nuclear program, Hecker wrote, including whether the North is really pursuing nuclear electricity; whether it's abandoning plutonium production; how it got such sophisticated centrifuge technology; and why it's revealing the facilities now.
"One thing is certain," he said. "These revelations will cause a political firestorm."
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