North Korea told Hecker it began construction on the centrifuges in April 2009 and finished only a few days before the scientist's Nov. 12 visit.
Hecker said his first glimpse of the North's new centrifuges was "stunning."
"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," wrote Hecker, a Stanford University professor.
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."
