Hecker was shown 2,000 uranium centrifuges at Nyongbyon in 2010, but it's not clear whether the centrifuges have been reconfigured to make highly enriched uranium. It's also unknown what fuel North Korea used in its Feb. 12 test, its third since 2006; a confirmed uranium-based nuclear test would show that North Korea has centrifuges producing highly enriched uranium.
North Korea built its secret uranium program at its main nuclear facility without the knowledge of the U.S. intelligence community, Bruce Klingner, a former U.S. intelligence officer and now an analyst at The Heritage Foundation think tank in Washington, said in an email. "As such, we do not know how many covert uranium enrichment sites North Korea has nor how many uranium weapons they can produce per year."
Still, scientists can't make a uranium bomb overnight.
Even if the North's 2,000 centrifuges were configured properly and spinning 24 hours a day, every day for a year, they could only make one or two uranium bombs, said Kune Y. Suh, a nuclear expert at Seoul National University.
The North's plan to restart the plutonium reactor looked to some like an admission that Pyongyang hasn't made much progress in its uranium enrichment program.
"Why else would it go to the trouble of a time-consuming and expensive restart to plutonium production at a known and vulnerable facility?" Thielmann asked.