Reactor work underlines Pyongyang's nuclear threats North Korea has already begun construction at a closed plutonium reactor that it vows to restart, and it could be back in operation sooner than expected, a U.S. research institute said. Pyongyang announced its plans Tuesday, the latest in an almost daily string of threats toward the United States and South Korea since it faced international censure over its latest nuclear and missile tests. The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies has analyzed recent commercial satellite imagery of the Nyongbyon nuclear facility, where the reactor was shut down in 2007 under the terms of a disarmament agreement. A cooling tower for the reactor was destroyed in 2008. The analysis published Wednesday on the institute's website, 38 North, says that rebuilding the tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks. "Pyongyang may be poised to prove wrong conventional wisdom that it will take months to restart its reactor, and in the bargain it is also showing us that they mean business by accelerating the process of producing more material for nuclear weapons," said Joel Wit, 38 North editor and a former U.S. State Department official. North Korea also said it would restart a uranium enrichment plant. Both facilities could produce fuel for nuclear weapons. State Department spokeswoman Victoria Nuland told reporters that restarting the plutonium reactor would be "extremely alarming" but added: "There's a long way to go between a stated intention and actually being able to pull it off."