North Korea appears to be in the process of restarting a nuclear reactor used to produce weapons-grade plutonium, five years after shutting the facility down as part of international disarmament efforts.

New satellite imagery appears to reveal that the 5-megawatt reactor at Yongbyon, which experts believe can produce enough plutonium for one to two bombs a year, shows signs of being operational.

Analysts Nick Hansen and Jeffrey Lewis, writing for the website 38 North, say the satellites show:

"[W]hite steam rising from a building near the reactor hall. The building in question houses the gas-graphite reactor's steam turbines and electric generators."

North Korea blew up the cooling tower on the reactor in 2008 as part of an international agreement to dismantle its nuclear program. But in April, amid growing tensions between North Korea, South Korea and the United States, Pyongyang announced it would restart the reactor.

"This is the reactor that essentially produced the material to build North Korea's nuclear weapons in the past," Joel Wit, a senior fellow with the U.S.-Korea Institute at Johns Hopkins University, tells NPR's Geoff Brumfiel.

"Restarting the reactor means that over time they will be able to produce more material for more bombs," Wit says.

He said that sometime in July, North Korea completed construction of a new cooling system to make the reactor operational.

Now, he says, "They are either in the process of restarting or nearing operation."

As The Associated Press reports:

"When North Korea announced its plans in April, 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.' "

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