North Korea in April announced plans to restart the reactor at Nyongbyon that was shuttered in 2007 under a disarmament deal. Satellite imagery at the end of August showed steam coming from an adjacent generator building, suggesting North Korea was in the process of starting it up.
The US-Korea Institute at Johns Hopkins School of Advanced International Studies says that a satellite photo from September 19 shows hot water being released through a drainpipe that is part of a cooling system for the five megawatt reactor. That most likely shows the reactor is now operating and its generators are producing power.
North Korea has yet to announce that the reactor is working again. The United States has said that any move to do so would be a "very serious matter" and violate United Nations Security Council resolutions.
The reactor can be used both to generate electricity and produce fissile material for nuclear weapons. Once it is operating, it can produce about 6 kilograms of plutonium a year, enough for one or two bombs. North Korea has conducted three nuclear test explosions since 2006, and experts estimate it already has enough plutonium for between four to eight crude weapons.
The United States and South Korea today agreed to work together to strengthen the South's ability to deter threats from North Korea. Their defense chiefs also endorsed a new military strategy to better coordinate the response to a nuclear, chemical or other attack from Pyongyang.
In February, North Korea conducted its third nuclear test, prompting the Security Council to tighten sanctions. That angered the North, which threatened nuclear strikes on the US and its allies.
