The colour and volume of the steam indicated that the reactor was in or nearing operation, the institute said.

 

Pyongyang vowed² to restart facilities at its main Yongbyon nuclear complex in April, amid high regional tensions.

 

The reactor can produce plutonium, which North Korea could use to make nuclear weapons.

 

Analysts³ believe North Korea already possesses between four and 10 nuclear weapons, based on plutonium produced at the Yongbyon reactor prior to mid-2007, when the facility was closed down.

 

 

The report, which was published on the 38 North website on Wednesday, was written by the US-Korea Institute at Johns Hopkins University. The institute uses satellite imagery to monitor developments in North Korea.

 

The reactor uses steam turbines to generate electricity, and the steam seen in satellite imagery from 31 August indicated that the electrical system was about to come online, the report said.

 

"The reactor looks like it either is or will within a matter of days be fully⁵ operational, and as soon as that happens, it will start producing plutonium," report author Jeffrey Lewis told the BBC.

 

"They really are putting themselves in a position to increase the amount of material they have for nuclear weapons, which I think gives them a little bit of leverage in negotiations⁶, and adds a sense of urgency on our part," he added.

 

The five megawatt reactor can produce spent fuel rods that can be made into plutonium, which experts believe North Korea used for its nuclear tests in 2006 and 2009. North Korea conducted its third, most-recent test in February, but it is not clear whether plutonium or uranium was used.

 

In a November 2010 report following a visit to Yongbyon, US scientist Siegfried Hecker said that based on what he saw he believed North Korea could "resume all plutonium operations within approximately six months" at Yongbyon, then shut down, if so inclined.

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