VIENNA (Reuters) - North Korea may have shut down a recently restarted reactor which can yield plutonium for bombs, possibly for renovation or partial refueling, a U.S. security institute said, citing new satellite imagery.
North Korea announced in April 2013 that it would revive its aged five-megawatt research reactor at the Yongbyon nuclear complex, saying it was seeking a deterrent capacity.
The isolated East Asian state, which quit the nuclear Non-Proliferation Treaty two decades ago, defends its nuclear arms program as a "treasured sword" to counter what it sees as U.S.-led hostility.
On Thursday, So Se Pyong, North Korea's Ambassador to the United Nations in Geneva, told Reuters that Pyongyang was ready to resume the so-called six-party talks and was not planning a nuclear or missile test.
The United States responded that Pyongyang must first take meaningful steps toward decentralization and refrain from provocative acts.
Early last month, the International Atomic Energy Agency (IAEA) said in an annual report on North Korea that it had seen via satellite imagery releases of steam and water indicating that the Yongbyon reactor may be operating.
In line with this view, the Washington-based Institute for Science and International Security (ISIS) said commercially available satellite imagery from September 2013 until June this year had shown it was operating.
"This assessment was based on the presence of either steam venting from the reactor's turbine building or water being discharged into the river through a pipeline buried east of the reactor," the think-tank said.
However, in imagery from late August and late September "both these signatures are missing," it said on its web site. "For this reason, ISIS assesses that it is possible that the reactor is partially or completely shut down."
North Korea may be carrying out a partial refueling of the reactor's core or it may have shut it down for maintenance or renovation purposes, it said.
"The question of refueling requires closer scrutiny, because the plutonium would be expected, after separation ... to be assigned to nuclear weapons," it said.
The Yongbyon reactor had been technically out of operation for years. North Korea destroyed its cooling tower in 2008 as a confidence-building step in negotiations with South Korea, China, the United States, Japan and Russia.
When North Korea said it planned to revive it, experts said it would probably take about half a year to get it up and running, if it had not suffered significant damage from neglect.
South Korea insists Pyongyang denuclearize for the two sides to come closer together, but few believe the North will ever surrender the ultimate weapon because it provides security for both the country and the government itself.
On Saturday, North Korea sent its highest level delegation to South Korea to attend the Asian Games closing ceremony amid a flurry of diplomatic activity which has raised hopes for improved ties between the arch rivals.
Reporting by Fredrik Dahl; editing by Susan Thomas