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WASHINGTON — Satellite imagery suggests that North Korea has restarted a research reactor capable of producing plutonium for weapons at its Yongbyon nuclear complex, a U.S. research institute said on Wednesday.

U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said a satellite image from Aug. 31 shows white steam rising from a building near the hall that houses the plutonium production reactor’s steam turbines and electric generators.

“The white coloration and volume are consistent with steam being vented because the electrical generating system is about to come online, indicating that the reactor is in or nearing operation,” said the Washington-based institute.

The reactor can produce 6 kilograms (13.2 pounds) of plutonium a year, the report added.

There was no immediate comment on Wednesday from the International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog.

A spokesman for the State Department’s Bureau of East Asian and Pacific Affairs declined to respond the report, citing a policy of not commenting on intelligence matters, but said Pyongyang’s “nuclear program remains a matter of serious concern.”

The spokesman repeated Washington’s longstanding call for North Korea to comply with a 2005 aid-for-disarmament agreement signed by North Korea, its neighbors, and the United States. Under that pact, Pyongyang would have dismantled its nuclear program in exchange for economic and energy aid.

North Korea has conducted three nuclear tests since 2006.

“Acknowledging that we are not completely certain yet, this is very disappointing but not at all unexpected,” James Acton, an analyst for the Nuclear Policy Program at the Carnegie Endowment for International Peace, a Washington think tank.