'Deeply Troubling': North Korea has resumed nuclear reactor operation, says UN

North Korea appears to have restarted the operation of its main nuclear reactor used to produce weapons fuels, the U.N. atomic agency said, as the North openly threatens to enlarge its nuclear arsenal amid long-dormant nuclear diplomacy with the United States.

The development, disclosed in the agency’s annual report on North Korea’s nuclear activities, adds a new challenge to President Biden’s foreign policy agenda, alongside the dangerous U.S. withdrawal from Afghanistan and stalemated talks on restoring the 2015 deal on Iran’s nuclear program.

“Since early July, there have been indications, including the discharge of cooling water, consistent with the operation of the reactor,” said the report by the International Atomic Energy Agency.

Both the plant and the lab are located in North Korea's best-known nuclear complex, Yongbyon.

The IAEA and other independent analysts have previously reported on the observed activity at the radiochemical laboratory and believed it may have been part of a campaign to turn nuclear fuel into plutonium for nuclear weapons.

IAEA Director-General Rafael Grossi said in June that the duration of activity at the lab was consistent "with the time required for a reprocessing campaign."

The complex, which North Korea calls “the heart” of its nuclear program and research, has been at the center of international concerns for decades. It’s not clear exactly how much weapons-grade plutonium or highly enriched uranium has been produced at Yonbyong and where North Korea stores it.