North Korea is operating its main plutonium-producing nuclear reactor at the Yongbyon complex, a Washington-based think tank revealed Wednesday, citing thermal infrared imagery.

The CSIS Korea Chair said in a report it found the “clear and conclusive evidence” that the 5-megawatt reactor was operating based on thermal image analysis.

The 5-megawatt nuclear reactor at the Yongbyon Nuclear Complex is the core of Pyongyang’s proliferation efforts given that the reactor has been estimated to be able to produce around six kilograms of plutonium a year.

The think tank underscored the continued water discharge from the cooling system as key evidence of the operation of the main reactor at the Yongbyon complex.

Multiple thermal images taken from September to November showed warm water being dumped from the cooling system of the 5-megawatt nuclear reactor and funneled into the nearby Kuryong River.

Specifically, the images taken during the three months showed “continued and similar patterns of warm water discharge” from the reactor’s outflow channel into the river.

The CSIS Korea Chair identified temperature variations of up to 4 degrees Celsius in the Kuryong River when heated water was discharged into it.

“The warmest temperatures of the dispersal pattern are found near the mouth of the water discharge channel outlet and reveal clear indications that the reactor is in operation,” the CSIS Korea Chair, which runs the Beyond Parallel project, said.

The multiple thermal images provided by the CSIS Korea Chair report showed the heat concentration and a warm thermal pattern around the water discharge area in the Kuryong River. 

In general, the temperature increase was observed in some parts of the river as the heated water was dispersed from the edge of the river where the outflow channel is located.

The thermal infrared analysis also corroborated the previous satellite images, which showed “water discharge from the reactor and probable steam exhaust from the steam turbine and electric generator.”

In addition, the CSIS Korea Chair said the irradiated fuel rods discharged from the nuclear reactor would “most likely to be subsequently sent to the Radiochemical Laboratory for reprocessing” to produce additional weapons-grade plutonium.

Pyongyang can extract plutonium for use in nuclear weapons at the radiochemical laboratory by reprocessing spent nuclear fuel rods obtained from the 5-megawatt reactor.

But the US think tank concluded that there were “no strong indications of any significant activity at the Radiochemistry Laboratory or thermal plant” based on the combined imagery analysis.

The CSIS Korea Chair also said thermal patterns showed there were no indications of the operation of the IRT-2000 research reactor. The thermal reactor was obtained from the Soviet Union in the mid-1960s, and requires 36 percent enriched uranium.

Wednesday’s analysis is in line with the previous assessment of the International Atomic Energy Agency.

The UN’s nuclear watchdog in late November said Pyongyang had continued to operate the 5-megawatt nuclear reactor from early July this year. The reactor had been temporarily shut down from early December 2018 to that month.

But the IAEA said there were no signs of a new processing campaign and the operation of the Radiochemical Laboratory since early July 2021. 

Suspected nuclear activities were previously observed at the laboratory from mid-February to early July.

In November, US think tank 38 North also released satellite images showing the continued operation of the main nuclear reactor at the Yongbyon complex, pointing to the steam exhaust and water discharge as the key evidence.  

By Ji Da-gyum (dagyumji@heraldcorp.com)