North Korea continues to show signs suggesting it could be reprocessing spent nuclear fuel to harvest plutonium for nuclear weapons, a U.S. research institute said Friday, citing recent satellite imagery.

The May 22 imagery of the North’s Yongbyon nuclear complex shows nine tanks in front of the spent fuel reception building next to the reprocessing plant, and smoke from a coal-fired plant providing steam for reprocessing activity, the Institute for Science and International Security (ISIS) said.

“The activity of the coal plant and the presence of tanks in front of the spent fuel reception building are signatures consistent with spent fuel reprocessing,” the institute said in a report. “However, it remains difficult to determine whether plutonium separation has occurred, is occurring, or will occur in the near future.”

The imagery also showed no signs of full-power operation at Yongbyon’s 5-megawatt reactor that produces spent fuel, such as water discharge or steam venting from the reactor’s turbine building, but the absence of the two indicators does not mean the reactor is shut down, the institute said.

“Our assessment is that the reactor has been operating intermittently or at low power since mid-2014,” it said.

Director of National Intelligence James Clapper said in a worldwide threat assessment earlier this year that the North had restarted its 5-megawatt reactor and has since run it long enough to harvest plutonium “within a matter of weeks to months.”

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests, in 2006, 2009 and 2013. The North conducted its fourth nuclear test on Jan.6, claiming it successfully detonated a hydrogen bomb. (Yonhap)