Recent satellite imagery shows strong indications that North Korea has already begun or is set to start reprocessing spent nuclear fuel to harvest plutonium for nuclear weapons, the website 38 North said.

The April 11 imagery showed a loaded railroad flatcar at the Radiochemical Laboratory reprocessing plant, a key signature that was observed only on a few rare occasions in the early 2000s when Pyongyang reprocessed nuclear fuel to extract plutonium, 38 North said.

The flatcar is loaded with what appears to be four rectangular tanks or casks that could be used to supply chemicals used in a reprocessing campaign intended to produce additional plutonium, haul out waste products or a number of other related activities, it said.

Last week, 38 North reported spotting "exhaust plumes" from a building used to heat the reprocessing plant.

"The presence of a loaded flatcar, together with the earlier presence of exhaust plumes, suggests that North Korea is preparing or conducting a reprocessing campaign to separate more plutonium for weapons," 38 North said.

In addition, the North also built a new dirt access road and began excavating parallel to a building, known as Building 500, which was used to store waste from earlier reprocessing campaigns. 

The excavation could be aimed at expanding the building's storage capabilities or improving its design features in order to reactivate the facility for a new reprocessing campaign, 38 North said. It could also mean that a reprocessing campaign is already underway and the building has been reactivated for waste storage, it said.

The 5-megawatt nuclear reactor, which produces spent fuel, shows no indications of steam generation or cooling water exhaust, suggesting that the reactor is not operating or is running at an extremely low level, 38 North said.

"Should this be the case, it would support the above indications of a reprocessing campaign as the 5-megawatt reactor would typically have to suspend operations to unload the spent fuel rods to be sent to the Radiochemical Laboratory," it said.

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.

Last month, Director of National Intelligence James Clapper said in a worldwide threat assessment that the North had restarted its 5-megawatt reactor at Yongbyon and has since run it long enough to harvest plutonium "within a matter of weeks to months." (Yonhap)