This suspicious activity has been noted at the Yongbyon Radiochemical Laboratory complex which is used to produce plutonium for building nuclear weapons, reported 38 North, the North Korea monitoring project.
The plumes of exhaust has been captured commercial satellite images taken March 12 and over the preceding five weeks, the group says.
"This activity is unusual, the report by the Washington," added the report.
"Exhaust plumes have rarely been seen there and none have been observed on any examined imagery this past winter," the report says.
The exhaust plumes were observed possibly three times at the Radiochemical Laboratory's Thermal plant.
"This activity is unusual since exhaust plumes have rarely been seen there and none have been observed on any examined imagery this past winter.
The plumes suggest that the operators of the reprocessing facility are heating their buildings, perhaps indicating that some significant activity is being undertaken, or will be in the near future.