Researchers from U.S.-based groups examined satellite images from August 31. They showed two columns of steam rising from a building, believed to house the reactor’s steam turbines and electric generators.

The steam indicates that the reactor is in or nearing operation, wrote Nick Hansen and Jeffrey Lewis, in a 38 North blog post, a program of the U.S.-Korea Institute at SAIS.

Work to reopen the reactor progressed throughout this spring and summer, they wrote.

“This is something they (North Koreans) very much need to arm their arsenal, and that is really linked into what they see as marketing power on the international plane,” Jasper Kim, founder of Asia-Pacific Global Research Group, told CNN.

“Without this type of nuclear capability, or at least the perception of the threat of having nuclear capabilities, North Korea really has few bargaining chips.”