Story highlights Researchers point to steam at North Korean facility

They write that it's a sign that the reactor is near or in operation

North Korea has opened, shut, re-opened, reshut this reactor before

North Korea said in April it would restart nuclear facility in April

Satellite images of North Korea's Yongbyon nuclear facility have again raised questions about whether the country has restarted its plutonium production reactor -- regarded by western experts as a key component in the development of a nuclear weapon.

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."

But Pyongyang has gone down this path before.

First completed in the 1980s, North Korea agreed to shut the Yongbyon facility in 1994 after backing down from a threat to withdraw from the nuclear nonproliferation treaty. It restarted the reactor in 2002, before it was once again disabled in 2007 after a six-party agreement between China, the United States, North and South Korea, Japan and Russia.

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In April this year, North Korea stated that it would be "readjusting and restarting all the nuclear facilities" in Yongbyon, which includes the uranium enrichment plant and the reactor, the country's state-run news agency KCNA said.

"This work will be put into practice without delay," the North Korean report stated at the time.

Analysts say it appears North Korea has fulfilled its word.

David Albright and Robert Avagyan, from the Institute for Science and International Security , examined the latest satellite images and agreed the venting appeared most likely connected to the operation of the reactor.

They also wrote last month that North Korea appeared to have greatly expanded a building used in the uranium enrichment process.

The expansions and developments "provide North Korea with the ability to expand its stocks of plutonium, as well as produce more highly enriched uranium for nuclear weapons," wrote Albright and Avagyan.

They estimated that North Korea would likely need two to three years to produce plutonium, which is essential in creating nuclear weapons.

"There remains time to negotiate a shutdown of the reactor before North Korea can use any of this new plutonium in nuclear weapons" they added. "If a shutdown is achieved in the next six months, the reactor would have produced very little plutonium."