WASHINGTON (Yonhap) -- North Korea temporarily shut down its 5-megawatt nuclear reactor, a key facility in its production of plutonium, or operated it at reduced levels following problems with a dam providing cooling water, a U.S. think tank said Friday.

The website 38 North, run by the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies, made the analysis based on satellite imagery, citing the absence of "white foam discharge" that occurs when the reactor runs.

The foam was absent in the May 16 image, but reappeared on imagery from May 18-22, according to the website.

"Imagery from early April to mid-May indicates that the 5-MWe reactor was probably shut down or operated at reduced power levels for an undetermined amount of time," it said.

The reactor at the North's main Yongbyon nuclear complex has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods which, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The suspected shutdown or reduced operations came after a sand dam built in March across the Kuryong River to provide cooling water for the reactor failed in two places between April 7 and May 16, the website said.

When it became apparent the dam was failing, the reactor's operators could have "worried its loss could cause the river channels to shift further to the east away from their cistern, and shut down or reduced the power of the reactor for safety reasons," the website said.

"When that didn't happen, because the dam did not completely fail and the river water was higher than normal, the decision was made to restart it," said.

In addition to the 5-megawatt reactor, the North has also been building a larger-scale light water reactor at Yongbyon that experts say could give Pyongyang enough plutonium to make about five or six weapons a year.

The new reactor is "externally complete" but the facility has yet to become operational, according to 38 North.

It is unclear whether the start-up has been delayed or if the project is proceeding according to schedule, but if it has been delayed, one possible cause could be the difficulty the North Koreans face in producing light water reactor fuel assemblies, it said.

Another possibility could be the continued failure of the dam, meant to ensure a steady, reliable supply of water for the reactor's cooling system, it added.