North Korea's main nuclear reactor is believed to have been shut down for more than two months amid signs Pyongyang may be preparing to restart the nearby reprocessing facility to extract weapons-grade plutonium from spent fuel, a U.S. think tank said Wednesday.

Satellite imagery shows that the 5-megawatt reactor at the Yongbyon nuclear complex has been closed for about 10 weeks, longer than what is required for routine maintenance, according to 38 North, a website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies.

The assessment was made based on the continued absence of two key indicators of reactor operations: vented steam and discharged water. Neither was seen in imagery from Oct. 28 to Nov. 3, said 38 North in a report.

"New evidence is accumulating that suggests the shutdown may have allowed the North to remove a limited number of fuel rods, possibly failed, from the reactor, and Pyongyang may be preparing to restart the Radiochemical Laboratory," where spent fuel rods are reprocessed to extract weapons-grade plutonium, 38 North said.

It is not clear why the reactor was shut down, but speculation has focused on such possibilities as routine maintenance; more serious problems, possibly with the cooling systems; a partial unloading of fuel rods, possibly defective; and the removal of the entire core, the report said.

Evidence from the most recent imagery seems to reinforce the hypothesis of a partial unloading of fuel rods, the report said.

Those rods would be transported underground from the reactor to the spent fuel cooling pond and then moved to the reprocessing facility by vehicles, it said.

The shutdown assessment could run counter to that of South Korea's government. 

When a separate U.S. think tank, the Institute for Science and International Security (ISIS), said in early October that it believed the reactor was shut down temporarily, Seoul's foreign minister, Yun Byung-se, said he believes the reactor was up and running.

It was not clear if Seoul still stands by the assessment.

With regard to the reprocessing facility, there was little activity throughout the summer and into the fall. However, on Nov.

4, steam was seen rising from a large cooling tower at auxiliary buildings just southeast of the plutonium separation building, it said.

   Such steam is consistent with activities before commencing operations, it said.

   In the same image, three vehicles -- two trucks and one unidentified vehicle -- were seen on the road in front of the door to the reprocessing complex building, to which spent fuel rods are transported for reprocessing, the report said.

Satellite imagery also showed piles of gray material outside the old pilot fuel fabrication facility, which may indicate that a chemical process is taking place, possibly related to the production of new rods for the 5-megawatt reactor, the report said.

38 North said Pyongyang may also be trying to mask indicators of reactor operations.

The imagery taken on Oct. 28 indicates that the North may be building a new pipeline running from the pipe that carries hot water and steam from the reactor turbine building to the river, a move that would complicate future efforts to monitor reactor operations, 38 North said.

"Observing the white froth from the steam and water discharged into the river helped private analysts determine that the reactor restarted in August 2013 as well as the short period when it has been shut down over the past year," it said. "Without this indicator, determining what is happening at the facility will become more difficult."