The U.S.-Korea Institute at Johns Hopkins University said a building in the city of Yongbyon that had been renovated to produce fresh fuel rods for the reactor appeared to be operational, the New York Times reported.
The building had been shown to visiting American nuclear experts in 2008, the same year six-nation talks on ending Pyongyang's nuclear program collapsed.
At the time, the building was used to make fuel rods for a graphite-moderated reactor.
The following year, North Korea expelled U.N. nuclear monitors. Since then, the country has begun reactivating its nuclear facilities in Yongbyon.
The institute also identified another building it said could be used to produce fuel for a light-water reactor.
"The identification of these facilities indicates a more wide-ranging, extensive effort by North Korea to modernize and restart the Yongbyon complex," the institute said on its website, 38 North.
"Based on this analysis, it appears the North Koreans anticipated that future requirement before 2009 and knew the lead times required to produce the necessary fresh fuel. Once international inspectors were expelled in 2009, they moved forward with the renovation of the main pilot plant building as part of what was likely their contingency plan for reactivation," the website said.
North Korea has two operating nuclear reactors and a third is under construction.
North Korea is believed to have only 25 percent of the fuel rods necessary for the reactor to reach full power.