In the latest commercial satellite imagery from March 30, there were no clearly visible steam plumes coming from the Generator Hall of the 5-megawatt plutonium-producing reactor at the Yongbyon nuclear complex, 38 North said.
The North claims the nuclear facilities are aimed at producing electricity, but they have long been suspected of producing weapons-grade plutonium.
The website did not say what may have prompted the shutdown.
The report comes amid hopes for a resolution to the standoff over North Korea's nuclear weapons program.
To discuss the regime's denuclearization, South Korean President Moon Jae-in is due to meet with North Korean leader Kim Jong-un on April 27. US President Donald Trump is also expected to sit down with Kim before the end of May.
Still, 38 North reported other activities around the reactor, including a major excavation project near the cooling water outfall.
"When coupled with recent efforts to dam the river below this point to create a reservoir, (this) could indicate an attempt to provide a more steady flow of water into the facility," it said. "This would allow for the reactor to run more continuously and safely in the future."
Trucks have also been spotted near the reactor, possibly to carry out maintenance or repairs or transport spent fuel rods to the reprocessing plant known as the Radiochemical Laboratory, it noted.
"Despite the apparent reactor shutdown, there was no evidence (as of March 30) of plutonium reprocessing taking place at the Radiochemical Laboratory. However, this development should be monitored closely in the future," said the website. "Presently, imagery indicates only routine vehicle movements and no activities associated with preparations for, or an ongoing, reprocessing campaign at the Radiochemical Laboratory."
The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make one bomb a year.
The reactor has provided Pyongyang with plutonium that the regime used in its first three nuclear tests, in 2006, 2009 and 2013. The North conducted its fourth and fifth nuclear tests in January and September 2016, and its sixth and most powerful test last September.
38 North also said work continues on a building adjacent to the Experimental Light Water Reactor, "but there are no obvious signs that the reactor is approaching operational status."
The New York Times reported last month that the ELWR appeared to be coming online after years of construction, possibly presenting a new challenge to Trump.(Yonhap)