The 5 MWe nuclear reactor and surrounding buildings at the fissile material production facility at North Korea’s Yongbyon complex in North Pyongan province were refurbished this past summer with brightly colored exterior walls and ceilings. But the work means more than just a facelift for those aging, worn-down buildings.
An analysis of thermal infrared satellite imagery suggests that North Korea installed thermal cladding to the reactor building to keep heat inside. Although the 5 MWe reactor is evidently in operation (given the coolant flowing from the pump stations into the Kuryong River), no heat is detected around the reactor.
Since the reactor’s temperature is actually cooler than the auxiliary buildings, it could easily be assumed that the facility isn’t in operation. Recent thermal infrared images showed that the reactor area was much cooler than its surroundings, a highly irregular and unusual state of affairs.
North Korea uses thermal cladding on important facilities to block radiation of heat and prevent heat loss. The regime appears to have installed cladding consisting of urethane foam and other insulating materials to the reactor’s outer walls and roof as part of renovations that are obfuscating analysis of thermal radiation detected by satellites.
It’s unclear whether blocking the international community’s satellite-powered surveillance represents a strategic move that was the primary purpose of the thermal cladding or whether that was a secondary benefit of preventing heat loss. Going forward, these measures are expected to reduce the utility of thermal infrared satellite imagery for analyzing operations at North Korean nuclear facilities.
Yongbyon 5 MWe nuclear reactor still in operation
Recent satellite images of the Yongbyon nuclear complex in North Pyongan province indicate that the 5 MWe reactor is in operation. A high-resolution satellite image taken on Nov. 3 shows foam where water is entering the Kuryong River from a pump station connected to the reactor. This water is presumed to be coolant used during reactor operations.
There are no indications of coolant being released from the pump station connected to the 25 MWe light water reactor also located at the complex. That suggests this experimental light water reactor hasn’t entered full operations.
According to recent analysis by 38 North, a news website specializing in North Korea, the 25 MWe light water reactor is still in a pre-operational testing phase. This experimental reactor, built in the 2010s, entered the testing phase in October 2023. It produces five times as much electricity as the current 5 MWe reactor.
Renovation of the Yongbyon reactor building
A high-resolution satellite image (on the right) from early November shows that the outer walls and roof of the 5 MWe reactor building at Yongbyon now have a bright new color. Compared to a satellite photograph from Google Earth in early June, the reactor’s walls appear to have been repainted.
“The building’s roof was resealed or replaced and the exterior walls painted between April and August of 2025,” 38 North reported in a Nov. 21 article, adding that “the roofs and exteriors of buildings associated with the 5 MWe were repainted or refurbished.” The Nov. 3 satellite image shows that the roofs of a nearby spent fuel storage building and warehouse now have a new color.
The 5 MWe reactor, which was built in the mid-1980s, is an old and dilapidated facility that’s thought to be irregular and inefficient in its operations. But the reactor building seems to have gotten a facelift in the recent renovations.
Heat radiation blocked after new structure built at the reactor
Thermal infrared images taken from Landsat-9, a U.S.-operated earth observation satellite, were used to image the area around the Yongbyon reactor. On Nov. 3, no heat was being radiated from the reactor, which actually looked cooler than the surrounding area (see image on right). The reactor building is displayed in green, indicating a low temperature of 3–4°C, lower than the average in the area.
In high-resolution satellite imagery, coolant being released into the river is evidence that the 5 MWe reactor is in operation. Indeed, coolant near the shore is displayed in purple, at a higher temperature of 8°C.
It’s highly irregular and unusual for thermal infrared imagery to display an online reactor at such a low temperature. When the Yongbyon reactor was in operation, thermal infrared analysis typically showed a significantly higher temperature than the surrounding area. An analysis of the Apr. 25 image on the left (before the renovations) found the reactor area to have a temperature of 22–23°C (marked in lavender), which is higher than the average temperature (19°C), showing the facility was operating as normal.
Thermal cladding may be designed to block satellite surveillance
A careful review of the preceding results suggests that North Korea installed thermal cladding to block heat radiation on the roof and exterior walls of the 5 MWe reactor during the recent renovations. This may be part of a North Korean strategy of avoiding surveillance using thermal infrared satellite imagery.
Thermal cladding refers to a type of wall insulation designed to prevent heat loss while keeping the indoor temperature constant. Cladding typically consists of urethane foam or other materials attached to the outer walls.
Because it reduces the amount of heat passing through the outer walls and roof, thermal cladding improves energy efficiency and reduces the cost of heating and air conditioning. Another advantage is that it minimizes damage from condensation, mold and humidity. Such cladding has its downsides, though: since it’s flammable, it’s more prone to fires.
Our review of thermal infrared satellite imagery this summer suggests that refurbishment work on the walls and roofs of the 5 MWe reactor and nearby buildings at Yongbyon took place in June and July. The installation of thermal cladding on the Yongbyon facilities was apparently designed to suppress, insofar as possible, the international community’s detection and analysis of thermal radiation through satellite imagery. This is likely to partially limit future efforts to monitor and analyze North Korean facilities through thermal infrared images.
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