North Korea has started to expand a suspected nuclear facility near Pyongyang, according to NK Pro analysis of satellite imagery, after leader Kim Jong Un called for increasing nuclear weapon production last year.

Planet Labs and Airbus images of the main building at the Kangson site alleged to house uranium enrichment operations show workers have been building a new annex along the length of the rear (west) side of the building since last month.

A new rear wall around 395 feet long (120 meters) was built about 40 feet (12 meters) away from the rear of the building, while blue-colored roofing was installed covering the 10,760 square-foot (1,000 square-meter) area between March 16 and 23. 

The newly covered area is around 22% the size of the existing 48,430 square-foot (4,500 square-meter) main building. It could be related to increasing floor space for centrifuges if the construction involves knocking down outer walls.

North Korea built similarly sized expansions to centrifuge facilities at a uranium enrichment plant inside the country’s main Yongbyon nuclear complex in 2013 and again in 2021.

The construction of inner support walls in the rear space indicates that the new work did not just involve installing a simple cover to obscure the arrival of vehicles. It may point to the creation of at least eight separate 645 square-foot (60 square-meter) sections or rooms.

The construction also added a narrow rectangular section measuring around 150 by 13 feet (45 by 4 meters) to the southern end of the rear wall, which may be for storage or support equipment, according to a high-resolution Maxar image from March 17 reviewed by NK Pro.

The image also showed possible construction materials stacked around the side and front of the building.

Last September, DPRK leader Kim Jong Un demanded the nuclear sector “exponentially increase nuclear weapons production to realize all kinds of nuclear strike methods and deploy them to various military branches.” He also called for “steadily increasing the production of nuclear weapons” at a party plenum in December.

In the first public report on the Kangson site in 2018, NK Pro contributing analyst Ankit Panda said the main building is “assumed to contain the main gas centrifuge cascades, which output uranium highly enriched in uranium-235 and suitable for use in nuclear weapons.”

A U.S. intelligence source told Panda at the time that Washington started to suspect the site had a role in nuclear weapons production around 2010 and that it may have the capacity to produce more highly enriched uranium (HEU) than facilities at Yongbyon.

International Atomic Energy Agency (IAEA) Director General Rafael Grossi said Kangson was “relevant” to North Korea’s nuclear program in 2020 and included references to “ongoing indications of activities” as recently as Sept. 2022. However, he did not mention Kangson in his five briefs to the IAEA Board of Governors since then, including one earlier this month.

The U.N. Panel of Experts in charge of monitoring sanctions on North Korea called Kangson “an alleged clandestine uranium enrichment facility” in its latest report released last week, saying it detected “constant activity of several types of trucks” outside the main building in recent months.

North Korea has never publicly reported on the facility, and some analysts have found it puzzling that the Kangson compound is located right next to residential areas off the main highway, less than five miles (8.6 km) west of Pyongyang, while also having weaker security than some other weapons-related facilities nearby.

Meanwhile, North Korea started operating an experimental light-water reactor (ELWR) at the Yongbyon complex last October to potentially boost weapons-grade plutonium production.

Edited by Bryan Betts

Updated on March 27 at 4:02 p.m. KST to include a high-resolution Planet Labs image taken earlier the same day