According to an article in The Diplomat, North Korea has for a decade or so been operating a uranium enrichment plant hidden in plain sight just outside Pyongyang, only 1 km from the Pyongyang-Nampo expressway.
For years, large vehicles were spotted entering and leaving the site, while highly enriched uranium could have been sent to research centers, according to the Diplomat. Nearby is the Taesong Machine Factory where missiles are built.
The site’s proximity to Pyongyang is notable; the enrichment site was built a little more than five kilometers away from the city’s Mangyongdae neighborhood, North Korean founder Kim Il Sung’s birthplace, per North Korean propaganda. Thus, for more than a decade, North Korea has been enriching uranium in what is effectively a Pyongyang suburb, on the doorstep of Kim Il Sung’s mythologized birthplace.
The site’s relatively good connectivity may serve an important role in enabling its smooth operation. Large gas centrifuge facilities like Kangson require regular resupply and depleted uranium tail disposal. Satellite imagery from recent years seen by The Diplomat shows evidence of regular visits by a number of large vehicles and trucks. The western side of the main hall appears to have a large opening to allow trucks to enter the building as well. These vehicles could transport important materials for the uranium enrichment process in and out of the facility, including uranium hexafluoride and depleted ‘tails’. They could additionally transport highly enriched uranium to where its needed, including North Korea’s Nuclear Weapons Institute. (The Kangson site is also not far from the Chamjin Missile Factory.) [...]
By 2010, North Korea had established six cascades of 2000 P2 centrifuges at Yongbyon—a fact that they were happy to share with Siegfried Hecker, an American nuclear scientist and a former director of the U.S. Department of Energy’s Los Alamos National Laboratory. Hecker visited in November that year with low expectations. “Quite frankly, I was expecting not much, sort of a couple of dozen centrifuges,” he said in 2011, following his visit. Instead, he found a state-of-the-art facility at Yongbyon.
Kangson, meanwhile, had likely served as North Korea’s first major gas centrifuge facility, allowing the country’s nuclear scientists to experiment with initial gas centrifuge cascades before setting up the facility at Yongbyon knowing that it would one day be shown to foreign scientists. In the end, North Korea successfully kept a lid on Kangson....
So the North Koreans, it seems, had no intention of openly admitting to the existence of this Kangson site, admitting only to the Yongbyon site.
North Korea could have made a bet that the U.S. intelligence community did not know about its ongoing activities at this site and that submitting to a verifiable freeze in enrichment activity at Yongbyon would suffice. Per a recent report by NBC News, U.S. intelligence has calculated that North Korea is actively intending on deceiving the United States, likely by concealing key sites and other measures.
Further proof, along with the secret base depot for ICBMs hidden away in the mountains of Chagang Province, that denuclearisation was never an option for Pyongyang.