North Korea this month lifted the veil on one of its most closely guarded nuclear secrets, releasing the first public photos of centrifuges it uses to make bomb-grade uranium. The revelatory images of North Korean leader Kim Jong Un touring a vast centrifuge hall, along with the recent startup of a reactor that may be producing plutonium and tritium for atomic weapons, heighten concerns over the rogue nation’s growing arsenal. They also help bring its nuclear program into sharper focus.
Kim’s nuclear whistle stop, which also included images of a smaller centrifuge hall, follows a speech in which he reiterated a 2023 vow to “exponentially” increase his nuclear stockpile. He has suggested the effort will include large numbers of tactical nuclear weapons, lower yield devices designed for short- or medium-range missiles. “North Korea is deadly serious about deploying large numbers of tactical nuclear weapons,” says Jeffrey Lewis, a North Korea expert at the James Martin Center for Nonproliferation Studies. Tactical nukes would pose an especially grave threat to neighboring South Korea.
North Korea has already amassed enough fissile material for up to 90 warheads, according to an estimate published last summer by a team led by Hans Kristensen, director of the nuclear information project at the Federation of American Scientists (FAS). From 2006 to 2017, North Korea conducted six nuclear tests. Siegfried Hecker, a former director of Los Alamos National Laboratory, believes the blasts included two uranium devices that would be suitable for tactical weapons, and one thermonuclear bomb that likely used tritium to boost its yield with a jolt of nuclear fusion. FAS estimates North Korea has assembled about 50 nuclear weapons. But there is uncertainty over what kind they are, how many have been fitted into missiles, and whether any of those missiles can reliably target the U.S. mainland or nearer territories such as Guam and Hawaii.
Still, the recently released photos held rare clues to the nuclear effort. Seeing them, Hecker, a metallurgist who visited North Korea’s Yongbyon Nuclear Complex in 2010, had a moment of déjà vu. That trip—his seventh and last to North Korea—included a whopping surprise: a previously unknown lair filled with hundreds of centrifuges resembling those used by Pakistan. Hecker says the new photos reveal a centrifuge hall that “is almost a carbon copy of what I saw” but with modest upgrades, such as piping that would make the centrifuges spin faster.
The tight packing of the pictured centrifuges also indicates they perform the early stages of enriching uranium hexafluoride gas, in which cascades—typically of a couple thousand centrifuges—raise levels of fissile U-235 from about 0.7% to a few percent. Centrifuges that spin highly enriched uranium (HEU) up to weapons-grade levels, about 90% U-235, are spaced farther apart in cascades of a couple hundred units to reduce the risk of neutrons flowing between centrifuges and creating a self-sustaining fission reaction—criticality—that would damage the machines. “You worry about criticality,” says Olli Heinonen, a distinguished fellow at the Stimson Center and former head of safeguards at the International Atomic Energy Agency. North Korea, he says, apparently did not wish to reveal its HEU centrifuge cascades.
The location of the enrichment site is also hidden. According to Lewis, satellite images indicate the two centrifuge halls are part of Kangson, a complex a few kilometers southeast of Pyongyang that North Korea had endeavored to keep under wraps before it was outed in 2018. Others are not convinced. Another unknown, Heinonen notes, is where North Korea produces uranium hexafluoride gas.
Observers wonder whether the uranium enrichment halls are run by North Korea’s enigmatic Nuclear Weapons Institute (NWI), whose director accompanied Kim on the tour. Heinonen and others believe NWI is comprised of several laboratories, and suspect it is North Korea’s equivalent of the Los Alamos or Lawrence Livermore national laboratories—the brain trust for the U.S. nuclear weapons program. In a 2018 letter to then-President Donald Trump, Kim offered to shutter NWI if the two nations could come to terms on a nuclear deal. “That was a blockbuster offer,” Hecker says. “Imagine closing Los Alamos or Livermore. … That would have been [the end] for their nuclear weapons business.”
That deal never materialized. Since then, North Korea has conducted dozens of missile tests, including the latest round last week. And several months ago, it fired up a light water reactor ostensibly built to generate electricity for about 100,000 homes. More likely, Hecker says, North Korea has repurposed the reactor to also produce plutonium and tritium.
A new wild card is Russia. Analysts worry Russia’s increasing reliance on conventional munitions from North Korea to wage its war against Ukraine may lead to nuclear cooperation. “Russia would be careful about sharing expertise,” Heinonen says. “But I wouldn’t be surprised if they are providing raw materials and advanced electronics.” That fresh concern, coupled with North Korea’s latest homegrown nuclear advances, will make it harder than ever for the United States and its allies to achieve a long-standing aspiration: denuclearizing the Korean Peninsula.
