A Closer Look at North Korea’s Enrichment Capabilities and What It Means

Last Friday, nearly 14 years after the North Koreans surprised our Stanford group by unveiling an ultra-modern centrifuge facility in the Yongbyon nuclear complex, the Korea Central News Agency (KCNA) reported a visit by Kim Jong Un to a centrifuge plant, including photos. During our November 2010 visit, we were not allowed to take photos. That makes direct comparisons between what we saw and what KCNA published difficult. But the questions are clear—what did Kim Jong Un want to show, why now, and what difference does it make?

What and Where Is This Centrifuge Facility?

The centrifuge hall shown in the KCNA photos was not the same one we saw in November 2010. The overall building layout looked nearly identical, but the centrifuges and piping were different. The centrifuges appeared to be about the same diameter but possibly somewhat shorter. Their exterior housing (likely a high-strength aluminum alloy) has a gray/white patina, whereas the ones we saw were shiny aluminum. The most significant difference is the current photos show many more small-diameter pipes leading to and from the centrifuges.

The additional piping provides a hint about the new centrifuges. We concur with Heinonen et al. that these likely are cooling coils that lead inside the centrifuge housing to cool the rapidly spinning rotors. In 2010, we were told the centrifuges had maraging steel rotors, which is consistent with the performance characteristics they claimed for their centrifuges.[1] The new centrifuges may be spinning faster because North Korea may have switched to composite material rotors. Separation capacity also depends on rotor length, but the new centrifuges are of similar length. We believe the new centrifuges provide only a modest increased capacity because they are of similar length, and it has been reported that Iran has had great difficulty increasing centrifuge performance with composite rotors.

North Korea could, of course, increase enrichment capacity just by building more centrifuge plants. They apparently did so by 2013 when they added an annex of identical size to the building we visited, likely doubling the original 2,000 centrifuge capacity of 8,000 kg-SWU/year.[2] Based on our visit, we concluded that they had additional centrifuge facilities outside Yongbyon because they could not have built the one we saw without first demonstrating that their centrifuges performed satisfactorily linked in cascades.

The facility we visited in 2010 could have produced either low enriched uranium (LEU) to fuel the new experimental light water reactor (ELWR) under construction or highly enriched uranium (HEU) for bombs. We were told the facility was producing LEU, which was likely correct for that centrifuge hall as well as for the second one at Yongbyon.[3] Some of the LEU was used to make reactor fuel, but some was likely sent to an undeclared facility to stepwise boost the enrichment from approximately four percent Uranium-235 to weapons grade at 90 percent.

In the intervening years North Korea may also have built additional facilities outside Yongbyon dedicated primarily to HEU production. We do not know the location of the facility Kim visited, although it is almost certainly outside Yongbyon. Reports that it is near Pyongyang at Kangson, which may have housed centrifuge facilities in the past, remain in dispute.[4] However, the actual location of the facility is not crucial to the arguments we make here.

All estimates of the North’s enrichment capacity are highly uncertain. We have no independent confirmation of the centrifuge designs or rotor materials. All estimates use our assessment that their original centrifuges used maraging steel rotors with a 4.0 kg SWU/yr separation capacity. If we were wrong and North was not able to produce or acquire maraging steel, but rather had to use aluminum alloys as did Iran at the time, the estimates would be too high by a factor of four. That is one of the reasons we tried to convince successive American administrations that there is great benefit to a deal that allows access to Yongbyon, but to no avail.

Another reason for the high uncertainty is that we do not know how many centrifuge facilities North Korea has (nor where they all are). We have assumed North Korea has similar enrichment capacity outside Yongbyon in undeclared facilities to what is inside the complex. For reference, the total estimated capacity in Yongbyon is approximately 16,000 kg-SWU/yr, which, if dedicated to HEU, could produce about 80 kg/yr. However, some of the enrichment capacity is used for LEU fuel for the experimental light water reactor (ELWR), which appeared to be operational by the end of 2023 (over a decade after construction began). Therefore, we do not know how much of the enrichment capacity was dedicated to HEU for bombs. Moreover, we do not know how North Korea partitions its HEU inventory between tactical nuclear weapons and components of their hydrogen bombs. Given these uncertainties, we believe North Korea already has sufficient HEU for 50 or so tactical nuclear weapons. It could have considerably more, as some analysts have estimated, but it also could be a lot less if our assumptions are incorrect.

Do These Pictures Reveal Anything That Changes the Military Threat?

It does not fundamentally change the threat but is a stark reminder of just how menacing the North’s nuclear arsenal is. In his remarks at the facility, Kim stressed the need to increase the production of “nuclear materials necessary for the manufacture of tactical nuclear weapons.” This facility may increase the production of HEU by around 25 percent, but not exponentially as Kim Jong Un has previously called for.

Producing more HEU bomb fuel is important to bolster the North’s regional deterrent of tactical nuclear weapons, which can reach all of South Korea and much of Japan. However, it does not improve the sophistication or versatility of the North’s nuclear arsenal. We believe that North Korea has dedicated only two of its six nuclear tests (February 2013 and September 2016) to developing tactical nuclear weapons.[5] Without additional nuclear tests, it will be difficult to improve the performance of the warheads and mate them to the apparently great variety of short and medium-range missiles that North Korea has tested that are believed to be nuclear capable. Yet, Kim has previously shown off a room full of small tactical nuclear weapons (mockups, presumably) that are claimed to fit into eight different delivery platforms.[6]

More HEU also does little to enhance the North’s strategic nuclear arsenal, which we believe is based on plutonium and tritium. North Korea has produced rather modest amounts of plutonium and tritium in its 5 MWe Gas-graphite Reactor in Yongbyon over the years.[7] Production of plutonium and tritium may now be augmented by the operation of the ELWR. Enhancing the strategic nuclear arsenal will require more plutonium, more tritium, more missile testing (especially to gather reentry data), and more nuclear testing. All pose technical and political risks. One concern we have is that the new partnership with Russia may offer North Korea shortcuts in one or all of these.

What Does This All Mean?

The importance of the report on Kim’s appearance at the enrichment facility comes from the context in which it occurred. Since the end of August, Pyongyang has been ramping up attention to its defense sector and especially to Kim Jong Un’s focus on nuclear weapons. This has been a two-pronged effort, aimed at preparing the population for an uptick in tensions while signaling to Washington that Kim’s nuclear capacity, already strong, is getting stronger. Kim’s visit to the enrichment plant appears to be part of that effort.

With all of this as the overture, Kim has signaled a significant move to be made at the upcoming October 7 Supreme People’s Assembly meeting to declare new national borders. That will almost certainly increase tensions on the peninsula, especially in the West Sea.

Kim has several times in the past emphasized the ability to launch an overwhelming attack, including the use of tactical nuclear weapons, on key military and civilian infrastructure in the South. By showing the enrichment facility, he has signaled an increased ability to produce uranium warheads for use on the peninsula and, to a lesser extent, against Japan. With plutonium and tritium production for strategic weapons still limited, it’s unlikely he intends to use nuclear weapons against the continental United States from the start. But he may want to have enough capacity regionally with tactical nuclear weapons—and wants the US to think he has more than enough—to keep the US out of the fight, something he no doubt knows his grandfather failed to do in June 1950.

By drawing the attention of the domestic audience to military issues and by underlining his involvement in them, Kim appears to be moving into a dangerous new phase, signaling to the population that despite the regime’s focus on economic policy in recent months, there is a need to continue preparing for potential confrontation.

Robert L. Carlin is a nonresident scholar at the Middlebury Institute of International Studies at Monterey and a former chief of the Northeast Asia Division in the Bureau of Intelligence and Research at the US State Department, where he took part in US-North Korean negotiations.

Siegfried S. Hecker is a professor of practice at the Middlebury Institute of International Studies at Monterey, a professor of practice at Texas A&M University, and a former director of the Los Alamos National Laboratory and professor emeritus of Stanford University.

1. [1]

   Performance of centrifuges is measured in kilogram-separative work units per year (kg-SWU/year). High-strength aluminum rotors such as those used in first-generation Iranian centrifuges are typically 1.0 or less. The North Korean engineer claimed four kg-SWU/year, much more capable than what Iran had at that time.

2. [2]

   We estimated at the time that this capacity could produce up to two tonnes of LEU (low enriched uranium for reactor fuel comprising half a reactor load) or up to 40 kg highly enriched uranium (HEU), enough for nearly two nuclear weapons.

3. [3]

   At the time, it was still possible that North Korea would allow inspectors back into Yongbyon. It would have been to their advantage to be able to demonstrate that the facility was dedicated to peaceful nuclear energy.

4. [4]

   Ankit Panda claimed Kangson housed a centrifuge facility. David Albright was not convinced. Jeffrey Lewis claims that the new facility is at Kangson. Peter Makowsky et al. say it is not.

5. [5]

   Siegfried S. Hecker with Elliot A. Serbin, Hinge Points: An Inside Look at North Korea’s Nuclear Program, (California: Stanford University Press, January 10, 2022).

6. [6]

   Figure 1 in Hans M. Kristensen, Matt Korda, Eliana Johns and Mackenzie Knight, “North Korean nuclear weapons, 2024,” Bulletin of the Atomic Scientists, July 15, 2024, https://thebulletin.org/premium/2024-07/north-korean-nuclear-weapons-2024/.

7. [7]

   We have tracked plutonium production over the years and estimated tritium production as reported in Hinge Points: An Inside Look at North Korea’s Nuclear Program. We estimate the current inventory of plutonium at approximately 60 kg with sufficient tritium for only a few hydrogen bombs.