Yongbyon Enrichment Plant

North Korea is expanding its uranium enrichment plant at Yongbyon. We suspect this is related to Kim Jong Un’s announcement that North Korea will “push ahead with the production of super-sized nuclear warheads.” This analysis is the subject of a story by CNN’s Zachary Cohen, “Satellite images reveal North Korea expanding facility used to produce weapons-grade uranium.”

North Korea Expanding Uranium Enrichment Plant at Yongbyon

Jeffrey Lewis, Joshua Pollack, and David Schmerler

James Martin Center for Nonproliferation Studies
Middlebury Institute of International Studies at Monterey

September 14, 2021

Satellite images show that North Korea is expanding the size of its uranium enrichment plant at Yongbyon.  The expansion of the enrichment plant probably indicates that North Korea plans to increase its production of weapons-grade uranium at the Yongbyon site by as much as 25 percent. This follows open-source information suggesting that North Korea has resumed plutonium-production activities at Yongbyon, including reprocessing spent fuel and restarting its gas-graphite reactor.

This group of high-resolution satellite images taken by Maxar shows construction in an area adjoining the existing uranium enrichment plant. (See above.) A satellite image taken on September 1, 2021 shows that North Korea cleared trees and prepared the ground for construction. A construction excavator is also visible in this image.  In a second image taken on September 14, 2021, North Korea erected a wall to enclose the area, began work on a foundation, and removed panels from the side of the enrichment building to provide access to the newly enclosed area.  This construction (below, right) resembles the process in 2013 (below, left) when North Korea expanded the facility, at the time doubling the floor space.  North Korea will probably cover the enclosed area with a roof.

The new area is approximately 1,000 square meters, enough space to house 1,000 additional centrifuges.  The addition of 1,000 new centrifuges would increase the plant’s capacity to produce highly enriched uranium by 25 percent.  This calculation assumes that North Korea continues to operate early-generation centrifuges seen in 2010, an assumption that should be treated with caution. Open-source information indicates that North Korea has conducted research on advanced centrifuge rotors. (note 1)  If North Korea replaced the P2-type centrifuges seen in 2010 with models using more advanced materials such as carbon fiber, as Iran has done, this could increase the capacity of the plant substantially. (note 2)

The pair of September images also show that North Korea has removed the six cooling units used to control the temperature inside the existing uranium enrichment plant.  An overhaul of the cooling system would be consistent with an increase in the floorspace and number of centrifuges, which would require additional cooling. Changes to the cooling units were first observed in July by analysts writing at the 38 North website.

* “Significant Quantities.” The IAEA defines a “significant quantity” as “the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded.” For highly enriched uranium, this number is 25 kilograms of U-235 within highly enriched uranium.   See: IAEA Safeguards Glossary, 2001 Edition. IAEA/NVS/3/CD. This figure is probably well in excess of the actual amount required for a single fission device designed by a state, like North Korea, with experience conducting several nuclear explosive tests.

The most recent expansion at Yongbyon probably reflects plans to increase production of nuclear materials for weapons production.  In January 2021, Kim Jong Un outlined the “core plan and strategic tasks of crucial importance in rapidly developing and strengthening the national defence industry” in a speech before the Workers’ Party Congress.  The first two national defense tasks mentioned were to “make nuclear weapons smaller and lighter for more tactical uses” and to “continuously push ahead with the production of super-sized nuclear warheads.” Achieving these goals will probably require North Korea to increase the amount of weapons-grade plutonium and uranium available for weapons production. (note 4) The production of thermonuclear weapons (“super-sized nuclear warheads”), in particular, requires substantial amounts of highly enriched uranium. (note 5)

End notes

1. 김정훈, 김광준 “기체원심분리기의 분리효률개선,”  기술혁신, 2016년, 2호, pp.16-17. [Kim Chong-hun, Kim Kwang-chun, “Separation Efficiency Improvement for Gas Centrifuges,” Technology and Innovation, February 8, 2016, pp.16-17.]
2.  North Korean officials indicated to an American delegation that their centrifuges used “alloys containing iron,” apparently a reference to maraging steel.  Based on North Korean statements, each centrifuge had a separative work capacity of about 4 kg SWU per year.  For comparison, Iran is developing an advanced centrifuge, the IR-8, which it claims will have a separative work capacity of approximately 16 kg SWU per year.
3. According to one member of the delegation, Sigfried Hecker, “The chief process engineer told us (in response to persistent questioning) that the facility contained 2,000 centrifuges in six cascades (one thousand centrifuges and three cascades on each side). …  When pressed, he said the rotors were made of alloys containing iron. …  We were able to extract the most important detail, that is, the enrichment capacity, which he said was 8,000 kg SWU/year. The average enrichment level is 3.5% and the tails are 0.27%.” Siegfried S. Hecker,  A Return Trip to North Korea’s Yongbyon Nuclear Complex, Center for International Security and Cooperation, Stanford University November 20, 2010.
4. A much less likely explanation is that North Korea is expanding production of low-enriched uranium fuel for a yet-to-be-built fleet of nuclear power plants. In the same speech, before the Workers’ Party Congress, Kim also made reference to “plans for … launching in real earnest into the founding of the nuclear power industry …”  Given Kim’s relative emphasis on weapons production in  his speech and the lack of reactors requiring LEU fuel, we find this explanation far less plausible.
5. A “secondary” for a thermonuclear weapon typically requires an order of magnitude more highly-enriched uranium than a fission primary, on the order of several tens of kilograms.