Jeffrey LewisNorth Korea’s ELWR Now Appears Operating

Under constructing since 2010, North Korea appears to have finally begun operating a new reactor at Yongbyon that could be a significant source of plutonium for its nuclear weapons program — but many of questions remain.

North Korea’s Experimental Light-Water Reactor (ELWR) is Possibly Operating

Jeffrey Lewis and David Schmerler

North Korea’s Experimental Light-Water Reactor (ELWR) likely began operations in early October 2023. Under construction since 2010, the reactor may produce as much as 20 kg of additional weapons-grade plutonium a year for North Korea’s nuclear weapons program (note 1).

In November, IAEA Director-General Rafael Grossi noted that “Since mid-October 2023, a strong water outflow has been observed from the LWR cooling system. These observations are consistent with the commissioning of the LWR.”  In fact, fifty-eight consecutive satellite images taken over the seventy-three day period from October 10 through December 21 indicate a large discharge of water from a second discharge point. Moreover this discharge is occurring from a second discharge point that is distinct from the other, smaller discharge point where water out-flow has previously been seen.   The uninterrupted outflow of water from this new discharge point suggests the reactor is now possibly operating.

Our conclusion that the reactor is likely operating is based on the continuous operation of the reactor’s cooling system, which pumps river water into a cooling loop before returning that water to the Kuryong River, and the observation of steam captured above the main cooling water discharge pipe (caused by the temperature difference between the hot water discharge and surrounding temperature).  When the reactor is operating, cool river water is pumped from the river into the cooling loop and then returned to the river.  This cooling system is a series of underground pipes that has two discharge points located about 200 m downstream from the reactor. (It is unclear why the reactor has two discharge points, although construction photographs show both points are connected by piping to the reactor.)The International Atomic Energy Agency and others have previously noted cooling water being periodically discharged from the smaller discharge point, though not on a continuous basis (note 2). 

Since early October, however, larger volumes of water have been seen consistently emanating from a second discharge point about 40 meters (130 feet) south of the smaller discharge point.  Water discharge is visible in fifty-five consecutive satellite images taken by Planet Labs, Maxar, and Airbus since October 10, 2023.  Moreover, steam can be seen emanating from the reactor’s turbine hall, as well as from the water at the discharge point.

If the reactor is operational, the ELWR could produce for North Korea significant amounts of plutonium.  However, there are many unanswered questions about the design of the reactor that preclude an accurate estimate of the reactor’s potential to North Korea’s nuclear weapons stockpile.  North Korean officials told our colleague at the Middlebury Institute, Siegfried Hecker, that the reactor would be a 100 MW(th) light-water reactor with low-enriched uranium fuel. The reactor is not under international safeguards, however, and North Korean statements about its technical characteristics cannot be verified.

It is not obvious that the reactor is, in fact, a light-water reactor. DPRK state media has not referred to the “experimental light water reactor” since 2012 (note 3). In 2013 and 2015, North Korea released statements under the name of the Director of Atomic Energy Institute noting that North Korea had “decided to adjust and alter the uses of the existing nuclear facilities” at Yongbyon, but these statements did not make reference to the ELWR.

The last plausible reference to the DPRK’s development of light-water reactors was in a March 2013 plenary meeting of the Workers Party, guided by Kim Jong Un, that called for “work for developing light water reactor [to] be dynamically promoted to actively contribute to easing the strain on the electricity problem of the country.” Kim did not mention the nuclear power industry again until January 2021, when he referred to plans for the “launching in real earnest into the founding of the nuclear power industry” during his report to the 8th WPK Party Congress, but made no mention of the ELWR specifically or light-water reactors in general.

Although light-water reactors are generally considered more proliferation resistant than other designs, the United States Department of Energy has stated “a potential proliferating state could build a nuclear weapon from reactor grade plutonium that would have an assured, reliable yield of one or a few kilotons (and a probable yield significantly higher than that)” (note 4).  With six nuclear tests, North Korea could likely improve on this performance, particularly if it used composite pits with both Pu and HEU.

Hecker estimates North Korea could use 100 MWth reactor “to annually produce 10 to 15 kilograms of plutonium suitable for nuclear weapons by changing the typical long electrical-power reactor burn cycle to a shorter one.” (This would requires some modifications to the Yongbyon reprocessing facility to handle ceramic instead of metallic spent fuel). North Korea might also configure the reactor to produce weapons-grade plutonium using “driver” fuel to irradiate target elements.  David Albright has estimated that the reactor could be configured in this way to produce up to 20 kg of weapons grade plutonium per year. North Korea could also use the ELWR to produce other nuclear weapons material, such as tritium.

The ELWR, therefore, may be a significant source of nuclear material for North Korea’s nuclear weapons program. Moreover, the lack of clarity on the configuration of the ELWR, raises questions about our ability to estimate North Korea’s production of weapons-usable plutonium as Pyongyang’s arsenal expands.


  1.  North Korea has previously produced plutonium for its nuclear weapons program at the neighboring 25MW(th) gas-graphite reactor, which has been operating since 1986.  North Korea began constructing this second reactor in 2010.
  2. The UN Panel of Experts summarized instances of effluent from cooling operations as observed by a UN member state and the IAEA: “8. According to a Member State, possible testing of the cooling water system was detected in July 2022. The Director General of the International Atomic Energy Agency reported on 7 September and 16 November 2022 that the Agency had observed indications of possible tests of the cooling water system in July, late September and early October 2022. The Panel observed traces of effluent consistent with these observations near the Kuryong River during the same periods.” We were able to correlate these dates with satellite images showing water outfall, including images from Maxar ( July 22, 2022, September 30, 2022, and October 19, 2022) and Planet (September 18, 2022).  See also: Peter Makowsky, Jack Liu and Olli Heinonen, “Yongbyon Nuclear Scientific Research Center: Expansion Work Continues,” 38North, April 28, 2023.
  3. The last direct reference to the ELWR in DPRK state media occurred in November 2011: “The construction of experimental LWR and the low enriched uranium for the provision of raw materials are progressing apace in reliance on solid foundation of the self-supporting national economy and the country’s latest science and technologies making leaping progress.” See: “Experimental LWR Construction: FM Spokesman,” KCNA, November 30, 2011.  There is also brief reference to the decision to construct an ELWR in an account of the collapse of the 2012 Leap Day Agreement: “The U.S. extreme hostile policy aimed at depriving the DPRK of its sovereign right for peaceful use of the outer space, the right recognized by international law, called upon the DPRK’s self-defensive response, namely another nuclear test. It again led to the repetition of the vicious cycle of mistrust and confrontation; the U.S. imposed ever-harsh sanctions on the DPRK and the DPRK responded by starting the construction of light-water reactor (LWR) on its own and the production of enriched uranium to meet the fuel need for the LWR.” See: “DPRK Terms U.S. Hostile Policy Main Obstacle in Resolving Nuclear Issue,”: KCNA, August 31, 2012. 
  4.  United States Department of Energy, Final Nonproliferation and Arms Control Assessment of WeaponsUsable Fissile Material Storage and Excess Plutonium Disposition Alternatives, January 13, 1997, pages 37-39. 4 United States Department of Energy, Draft Nonproliferation Impact Assessment for the Global Nuclear Energy Partnership Programmatic Alternatives, December 2008, pages 68-69.
Images Consulted
DateSourceSystemVisible discharge
December 21PlanetSkysatYes
December 18AirbusPléiadesYes
December 17PlanetSkysatYes
December 17AirbusPléiadesYes
December 12AirbusPléiadesYes
December 10PlanetDoveYes
December 10MaxarYes
December 9PlanetDoveYes
December 9MaxarYes
December 7PlanetDoveYes
December 3MaxarYes
December 1PlanetDoveYes
December 1MaxarYes
November 30PlanetDoveYes
November 30AirbusPléiadesYes
November 29PlanetDoveYes
November 28PlanetDoveYes
November 28MaxarYes
November 25PlanetDoveYes
November 25PlanetSkysatYes
November 24PlanetDoveYes
November 24PlanetSkysatYes
November 20PlanetDoveYes
November 19PlanetDoveYes
November 18PlanetDoveYes
November 18MaxarYes
November 17PlanetDoveYes
November 14PlanetDoveYes
November 13PlanetDoveYes
November 13MaxarYes
November 12PlanetDoveYes
November 11PlanetDoveYes
November 10PlanetDoveYes
November 7PlanetDoveYes
November 7MaxarYes
November 4PlanetDoveYes
November 4MaxarYes
November 3PlanetDoveYes
November 3MaxarYes
October 30PlanetDoveYes
October 30MaxarYes
October 28PlanetDoveYes
October 28AirbusPléiadesYes
October 27PlanetDoveYes
October 24PlanetDoveYes
October 24MaxarYes
October 23PlanetDoveYes
October 23PlanetSkysatYes
October 23MaxarYes
October 23AirbusPléiadesYes
October 21PlanetDoveYes
October 21AirbusPléiadesYes
October 16PlanetDoveYes
October 16MaxarYes
October 12PlanetSkysatYes
October 11PlanetDoveYes
October 10PlanetDoveYes
October 10MaxarYes
October 7MaxarNo
October 4MaxarYes
October 4PlanetDoveNo


  1. Christopher (History)


    ‘Although the chief engineer assured him they excavated down to the bedrock, there was little visible evidence that such an analysis had occurred.[11] In addition, the method of laying the foundations for the reactor’s containment structure appeared inadequate. Best practices for reactor design requires special reactor-grade concrete poured in large, unbroken units whose drying must be carefully watched through close temperature control. However, only a small mixer was visible at the site and the concrete containment shell was being poured only one meter at a time.’

  2. Patrick Huber (History)

    We would like to point out that a ton-scale neutrino detector on the ground at Yongbyon but outside the fence of the facility could provide near-real time information on the reactor status, see for instance here

  3. David Schmerler (History)

    Great work!