Satellite Imagery Analysis: North Korea’s Development of Nuclear-Powered Submarines and Russian Assistance

Developments surrounding North Korea’s nuclear-powered submarine (SSN) program have accelerated in recent years. At the end of 2025, Spanish investigators reported that a Russian cargo vessel carrying reactor components from a decommissioned Russian nuclear submarine to North Korea had sunk in the Mediterranean Sea while en route to its destination.[1] In the same year, South Korea’s Minister of National Defense also suggested before the National Assembly that Russia may be providing assistance to North Korea’s nuclear-powered submarine development efforts. Furthermore, at the end of that year, the Korean Central News Agency (KCNA) reported that General Secretary Kim Jong Un had inspected a nuclear-powered submarine reportedly under construction.[2] Kim implied that the vessel was a strategic nuclear submarine capable of carrying submarine-launched ballistic missiles (SLBMs) equipped with nuclear warheads. Although photographs of the submarine were released, questions have emerged regarding whether it could become operational in the near future and whether a reactor has actually been installed, as the images provided no indication of when they were taken.

Unlike conventionally powered submarines, which rely on diesel engines and batteries and must surface periodically to replenish their power supply, nuclear-powered submarines utilize nuclear fission in much the same way as commercial nuclear reactors and can remain submerged for extended periods once fueled. Nuclear-powered submarines are generally classified into two categories: strategic ballistic missile submarines (SSBNs), whose primary mission is to ensure a retaliatory nuclear strike capability following an enemy nuclear attack, and attack submarines (SSNs), which are designed to engage enemy surface vessels and submarines. If the submarine recently publicized by North Korea is indeed a strategic nuclear submarine, Pyongyang’s objective is likely to strengthen its deterrence posture against the United States.

Additional developments potentially related to the nuclear-powered submarine program have also been observed. In August 2025, a new building believed to house a uranium enrichment facility was constructed at Yongbyon in northwestern North Korea. The facility could be intended not only for the production of highly enriched uranium for nuclear weapons but also for the manufacture of fuel for naval reactors. Around the same time, a reactor at Yongbyon entered operational status. According to the International Atomic Energy Agency (IAEA), this reactor is a light-water reactor (LWR),[3] the same type commonly employed in naval propulsion systems. In addition to electricity generation, the facility may serve as a training platform for personnel expected to operate nuclear-powered submarines in the future.

Nevertheless, possessing the capability to develop and operate commercial nuclear reactors does not automatically translate into the ability to design reactors suitable for submarine propulsion. A naval reactor must be sufficiently compact and lightweight to fit within a submarine while still maintaining adequate power output. Operational requirements also differ considerably. Whereas a commercial power reactor typically operates at a stable output level close to full capacity once it has entered routine service, a submarine reactor must frequently adjust its power level in response to changing operational conditions. For North Korea, which only recently began operating a domestic light-water reactor, developing a naval reactor and acquiring the necessary operational expertise independently would be a formidable challenge. Russian technical assistance is therefore likely to be indispensable.

Should North Korea succeed in deploying nuclear-powered submarines capable of carrying nuclear warheads, the implications for the security environment in Northeast Asia would be profound. As a result, careful analysis of the country’s submarine development efforts has become increasingly important. This paper examines the status of North Korea’s nuclear-powered submarine program, its light-water reactor, and uranium enrichment facilities through satellite imagery and other open-source evidence, while incorporating expert assessments to evaluate the program’s progress and strategic significance.

The Russian cargo vessel reportedly transporting nuclear reactor components for North Korea’s nuclear-powered submarine program was the Ursa Major, a ship owned by a company affiliated with the Russian Ministry of Defense. The vessel had previously been placed under U.S. sanctions and monitoring after allegedly transporting cargo to Crimea in 2022. When the ship sank in the Mediterranean Sea, sixteen crew members were killed and two others were reported missing.[4]

In May 2026, the Spanish government submitted a letter to parliament detailing the circumstances surrounding the incident. According to the report, interviews with the vessel’s captain revealed that the ship had been carrying “components for two reactors of the same type used in nuclear-powered submarines.” Although the Ursa Major had officially declared Vladivostok in Russia’s Far East as its destination after departing from St. Petersburg, the Spanish authorities concluded that its ultimate destination was most likely the North Korean port of Rajin.[5] As this assessment was presented by the government to parliament, it effectively indicates that the Spanish government concluded that Russia had been attempting to transfer nuclear submarine reactor components to North Korea.

Spanish investigators further identified the reactors involved as the Soviet-era VM-4SG model.[6] This pressurized-water reactor (PWR) was deployed aboard second-generation Soviet nuclear-powered submarines and operates on uranium enriched to approximately 20 percent (see Table 1 below). It is capable of producing approximately 90 megawatts (MW) of power. By comparison, commercial nuclear power reactors currently operating in Japan typically have generating capacities of around 1,000 MW. Although significantly smaller, the VM-4SG is specifically designed for naval propulsion applications that require compactness and operational flexibility. (For uranium enrichment levels and their respective applications, see Figure 2 below.)

Since North Korea dispatched troops to support Russian military operations in Ukraine in October 2024, the author and Dr. Yu Koizumi, Senior Fellow at the Sasakawa Peace Foundation, have monitored developments under the assumption that Moscow could provide assistance to Pyongyang in areas such as nuclear-powered submarine development in return. One of the primary locations examined through satellite imagery has been Dunay Port in Russia’s Primorsky Krai, near the North Korean border, where reactor compartments removed from retired nuclear submarines are reportedly stored (Satellite Image 1).

Within the image, the spent reactor compartments from decommissioned submarines are believed to be housed inside the red containers visible on the right-hand side. In November of last year, some of the containers located in the second row from the bottom (indicated by the blue arrow in Satellite Image 1) were observed to have been relocated to the port area on the left side of the image. Given the possibility that these components could eventually be transferred to North Korea, continued monitoring was undertaken. Subsequent movements of containers within the port complex were also observed. However, due to limitations in the availability and quality of commercially accessible satellite imagery, no conclusive evidence has yet emerged, as of August 2026, demonstrating that the containers were removed from the facility and transported elsewhere.

Based on the contents of the Spanish government’s report to parliament, another possibility is that the reactor components destined for North Korea were transported not from Dunay Port but from Sayda Bay, another Russian storage site for decommissioned submarine reactors, via a route passing through the Mediterranean Sea (see Figure 1). Sayda Bay is located on Russia’s Kola Peninsula near the Finnish border. Continued observation of activities at this facility will therefore also be necessary.

At present, there is no definitive proof that Russia has successfully transferred a naval reactor to North Korea. Nevertheless, an important question remains: to what extent would the provision of a reactor, reactor components, or related technical materials contribute to North Korea’s submarine development efforts? According to Tomonori Iwamoto, Secretary General of the Japan Chapter of the Institute of Nuclear Materials Management (INMM), “In the nuclear field, obtaining equipment itself or gaining access to design blueprints can substantially accelerate technological development. Iran’s advances in uranium enrichment following its covert acquisition of centrifuge designs from Pakistan clearly demonstrate this point.”[7] Access to Soviet-era naval reactor technology could therefore significantly reduce the technical barriers facing North Korea as it seeks to develop an indigenous nuclear-powered submarine capability.

On December 25, 2025, North Korea released new images of what it claimed to be the country's first nuclear-powered submarine. According to the Korean Central News Agency (KCNA), the vessel depicted in the images has a displacement of approximately 8,700 tons, making it comparable in size to, or even larger than, many nuclear-powered submarines currently operated by other countries.

The development of a nuclear-powered submarine has been one of North Korea’s strategic objectives since Kim Jong Un first publicly referenced the program at the Eighth Congress of the Workers’ Party of Korea in 2021. At that time, the acquisition of a nuclear-powered submarine was formally incorporated into the country’s five-year defense modernization plan.[8]

The principal advantage of a nuclear-powered submarine lies in the endurance provided by its propulsion system. Because the onboard reactor can operate for extended periods, the submarine can remain submerged far longer than conventionally powered vessels. However, only a limited number of countries possess the technological expertise required to develop and operate naval nuclear reactors. At present, only six countries, the United States, Russia, China, France, the United Kingdom, and India, operate nuclear-powered submarines.

The degree of endurance achieved depends largely on each country's reactor and fuel-cycle strategy. As shown in Table 1, the United States and the United Kingdom utilize highly enriched uranium (HEU) with enrichment levels exceeding 90 percent, allowing their submarines to operate throughout their service lives without refueling. By contrast, the VM-4SG reactor discussed earlier, which North Korea is believed to have sought to acquire, uses uranium enriched to approximately 20 percent and is designed to undergo periodic refueling.

Source: Compiled by the author based on various sources, including Japan Atomic Energy Agency (JAEA), "India's Uranium Enrichment Program" (In Japanese) March 2007.

New developments have also been observed at North Korea's inland nuclear facilities. In September 2024, the Rodong Sinmun, the official newspaper of the Workers' Party of Korea, reported that Kim Jong Un had visited and provided on-site guidance at a nuclear production facility. The sites inspected reportedly included a uranium enrichment facility responsible for producing nuclear material for nuclear warheads. This marked the first time North Korea publicly disclosed photographs of one of its uranium enrichment facilities.

The Rodong Sinmun did not identify the location of the facility visited by Kim. While a uranium enrichment facility at Yongbyon has long been known to exist, the International Atomic Energy Agency (IAEA) reported in June 2024 that a facility resembling the Yongbyon enrichment plant appeared to be nearing completion at the Kangson nuclear-related complex on the outskirts of Pyongyang.[10] The following year, in August 2025, the IAEA further reported the construction of a new building at Yongbyon that appeared consistent with a uranium enrichment facility and could indicate an expansion of North Korea's enrichment capacity.[11]

As shown in Table 1, the enrichment levels used in naval reactor fuel vary considerably among states operating nuclear-powered submarines. Given that North Korea has already acquired the capability to produce weapons-grade highly enriched uranium, it is reasonable to assume that the technical requirements for manufacturing uranium fuel suitable for naval propulsion would not constitute a major obstacle.

There have also been notable developments involving Yongbyon's light-water reactor (LWR). At the end of December 2023, the IAEA stated that there were indications that the light-water reactor constructed at Yongbyon had reached criticality (Satellite Image 2).[12] Construction of this reactor was publicly announced by North Korea in 2009 as an indigenous project. The facility is believed to possess a generating capacity of approximately 100 megawatts. Significantly, it is a pressurized-water reactor (PWR), the same reactor type employed in the VM-4SG naval reactor discussed earlier, and its power output is also broadly comparable.

The IAEA has monitored the operational status of the reactor through satellite imagery, analyzing indicators such as water discharge from the cooling system and the temperature of discharged water (see Satellite Image 2). When a reactor is operating, coolant water circulates through the system to remove heat from reactor-related equipment, resulting in the discharge of warm water. Using these indicators, the IAEA assessed reactor operations and published its findings in an August 2025 report on North Korea's nuclear activities. According to the report, the reactor ceased operations between September and October 2024, resumed operation from November 2024 through April 2025, shut down once again, and subsequently restarted in May 2025.[13] Furthermore, the reactor operations were temporarily suspended from August to November 2025 and from March to April 2026, and then resumed.

Commercial pressurized-water reactors typically utilize fuel enriched to approximately 5 percent uranium-235, allowing them to operate continuously for several years before refueling becomes necessary (see Figure 2). Naval reactors likewise employ enriched uranium fuel, and if enrichment levels exceed approximately 80 percent, refueling may not be required throughout the submarine's operational lifetime.

Against this backdrop, questions arise as to why North Korea's pressurized-water reactor, even assuming it remains in a testing or commissioning phase, needed to undergo four shutdown-and-restart cycles within less than three years after becoming operational at the end of 2023. One possible explanation is the existence of technical difficulties. However, Tomonori Iwamoto, Secretary General of the Japan Chapter of the Institute of Nuclear Materials Management (INMM), offers another interpretation: “The periods during which reactors are most susceptible to operational problems are startup and shutdown. It is possible that North Korea is using this light-water reactor, which shares the same reactor type as those employed for naval propulsion, as a training platform to acquire the expertise necessary for stable startup procedures and reactor operation in anticipation of operating nuclear-powered submarines."[14]

Some elements of the analysis presented in this paper remain hypothetical. Nevertheless, the findings of the Spanish government's investigation and the statements made by South Korea's Minister of National Defense before the National Assembly can be regarded as official assessments. Taken together, they strongly suggest that Russia is providing technical assistance to North Korea's nuclear-powered submarine program and that North Korea is advancing not only the development of the submarine itself but also related capabilities, including the expansion of uranium enrichment activities that are essential for the operation of naval reactors. Russia and North Korea share a common strategic objective of countering the United States. Should North Korea ultimately succeed in deploying a strategic nuclear submarine of the type implied by the vessel unveiled by Kim Jong Un, the result would likely be a significant enhancement of Pyongyang's nuclear deterrent against the United States. Such a development would inevitably affect Japan and South Korea, both of which rely heavily on the credibility of U.S. extended nuclear deterrence for their national security.

At the same time, neither Japan nor the broader international community possesses an effective means of halting North Korea's nuclear-powered submarine program. North Korea expelled International Atomic Energy Agency (IAEA) inspectors many years ago, and no international mechanism currently exists to monitor either its submarine development efforts or the expansion of related uranium enrichment facilities. As a consequence, the international community's visibility into North Korea's nuclear activities continues to deteriorate. Under these circumstances, improving situational awareness and strategic foresight should become a priority for Japan and other concerned states. Although significant uncertainty will remain, governments, research institutions, and analytical organizations should strengthen cooperation in the collection, analysis, and exchange of information, including the systematic use of satellite imagery and other open-source intelligence. Enhancing transparency regarding North Korea's activities, even incrementally, will be essential for reducing uncertainty and supporting informed policymaking in Northeast Asia and beyond.