North Korea's Submarine Program and South Korea's Response: A New Balance in Northeast Asian Maritime Security

On December 25, 2025, North Korea's state media released footage of General Secretary Kim Jong-un inspecting an "8,700-ton nuclear-powered strategic guided missile submarine" at the Sinpo shipyard. This is the most detailed public footage of a plan that Kim has prioritized since 2021. Analysts based in Seoul point out that the fully assembled appearance suggests that the installation of the reactor is likely complete, and a former South Korean submarine officer estimates that sea trials could be possible within a few months.

This development could fundamentally change the maritime security environment in Northeast Asia. In nuclear deterrence theory, a state that has completed the capability to launch nuclear weapons from land, air, and sea is referred to as having a "nuclear triad." North Korea is now steadily advancing toward the completion of this triad. John Ford, a researcher at the James Martin Center for Nonproliferation Studies in California, describes what North Korea is building as a "rudimentary triad." While not advanced, it could function sufficiently to prompt adversaries to reconsider attacking a country with second-strike nuclear capabilities.

In the past, North Korea has independently achieved complex technological goals that observers believed would take years to accomplish, repeatedly surprising stakeholders. Progress in the production of nuclear bombs, miniaturization, missile integration, and the development of functional reentry vehicles are examples of this. However, Ford points out that submarine technology is a challenge of a different dimension. "All previous technologies need to be integrated and amplified. This time, it has to function underwater." The skills required for actual underwater operation are something North Korea does not yet possess, and it would take an extremely long time to acquire them unless Russian Navy submarine crews are on board for training.

This raises the possibility of technology transfer from Russia to North Korea. On February 10, 2026, during a UK parliamentary hearing on underwater activities, Peter Roberts, an associate researcher at the University of Exeter's Centre for Public Understanding of Defence and Security, issued a warning. As North Korea's role in the Ukraine war becomes essential for Russia, Moscow is running out of what it can offer in exchange for Kim Jong-un's support and may be willing to trade submarine technology, which is considered "military knowledge of the highest secrecy."

According to South Korean intelligence, North Korea has fought for Russia, resulting in 6,000 casualties, while supplying millions of shells, ballistic missiles, and various vehicles and equipment. Roberts testified in the UK Parliament: "Previously, they were trading ballistic missile technology and knowledge to obtain North Korean weapons and soldiers fighting for Putin in Ukraine. But that has already happened. Now, there is a possibility of providing submarine expertise to countries like North Korea and Iran in exchange for technicians to produce large quantities of drones and send them to the front lines in Ukraine."

In September 2025, the South Korean military announced that it had obtained information indicating that Russia supplied North Korea with 2 to 3 nuclear submarine propulsion modules. These were reportedly recovered from decommissioned Russian submarines and include reactors, turbines, and cooling systems. Seoul stated at the time that it was in the process of verifying this information.

However, Ford expressed "great skepticism" that Russia would hand over the most classified elements. "Designing nuclear weapons is impossible. The propulsion system is likely, but still very unlikely." As a principle, states do not trade their most tightly guarded military knowledge. Russia has a special reason to be cautious. Information transferred to North Korea could leak to Seoul through human intelligence networks, effectively providing Moscow with a window into South Korea's reactor designs.

Even if Russia were to share information about nuclear submarine propulsion systems, North Korea would need to bridge the gap between theoretical knowledge and actual implementation. Ford points out that it took decades for the Soviet Union and the United States to master this during the Cold War. There is a difference between handing over blueprints and providing iterative technical support to answer questions about whether components are correctly placed.

As North Korea's submarine development progresses, South Korea is being forced to reconsider its maritime security strategy. According to an analysis published on February 20, 2026, the most realistic option for South Korea to acquire nuclear submarines is to obtain or jointly construct the U.S. Virginia-class Block IV submarines.

The South Korean Navy is facing increased demands for long-range patrols, a deteriorating underwater balance against North Korea, and a rising probability of simultaneous bilateral emergencies involving China and North Korea. The limitations of existing conventional diesel submarines have become apparent. Nuclear submarines offer durability, speed, and operational flexibility that cannot be replicated by conventional platforms.

However, as pointed out by Kim Joo-hyun, director of the Korea Security Management Institute, the core issue facing Seoul is not whether nuclear submarines are useful. Given the existing legal, political, financial, and alliance constraints, the question is whether they are realistically achievable.

South Korea's independent nuclear submarine program must simultaneously overcome three interconnected barriers: domestic legal reform, independent development of reactors, and authorization under the international nuclear nonproliferation regime. Each of these is challenging on its own, and solving all three simultaneously is nearly impossible.

The regulatory framework in South Korea was not originally designed with mobile military reactors in mind. Therefore, extensive amendments to the Nuclear Law and the Nuclear Safety Law will be unavoidable. This is not merely a technical amendment but a politically sensitive change that will inevitably provoke backlash from environmental groups, civil society organizations, and parts of the political class. The legislative process itself is likely to be prolonged across multiple administrations, and continuity is uncertain.

Technical challenges further complicate the issue. Developing a reactor for submarines is not comparable to adapting a civilian small modular reactor. It requires entirely different safety assumptions, shock resistance standards, and operational lifecycles. Moreover, the costs are enormous. As demonstrated by the KF-21 fighter development case, South Korea allocated $3.4 billion for engine development. A naval reactor will require far more resources, which is at a stage before the submarine hull is assembled.

The international aspect is also decisive. Under the Nuclear Non-Proliferation Treaty (NPT), naval nuclear propulsion occupies a very narrow legal space. While it is not explicitly prohibited, it must be recognized as military activity not banned under IAEA safeguards agreements. Such recognition is not automatically granted; it is a political issue that heavily depends on the judgment of major powers, particularly the United States.

Australia secured such recognition after years of negotiations under the AUKUS agreement and agreed to operate submarines using weapon-grade uranium fuel, which is exclusively managed by the supplier. India's nuclear submarines operate under an entirely different logic made possible by Russia's willingness to accept proliferation risks. South Korea does not fall into either of these categories.

If South Korea, a non-nuclear state facing a nuclear-armed North Korea, were to choose low-enriched uranium and attempt to develop its own reactor, it would likely provoke intense scrutiny in Washington and face significant resistance in the U.S. Congress. In short, the likelihood that the U.S. would approve a South Korean-designed reactor operating outside U.S. control is extremely low.

In this context, the idea of construction at the Philadelphia shipyard in the U.S. is worth careful consideration. However, the practical aspects are extremely complex. The Philadelphia facility is unsuitable for building nuclear submarines. It lacks dedicated submarine dry docks, nuclear material handling infrastructure, and the geographical isolation that characterizes existing U.S. nuclear shipyards. While the Newport News shipyard, which has been building nuclear submarines since the 1950s, is about 10 kilometers from urban areas, the Philadelphia Naval Shipyard is only 7 kilometers from densely populated areas.

More importantly, the U.S. would not permit the manufacture of multiple reactor types there. One conclusion can be drawn here: nuclear submarines built at U.S. facilities will require reactors designed by the U.S., placed under U.S. safeguards, and subject to U.S. political constraints. There is no realistic path for South Korea to initiate its own reactor program alongside U.S. production lines.

This reality leads the discussion to a logical conclusion. If South Korea were to acquire nuclear submarines, it would only be possible through platforms that the U.S. already trusts, which the U.S. Congress has approved for transfer to allies, and that are currently operated by the U.S. Navy. The Virginia-class submarines meet these requirements, and particularly the Block IV variant offers the most realistic balance between capability and cost.

The Block IV variant retains reliable vertical launch capabilities while avoiding the cost burden of the Block V Virginia Payload Module and associated industrial pressures. Additionally, it has the advantages of a mature production line, proven reactor durability without refueling, and full compatibility with U.S. logistics, training, and maintenance systems. From Washington's perspective, cooperation with South Korea on Virginia-class submarines aligns with the precedent set by Australia. From Seoul's perspective, it would allow for the avoidance of years of legal disputes, technical uncertainties, and political deadlock.

Opposition to this path is likely to arise from industrial nationalism, the belief that strategic autonomy requires ownership of unique designs. However, such logic misunderstands the essence of nuclear propulsion. Autonomy does not stem from reactor blueprints but from operational capability. Submarines that are not realized do not provide deterrence. Practical deterrence is created by submarines that are stably deployed and integrated into allied operations, increasing the complexity of adversaries' strategic calculations.

From Japan's perspective, this development has multiple implications. First, the progress of North Korea's nuclear submarine program signifies a qualitative change in the nature of potential threats in the waters surrounding Japan. Unlike conventional diesel submarines, nuclear-powered submarines have the capability to conduct prolonged covert operations within Japan's vast exclusive economic zone.

Second, if South Korea acquires Virginia-class submarines, it could provide a new opportunity to deepen trilateral maritime security cooperation among Japan, the U.S., and South Korea. Operating the same platform would enhance interoperability and make joint training and information sharing more effective. This is crucial in the Northeast Asian security environment, which must simultaneously address the dual challenges of China's maritime expansion and the increasing threat from North Korea.

Third, the possibility of technology transfer from Russia to North Korea raises new concerns for Japanese defense planners. If Russian submarine technology flows into North Korea, it not only means an enhancement of Pyongyang's capabilities but also increases the risk of that technology further proliferating to third countries. Japan needs to strengthen information sharing with the U.S. and South Korea to monitor this threat.

The underwater environment in Northeast Asia is rapidly changing. North Korea's nuclear submarine program, South Korea's consideration of acquiring nuclear submarines, and the deepening military technology cooperation between Russia and North Korea could fundamentally alter the balance of maritime security in the region. Japan must strengthen its cooperation with South Korea while centering on the U.S.-Japan alliance to build a comprehensive strategy to address these challenges. The competition to form a new balance in maritime security has already begun.