In Defense of South Korea's Nuclear Submarine Program
Addressing misconceptions about the legal basis and strategic imperative behind Seoul's decision to build nuclear-powered submarines
South Korea’s pursuit of nuclear-powered submarines has already sparked debates about nonproliferation and regional stability. Much of the criticism, however, overlooks two critical realities: South Korea is facing security threats that necessitate robust military capabilities and that indications given so far signal the country looks to remain within the boundaries of the Nuclear Non-Proliferation Treaty (NPT).
Nuclear Propulsion, Not Weapon
The NPT explicitly permits what South Korea plans to do. While the NPT prohibits non-nuclear weapon states from acquiring nuclear weapons and requires International Atomic Energy Agency (IAEA) safeguards on nuclear material, the NPT does not prohibit non-explosive military uses of nuclear material. Naval propulsion is a prime example of such permitted use.
The standard NPT safeguards agreement, paragraph 14 of INFCIRC/153, provides for safeguards to be suspended under paragraph 14 procedures while nuclear material remains in non-proscribed military use. States must inform the IAEA that the material will be used only in peaceful nuclear activity and not for weapons production, then arrange the terms with the Agency. When Canada initiated this process in 1987, it established that nuclear submarines qualify as “non-proscribed military activity.” Simply, South Korea is following established international law.
Low Enriched Uranium: A Proliferation-Resistant Choice
Here’s what critics often miss: South Korea has been developing small modular reactors (SMR) powered by low enriched uranium (LEU) rather than the highly enriched uranium (HEU) used by American and British submarines. This distinction is crucial.
LEU contains less than 20% U-235 and is not weapons-grade. France has powered its submarines with LEU starting in the 1980’s with its Rubis-class attack submarines. China is also understood to use LEU for naval propulsion, while Brazil is known to be considering LEU for its ongoing nuclear submarine program. Such approaches minimize the proliferation risks associated with weapons-grade uranium while still providing the operational advantages of nuclear propulsion.
South Korean officials have explicitly stated they plan to build submarines with conventional armament and expect to receive enriched uranium fuel from the United States (US). Defense Minister Ahn Gyu-back told lawmakers that South Korea would build its own submarines and modular reactors while receiving fuel supply from America — a model that prevents domestic enrichment capabilities from emerging.
Normalization of Nuclear Propulsion
The debate about naval nuclear propulsion misses a higher level transformation underway: nuclear propulsion is increasingly viewed as essential technology for decarbonizing global commercial shipping, not just powering naval vessels.
The shipping industry faces enormous pressure to eliminate carbon emissions. Nuclear propulsion offers zero-emission operation with fuel cores lasting 25 years—the typical operational life of a merchant ship. Recent developments demonstrate this isn’t science fiction but near-term reality.
In South Korea itself, nine organizations including major shipping companies and the Korea Atomic Energy Research Institute are collaborating to develop large commercial ships powered by small modular reactors. HD Korea Shipbuilding & Offshore Engineering announced plans in early 2025 to develop nuclear reactor cargo ships, aiming for a business model by 2030. The Norwegian consortium NuProShip is targeting a prototype by 2030. China revealed plans for a thorium molten salt reactor-powered cargo ship that would be the largest ever built.
The American Bureau of Shipping completed a landmark study for the US Department of Energy modeling nuclear propulsion for 14,000 TEU containerships and 157,000 dwt Suezmax tankers. The study found that advanced small modular reactors address traditional concerns about safety, cost, waste, and proliferation while enabling net-zero shipping.
Nuclear propulsion for commercial vessels has proven reliable. The NS Savannah, America’s first nuclear merchant ship, achieved 99% power plant availability during operations. Russia’s Sevmorput has successfully operated in the Arctic for 36 years. The technology works — what’s changing is that climate imperatives and advanced reactor designs are making it commercially viable.
The SMR’s being developed for commercial shipping will use the same basic technology South Korea seeks for submarines. The expertise, supply chains, and infrastructure being built for merchant ships will in all likelihood support naval applications.
Such normalization is inevitable. The global transition to LEU fuel for both commercial and naval reactors could reduce proliferation concerns while enabling sustainable maritime transportation.
The Real Triggers
Before criticizing South Korea’s pursuit of nuclear propulsion, it’s essential to understand the country’s motivations. The problems for Seoul start with two nuclear-armed neighbors: China and North Korea. These two are the triggers for regional military buildup, not South Korea or Japan.
The world watched as North Korea withdrew from the NPT in 2003 and became a nuclear power, conducting six nuclear tests between 2006 and 2017, culminating in a thermonuclear test with approximately ten times the explosive power of the Hiroshima bomb. The international community’s inability to prevent this proliferation — or even meaningfully respond to it — has altered the security calculus in North-East Asia, particularly for Seoul.
North Korea now possesses nuclear warheads, a suite of ballistic missiles to deliver them, and has publicly revealed its own nuclear-powered submarine program. Pyongyang has also demonstrated submarine-launched ballistic missile (SLBM) capability in 2015 and continues to advance this technology.
Meanwhile, China operates arguably the world’s largest naval force equipped with a growing fleet of nuclear-powered submarines, consistently engages in maritime aggression throughout East Asian waters, and maintains the world’s third largest nuclear arsenal. In so doing, Beijing threatens the maritime domains that are existential concerns for maritime nations like South Korea and Japan.
In light of such developments, South Korea has been left with little or no choice but to pursue capabilities such as nuclear-powered submarines. When a neighboring state withdraws from the NPT, builds nuclear weapons, and develops submarine-launched nuclear delivery systems while the international order proves powerless to stop it, deterrence requires more robust measures. South Korea’s diesel-electric submarines, however capable, have limited underwater endurance compared to the nuclear threats they must track.
Nuclear propulsion provides submarines the ability to remain submerged for months, dramatically improving surveillance and deterrent capabilities. South Korean President Lee Jae Myung explained to President Donald Trump that conventional submarines have operational limitations in tracking submarine threats from North Korea and China, and that nuclear propulsion with conventional armament would reduce the burden on US forces.
If Japan follows South Korea’s path toward nuclear submarines, that too will be a response to the same threats. The narrative that South Korea and Japan are destabilizing the region gets causation backwards. China’s nuclear buildup and North Korea’s brazen NPT violation and subsequent nuclear armament are what destabilized East Asia. Responses to those threats are just that — responses.
The Path Forward
By developing nuclear submarines, South Korea is simply fulfilling its fundamental obligation to defend its territory and people, all the while maintaining close coordination with its most important ally, Washington. Seoul is not acting unilaterally or recklessly — it is working within established legal frameworks, choosing proliferation-resistant LEU fuel, and sourcing nuclear material from the US rather than building independent enrichment capabilities. Such approach demonstrates South Korea’s commitment to threading the needle between legitimate security requirements and nonproliferation obligations.
When a nation borders hostile nuclear-armed neighbors, pursuing enhanced military capabilities is not provocative behavior. It is responsible governance. Naysayers should think twice before advocating South Korea naively exercise restraint in arguably the world’s most dangerous neighborhood.