Washington Says "Yes" to South Korean Nuclear Submarines
Why America just allowed South Korea to build nuclear submarines — and what it means for China (and Philadelphia)
On October 29, 2025, President Donald Trump made an announcement that will impact the balance of power in the Indo-Pacific: South Korea will build nuclear-powered submarines. The decision, announced during the APEC summit in Gyeongju, breaks decades of United States (US) policy limiting nuclear propulsion technology to its closest allies, while placing South Korea alongside only six other nations authorized to operate nuclear-powered submarines.
South Korea’s Readiness
Over the past few decades, South Korea has steadfastly developed some of the core technology needed to design and build nuclear submarines. The country’s submarine design and construction capabilities are world-class as evidenced by the country’s indigenously designed KSS-3 submarines, while the KSS-1 (a.k.a Type 209) and KSS-2 (a.k.a Type 214) have proven their capabilities through multinational exercises, including on more than one occasion when the KSS-1 sunk US aircraft carriers.
Meanwhile, South Korea has been developing small modular reactors (SMR) at the Munmu the Great Research Institute, with plans for a 70MW reactor by 2027 — technology directly applicable to submarine propulsion. Furthermore, South Korea’s silent pursuit of “mini reactors” is known to predate these SMR’s.
The Nuclear Fuel Breakthrough
However, the real bottleneck has been to secure nuclear fuel. The 2015 Korea-US Nuclear Cooperation Agreement prohibits South Korea from enriching uranium beyond 20% or reprocessing spent nuclear fuel for military purposes without US approval. South Korea currently imports 4-5% enriched uranium for civilian reactors, which is insufficient for submarine propulsion.
Previous South Korean administrations have persistently attempted to change Washington’s mind, weathering repeated rejections. The US, concerned about nonproliferation precedents, repeatedly said no.
Why Washington Said Yes This Time
The October 29 breakthrough reflects a fundamental shift in how the US views burden-sharing and alliance dynamics in the Indo-Pacific. This time, the calculation was different. Washington didn’t simply grant Seoul’s long-standing request — it recognized mutual strategic benefit.
South Korea has understood the operational advantages of nuclear submarines for years: persistent surveillance capabilities against Chinese and DPRK submarines, extended patrol ranges throughout the Indo-Pacific, and the ability to operate without the vulnerability windows that constrain diesel-electric boats. These benefits were apparent to South Korean planners decades ago. What’s new is that Washington now sees tangible American benefits that align with its own strategic priorities.
Beyond these strategic calculations, the deal structure reveals how each stakeholder benefits.
Who Wins What
For the US Navy’s (USN) submarine community and Hanwha Philly Shipyard (“Hanwha”), this arrangement is transformational. Currently, only two US shipyards build nuclear submarines — General Dynamics Electric Boat in Connecticut and Huntington Ingalls Newport News Shipbuilding in Virginia. Both are stretched to capacity, struggling to meet the USN’s own requirements while also supporting the AUKUS commitment to build submarines for Australia.
Hanwha would become the third alternative, giving the USN crucial industrial diversification. This is particularly significant because submarine construction requires highly specialized infrastructure — nuclear-qualified welders, radiation shielding facilities, reactor compartment assembly capabilities — that can’t be replicated at just any shipyard. Unlike surface ship construction, which can be performed at numerous yards, submarine work demands much rare and specialized expertise.
For the USN, the timing couldn’t be better. American submarine construction is in crisis. The USN needs to produce 2.3 Virginia-class submarines per year to meet its requirements and AUKUS commitments, but current production barely exceeds one per year. ADM Daryl Caudle, nominated to lead the USN, told Congress in 2025 that meeting these obligations requires “not a 10 percent improvement, not 20 percent, but a 100 percent improvement” in production capacity.
Hanwha’s massive infrastructure investments will help address this industrial bottleneck. More importantly, the USN gains a proven shipbuilding partner. South Korea’s shipbuilding industry is arguably the world’s best — efficient, technologically advanced, and capable of meeting demanding schedules. Korean shipyards account for over 50% of global merchant ship tonnage. This partnership brings that expertise into America’s submarine industrial base.
For South Korea and the Republic of Korea Navy (ROKN), this means closing a critical capability gap. Nuclear-powered submarines can remain submerged for months, travel at sustained speeds of 20+ knots underwater, and operate throughout the Indo-Pacific without the range limitations that constrain diesel-electric boats. Even the best diesel submarines must eventually surface or snorkel to recharge batteries — a vulnerability that nuclear propulsion eliminates entirely.
Strategic Benefits
Whether on land or at sea, the geo-strategic advantages of the Korean peninsula have been recognized for centuries. It’s no different, when it comes to contemporary naval operations.
The ROKN installations at Jeju Island and Jinhae aren’t just convenient locations, they’re strategic chokepoints. Jeju sits at the entrance to the East China Sea, providing direct surveillance of sea lanes connecting China’s submarine headquarters in Qingdao to the Pacific and the rest of East Asia. Jinhae, on the southern coast, offers access to both the Korea Strait and the East Sea (a.k.a Sea of Japan). Together, they form a persistent maritime surveillance network that, when equipped with nuclear submarines, could impact the regional military balance.
This isn’t theoretical. During the Cold War, Japan’s sophisticated submarine force effectively monitored and constrained Soviet Pacific Fleet submarines attempting to transit from Vladivostok through the West Sea (a.k.a Sea of Japan) into the Pacific. The geographic reality of island chains and narrow straits gave Japan’s submarines outsized strategic influence despite operating a smaller fleet than the Soviets.
South Korea is now positioned to somewhat replicate this playbook. Nuclear-powered submarines operating from Jeju and Jinhae could monitor Chinese submarine movements, gather intelligence on their capabilities and operating patterns, and — in a crisis — potentially constrain their access to open ocean. This capability would contribute not just to South Korea’s national defense, but also to the broader allied effort to maintain maritime security throughout the Indo-Pacific.
Depending on how many units are ultimately built, ROKN nuclear submarines could become key nodes in a regional underwater surveillance network, operating alongside US, Japanese, and potentially Australian submarines to monitor the first island chain.
Conclusion
On October 29, 2025, Washington finally recognized what Seoul has known for years: South Korean nuclear submarines serve American strategic interests. The announcement in Gyeongju was by no means charity — it was recognition that South Korea’s geographic position, industrial capacity, and demonstrated naval power make it the ideal partner for sharing America’s most sensitive submarine technology. South Korea earned this through decades of preparation, until the country became strategically indispensable and rejection was no longer defensible.