U.S. approves South Korea's plan to build a nuclear-powered submarine in Philadelphia.

The United States has authorized South Korea to construct a nuclear-powered but conventionally armed submarine at Hanwha Philly Shipyard in Philadelphia, following a meeting between U.S. President Donald Trump and South Korean President Lee Jae Myung.

Following the October 29, 2025, meeting between U.S. President Donald Trump and South Korean President Lee Jae Myung in Gyeongju, Washington approved Seoul’s plan to build a nuclear-powered submarine in the United States. The vessel, to be assembled at Hanwha’s recently acquired Philadelphia facility, will operate with nuclear propulsion and conventional armament. The announcement is part of a broader new economic and security package covering trade, industrial investment, and defense cooperation.
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Nuclear reactors allow submarines to remain submerged for months without surfacing, reducing the vulnerability to detection, but also permit a greater power generation to improve situational awareness and target tracking. (Picture source: US Navy)

President Donald Trump stated that the South Korean nuclear-powered submarine will be built at Hanwha Philly Shipyard in Philadelphia following his meeting with South Korean President Lee Jae Myung in Gyeongju. The announcement coincided with a broader bilateral agreement that reduces U.S. tariffs on South Korean automobiles and auto parts to 15 percent and includes up to $350 billion in South Korean investments in the U.S. economy. Of that total, $150 billion is designated to enhance American shipbuilding capacity. According to the South Korean presidency, the project aims to produce a conventionally armed submarine equipped with nuclear propulsion, primarily to improve the country’s ability to track North Korean and Chinese submarines and to reduce operational dependence on U.S. assets. The White House did not specify any construction schedule or timeline for the initiative.

Currently, Hanwha Philly Shipyard, situated within the former Philadelphia Naval Shipyard area, has primarily produced commercial vessels, including oil tankers and containerships, and has no prior experience in naval or nuclear construction. Hanwha acquired the yard for $100 million in December 2024 and announced a $5 billion expansion plan to add docks, cranes, and quays to support larger projects. The company currently holds contracts for ten medium-range tankers and two LNG carriers, and it has indicated interest in participating in future naval programs through industrial cooperation. Building a nuclear submarine at this facility would require radiological licensing, the creation of secure perimeters, and training a specialized nuclear-qualified workforce. Additionally, Hanwha’s U.S.-linked subsidiaries are currently affected by Chinese trade restrictions, which could delay the execution of the program.

The submarine decision formed part of a wider package combining economic and security measures. During the Gyeongju summit, Trump and Lee presented the agreement as a renewal of the alliance’s economic and industrial foundations. Trump emphasized that the project would contribute to revitalizing shipbuilding in the United States and reiterated that South Korean investment would have a direct industrial impact on Philadelphia and other maritime hubs. The South Korean presidential office stated that it viewed the approval of nuclear-powered submarines as a cooperative measure to enhance national security and modernize the alliance, while clarifying that Seoul seeks access to propulsion fuel and does not intend to pursue nuclear armament. Both sides agreed to hold further consultations regarding technical implementation, industrial coordination, and regulatory compliance concerning nuclear fuel supply.

For now, the South Korean Navy operates around 21 conventionally-powered submarines divided among three programs: nine Type 209/1200 Jang Bogo-class (KSS-I), nine Type 214 Son Won-il-class (KSS-II), and three indigenous 3,000-ton Dosan Ahn Chang-ho-class (KSS-III) units either commissioned or fitting out. The KSS-I boats, built under German license between 1993 and 2001, remain in service with upgrades, while the KSS-II class, introduced from 2007 to 2020, employs air-independent propulsion for longer submerged endurance. The KSS-III program, designed and built domestically, includes nine submarines in three batches: Batch I (3,000 tons) and Batch II (3,600 tons with 10 VLS cells) under construction at Hanwha Ocean. The first two, ROKS Dosan Ahn Chang-ho (SS-083) and ROKS Ahn Mu (SS-085), were commissioned in 2021 and 2023, and the third is fitting out. Batch II units will integrate lithium-ion batteries and launch sequentially between 2025 and 2031. Completion of all KSS-III boats by 2029–2031 will allow gradual retirement or modernization of KSS-I units, strengthening endurance, surveillance, and deterrence in regional waters.

Compared to diesel-electric submarines, nuclear-powered submarines possess several operational advantages that explain Seoul’s interest in propulsion technology. Nuclear reactors allow submarines to remain submerged for months without surfacing, eliminating the need for frequent snorkeling and reducing vulnerability to detection. Their propulsion systems provide much higher sustained speeds, often exceeding 25 knots underwater, and enable rapid repositioning across long distances without refueling. The increased endurance also allows greater power generation for advanced sonar, sensor arrays, and electronic systems that improve situational awareness and target tracking. These factors make nuclear-powered submarines especially suitable for extended patrols in the Pacific and for monitoring both strategic and tactical naval movements by adversaries. For South Korea, such capabilities would offer a qualitative leap in underwater deterrence and intelligence-gathering compared to conventional platforms.

Recently, North Korea has publicized progress on a nuclear-powered submarine hull and has tested sea-launched cruise missiles, while China continues to expand its naval and island-building activities in the Yellow Sea and East China Sea. President Lee reportedly told Trump that conventional diesel-electric submarines possess limited endurance for long-duration patrols, making nuclear propulsion a key requirement to strengthen surveillance around the Korean Peninsula. Trump’s subsequent statement about the project emphasized that it represented a defensive measure aligned with regional developments. South Korea reiterated that the request concerned nuclear propulsion technology rather than armament, emphasizing the necessity of enhanced deterrence amid evolving maritime threats.

Until now, the bilateral U.S.–ROK 123 Agreement, updated in 2015 and effective into the 2030s, prohibits South Korea from enriching uranium above 20 percent or using U.S.-origin nuclear materials for military purposes without explicit U.S. authorization. This restriction has long prevented Seoul from advancing propulsion-related fuel research independently. South Korean officials now expect to begin technical and diplomatic consultations on how nuclear propulsion could proceed within the existing framework or under a separate arrangement. Some observers view the trilateral AUKUS model, through which Australia accesses nuclear propulsion under strict safeguards without direct transfer of classified design data, as a potential precedent. Domestic debate in South Korea remains divided, balancing proponents of nuclear latency for deterrence with those emphasizing compliance and alliance stability.

The industrial component of the project could also face substantial constraints. The United States Navy is experiencing production delays of two to three years on Virginia-class submarines and up to two years on the first Columbia-class ballistic missile submarine, largely due to workforce shortages and supplier bottlenecks. Introducing a new program for a foreign partner would require qualified personnel, specialized components, secure supply chains, and oversight infrastructure currently lacking at commercial facilities. Hanwha’s management has expressed readiness to cooperate with U.S. authorities to bridge these gaps, but establishing a nuclear-certified construction line would likely take several years. Local industry actors such as Rhoads Industries, which produces submarine components for Electric Boat, are expanding operations at the Philadelphia Navy Yard; however, such support industries cannot replace the need for comprehensive nuclear certification and oversight required for propulsion systems.

South Korea’s domestic industrial base provides partial groundwork but would still need adaptation. The country operates the KSS-II and KSS-III series of diesel-electric submarines with lithium-ion batteries and air-independent propulsion, representing advanced conventional technologies. Transitioning to a nuclear-powered design would require new engineering capacities for reactor integration, pressure hull design, and long-duration underwater systems. Experts describe five essential prerequisites for a nuclear submarine program: compact reactor design, nuclear-qualified industry, safety regulation, skilled human resources, and legal authority to possess propulsion fuel. South Korea possesses advanced shipbuilding and civilian nuclear expertise but remains legally restricted from handling or enriching fuel for military propulsion. Achieving these capabilities under U.S. supervision would require new bilateral mechanisms and international safeguards compatible with nonproliferation standards.

Further steps will depend on how the two governments define technical and procedural frameworks. Joint committees are expected to examine reactor safety, industrial capacity, and nuclear fuel supply mechanisms in the coming months. Any construction in Philadelphia would require radiological licensing, security zoning, and infrastructure reinforcement before hull fabrication could begin. Regional responses from China and North Korea will be closely monitored, given their opposition to allied nuclear propulsion projects. Integration with U.S. and allied naval doctrine would also require coordination of communications, maintenance, and logistics. Until those technical, legal, and regulatory aspects are finalized, the Philadelphia submarine remains a conceptual project symbolizing intent rather than an executable program. Its realization will depend on compliance with nonproliferation obligations, industrial certification, and the ability of both nations to synchronize shipbuilding timelines within an already congested naval production environment.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.