International Cooperation for South Korea's Nuclear Submarine Ambitions in the Trump 2.0 Era

                                                                                                                                                                                                                                                                                                  Seong Chang Cheong

softpower@sejong.org​

Principal Research Fellow

Sejong Institute

Peter Ward

pward89@sejong.org

 Research Fellow

Sejong Institute

1. The Strategic Challenge

North Korea has made striking progress in its development of nuclear-powered ballistic missile submarines (SSBNs). At the 8th Party Congress in January 2021, Kim Jong Un declared the development of a strategic nuclear submarine a national priority, calling it essential to enhancing the country’s long-range nuclear strike capability. In December 2023, he reaffirmed this commitment, instructing the shipbuilding sector to accelerate the naval modernization program as part of the five key national defense goals. Until recently, many external analysts believed North Korea's SSBN program was facing technical hurdles. However, in March 2025, North Korea publicly revealed the hull of a new strategic nuclear submarine, suggesting that it may soon acquire a credible sea-based second-strike capability.

Despite the immense technical challenges involved, North Korea could achieve this milestone sooner than expected if Russia provides small reactor technology in return for North Korean support in the Ukraine war. While some experts believe Russia would refrain from transferring such sensitive technologies, historical precedent suggests otherwise—Russia has previously supported nuclear submarine development in India and even considered assistance to South Korea in the 1990s.

Nuclear submarines are broadly categorized into SSNs (nuclear-powered attack submarines) and SSBNs (ballistic missile submarines capable of launching SLBMs). While all current nuclear submarine powers operate SSBNs, countries like Brazil and Australia are developing SSNs without nuclear weapons. South Korea's past proposals under the Roh Moo-hyun and Moon Jae-in administrations focused on SSNs as well. Some variants like SSGNs also carry cruise missiles rather than ballistic missiles.

In response to North Korea’s growing underwater threat, South Korea must consider acquiring multiple nuclear-powered submarines. Retired Vice Admiral Kim Pan-gyu argued in 2022 that South Korea should build three to six nuclear-powered strategic submarines to prepare for future threats. Estimates vary on the development timeline: some experts suggest it could take five years using existing domestic technology, while others believe a decade or more may be needed. Regardless, the Lee administration should raise the issue in summit talks with President Trump.

If North Korea succeeds in fielding an SSBN capable of covertly reaching the U.S. mainland, it will fundamentally alter the strategic landscape by giving Pyongyang a secure second-strike capability—posing a severe threat to the security of South Korea, Japan, and the United States. This makes it all the more urgent for South Korea and Japan to pursue nuclear-powered submarine programs of their own.

Finally, the growing constraints on U.S. shipbuilding and submarine production—driven by industrial decline—create a window of opportunity for expanded U.S.-ROK cooperation in this domain. This brief explores the rationale for South Korea’s pursuit of nuclear-powered submarines, the broader context of U.S. naval challenges, and the international cooperation pathways that could support this goal.

2. North Korea’s Submarine Threat and the Case for South Korean Submarines

North Korea has increasingly emphasized submarine-based nuclear capabilities as part of its strategic posture. In September 2023, it launched the Kim Kun Ok Hero, a diesel-powered submarine modified to carry tactical nuclear weapons. Kim Jong Un simultaneously revealed plans to convert all of North Korea’s mid-sized submarines into nuclear-armed attack submarines as part of a "low-cost modernization strategy." North Korea reportedly possesses over 20 Romeo- and Gorae-class submarines, and if each were fitted with 10 SLBM launch tubes for tactical nuclear warheads, it could be armed with up to 200 nuclear missiles. The Kim Kun Ok Hero, while diesel-powered, is designed for regional operations, unlike the strategic SSBN currently under development, which may have a displacement of 5,000–10,000 tons based on images released in March 2025.

Given its size, this SSBN is likely to be powered by a nuclear reactor and built for long-endurance missions—suggesting an intent to secure a credible second-strike capability. Although North Korea lags behind South Korea in overall shipbuilding capacity, its extensive submarine construction experience, Kim Jong Un’s strong determination, and substantial state investment make it increasingly plausible that it will field an operational SSBN in the near future. Once deployed, such a submarine could operate in the Indo-Pacific, potentially armed with both strategic and tactical nuclear weapons—making it a true “game changer” for the regional security environment and a serious challenge to the U.S.-ROK alliance.

North Korea is already the world’s largest operator of submarines by number, reportedly fielding over 80—including midget submarines—outpacing even the U.S., China, and Russia. While those nations possess nuclear-powered submarines and vastly superior capabilities, North Korea’s acquisition of a true SSBN would significantly alter threat perceptions, particularly for South Korea, which relies solely on diesel-electric submarines.

This development compounds the threat already posed by North Korea’s land-based ICBM capabilities. If North Korea fields SSBNs capable of operating near the U.S. mainland, it would raise doubts about America’s willingness to retaliate against a nuclear strike on South Korea—given the risk of nuclear reprisal against U.S. cities. Former Pentagon official Elbridge Colby underscored this dilemma in 2024, stating that the U.S. public is largely unaware that extended deterrence could put millions of Americans at risk, and that America might not uphold its nuclear umbrella in practice.

Strategic nuclear submarines enhance North Korea’s ability to deliver reliable nuclear threats—raising the credibility and coercive power of its deterrent. In this context, it becomes urgent for South Korea to pursue nuclear-powered submarines (SSNs). Although not nuclear-armed, SSNs would enable South Korea to track and deter both North Korea’s modified diesel submarines and its future SSBNs. Nuclear submarines provide critical advantages over diesel-electric ones: greater speed, longer endurance, stealthier operation, and larger payload capacity.

SSNs can sustain speeds over 46 km/h—more than three times faster than conventional submarines—allowing for rapid attack and repositioning. Their operational range is also unmatched: during the Falklands War, a British SSN arrived in theater in 10 days, while a diesel sub took over five weeks. Modern SSNs are also quieter than ever, equipped with advanced noise-reduction technologies that outperform even diesel boats in stealth.

In contrast, conventional submarines must surface or snorkel frequently to recharge batteries, exposing themselves to detection by satellites or anti-submarine aircraft. Even South Korea’s advanced Son Won-il–class submarines can only stay submerged for about two weeks at low speed, with effective operational endurance near enemy bases limited to a matter of days. Even air-independent propulsion (AIP) systems, while extending endurance, fall far short of the months-long deployments enabled by nuclear propulsion.

Given these realities, and the limitations of conventional submarines in endurance, survivability, and mobility, South Korea must seriously consider acquiring SSNs. Doing so would provide a decisive edge in tracking and countering North Korea’s emerging underwater nuclear threats—and help reinforce deterrence at a time of strategic instability in the Indo-Pacific.

3. The Decline of U.S. Shipbuilding and Challenges to Naval Power

The decline of U.S. shipbuilding has reached a critical stage. Once the world’s leading shipbuilder in 1975, the U.S. ranked 19th by 2023. Historically, America’s shipbuilding prowess—bolstered by Theodore Roosevelt’s “Great White Fleet” and scaled massively during World War II—was instrumental in establishing global naval dominance. Today, however, the U.S. builds fewer than 10 commercial vessels per year, in stark contrast to China’s output of over 1,000 ships annually. Of the roughly 44,000 merchant vessels worldwide, fewer than 200 are U.S.-flagged.

America’s global share in shipbuilding began declining with Japan’s rise in the 1950s, followed by South Korea in 2000, and China surpassing both by 2010. Ironically, protectionist laws like the Jones Act—which mandates that vessels operating between U.S. ports must be domestically built—have further insulated and weakened U.S. competitiveness.

The U.S. Navy also suffered post–Cold War cuts. From a peak of 526 vessels in 1991, the fleet fell to 291 ships by 2023. The number of naval shipyards dropped from 24 in 1945 to just four by 2013, only two of which can construct or repair large warships. In contrast, China operates over 100 major shipyards and has an estimated 233 times the shipbuilding capacity of the U.S.

Although the Navy now aims to reach a 355-ship fleet by 2034—a reduction from the original 459-ship goal due to budgetary pressures—current construction rates fall short. By 2024, the Navy operated 297 ships, but retirements are outpacing new deliveries. Attack submarine (SSN) capacity is particularly strained. Since 2011, the Navy has aimed to procure two SSNs annually, yet U.S. shipyards have averaged only 1.2 SSNs per year since 2022. Moreover, over 30% of the SSN fleet has been non-operational at any given time since 2018 due to maintenance delays, far exceeding the Navy’s goal of keeping downtime under 20%.

The U.S. plans to deliver 3–5 Virginia-class SSNs to Australia under the AUKUS agreement, but doing so without undermining its own force posture requires achieving two-per-year SSN construction by 2028—still an uphill battle. Future projections are grim: the Congressional Budget Office estimates the Navy will need $401 billion over 30 years to implement its 2025 shipbuilding plan, a 46% increase in average annual funding compared to the previous five years. The plan envisions a force of 381 manned ships and 134 unmanned surface and undersea platforms, including 59 SSNs (45 Virginia-class and 14 of the future SSN(X)).

Costs are ballooning. The Virginia-class SSN, which cost $2.8 billion per unit before 2019, now costs $4.5 billion due to upgrades like Tomahawk missile modules. These increases come despite the U.S. running peacetime budget deficits of 6–7% of GDP. While the Trump administration has called for defense budget increases—at times even suggesting a $1 trillion annual defense budget—long-term sustainability is uncertain due to America’s worsening fiscal position.

As doubts grow over the U.S. Navy’s long-term presence in East Asia, its ability to deter emerging threats like North Korea’s nuclear-powered submarines is also being questioned. The Navy currently has only three nuclear submarines stationed in Guam, the closest major base to the Korean Peninsula. If the U.S. must simultaneously counter China’s naval rise in the Taiwan Strait and respond to North Korean provocations near the peninsula, its overstretched SSN fleet may not be able to meet both demands.

Although the U.S. maintains a qualitative edge in SSNs, China's shipbuilding advantage and rapidly growing fleet mean the U.S. could soon be outmatched in East Asia. As such, South Korea’s vulnerability to nuclear threats from North Korean submarines may increase—especially if U.S. naval power becomes less able to maintain credible forward presence in the region.

4. International Cooperation Possibilities

a. South Korea’s Independent Development Efforts and Capabilities

Since the 1990s, successive South Korean administrations have covertly explored the development of nuclear-powered submarines (SSNs) in response to the North Korean nuclear threat. In 1994, during the first nuclear crisis, President Kim Young-sam ordered a secret SSN program, allocating 48 billion won and seeking Russian small reactor designs due to their experience with over 200 nuclear submarines. Plans were made to build 3,000-ton SSNs following the deployment of 209-class submarines, but technical challenges, particularly hull design, stalled progress.

Efforts were revived under the Roh Moo-hyun government through the so-called "362 Project," which completed a basic reactor design by 2004. However, IAEA inspections and media leaks, along with budgetary and manpower constraints, led to another suspension. The Moon Jae-in administration included SSN development in its 2021–2025 mid-term defense plan but failed to make substantial progress, mainly due to the U.S. refusal to share nuclear propulsion technology.

Recent developments, including North Korea’s advancing strategic submarine program, have opened the possibility of a more flexible U.S. stance. A commander from U.S. Indo-Pacific Command hinted at receptiveness, though ambiguously. Meanwhile, South Korea’s own submarine capabilities have grown significantly. The domestically designed 3,000-ton KSS-III class demonstrates world-class technology, boasting lithium-ion batteries, AIP systems, and 80% localization. However, a “black project” approach to SSN development may delay progress due to inter-agency coordination issues. The government should shift to an open, state-backed model like Brazil or Australia and allocate sufficient funding to avoid delays.

While concerns over cost remain, experts estimate that a South Korean SSN could be built for about 2.5 trillion won—comparable to U.K.’s Astute-class and significantly cheaper than U.S. Virginia-class submarines. Moreover, South Korea’s lower labor costs and mature shipbuilding infrastructure could shorten construction time and reduce costs. The government is also developing small modular reactors (SMRs), which, if adapted for military use, could serve as SSN propulsion units—though fuel sourcing remains a hurdle. With U.S. enrichment restrictions in place, alternatives such as French cooperation should be explored, highlighting the need for diversified international collaboration.

b. Pathways for International Cooperation

(1) United States

The U.S. faces severe constraints in sustaining its SSN force due to industrial decline and AUKUS commitments. The U.S. plans to deliver 3–5 Virginia-class SSNs to Australia in the 2030s while struggling to maintain its own force levels. Given this, cooperation with South Korea in SSN parts production, maintenance (MRO), or even licensed co-construction is increasingly relevant. South Korea’s involvement could help ease U.S. naval burdens while providing Seoul with strategic benefits.

Though the U.S. traditionally restricts nuclear propulsion technology transfers (the last being to the U.K. in 1958), AUKUS marks a historic exception. South Korea, aiming to build non-nuclear-armed SSNs similar to those planned by Australia and Brazil, could seek participation in an expanded AUKUS-plus model or a parallel bilateral arrangement. U.S. officials have emphasized that SSNs do not violate the NPT as long as they do not carry nuclear weapons. South Korea’s advanced shipbuilding industry also makes it an ideal partner.

Moreover, trilateral cooperation among South Korea, the U.S., and Japan could be pursued. If Japan and South Korea jointly developed SSNs under a three-member consortium, their assets could complement U.S. naval strategy in contingencies such as a Taiwan Strait crisis. Such collaboration aligns with Trump-era preferences for allies to take on more defense responsibilities and may be more viable under a potential second Trump administration, which may reduce direct U.S. naval presence in favor of allied burden-sharing.

The acquisition of the Philadelphia Naval Shipyard in 2024 by Hanwha Group further strengthens its credentials. The U.S. Navy has acknowledged this as a milestone, suggesting Korea is not merely seeking assistance but can actively contribute to U.S. maritime strategy.

(2) France

France presents an attractive alternative partner. Brazil’s SSN program has relied heavily on French design and technology since the two countries signed a strategic defense agreement in 2008. While France has been cautious in sharing sensitive reactor technologies, President Macron signaled openness to supporting Brazil’s SSN development in full compliance with nonproliferation norms.

France’s Naval Group has shown flexibility in cooperating with non-nuclear-weapon states. For Korea, which already possesses a completed basic design for an SSN reactor and advanced shipbuilding capabilities, working with Naval Group could accelerate development—particularly if U.S. cooperation remains limited. France’s use of low-enriched uranium in its submarines would also help Korea stay within NPT bounds. However, past issues with cost overruns and cybersecurity in Australia’s canceled submarine deal with France caution the need for clear, well-managed cooperation.

(3) India

India, while already possessing strategic SSBNs, has no SSNs in operation. Its past leasing of nuclear submarines from Russia may not be replicable for Korea due to geopolitical constraints. However, India’s growing interest in defense industrial cooperation with South Korea creates a foundation for deeper strategic collaboration. India’s enrichment and reactor capabilities could also help shorten Korea’s SSN development timeline and stabilize fuel supply chains. Although India is not a primary partner, it could play a complementary role in technology sharing and fuel logistics.