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Three Key Diplomatic and Security Challenges in Pursuing Nuclear-Powered Submarines |
| December 26, 2025 |
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Bong-Geun JunVisiting Research Fellow, Sejong Institute | jun2030@mofa.or.kr
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At the ROK–U.S. summit held in Gyeongju on October 29, 2025, the two leaders agreed to expand the alliance into a “future-oriented, comprehensive strategic alliance.” Among the summit’s various outcomes, media outlets and experts focused intensely on South Korea’s move to introduce nuclear-powered submarines (hereafter “SSNs”).1)
Following bilateral coordination, the Joint Fact Sheet released on November 14 explicitly stated that the United States would “approve the construction of South Korea’s SSNs and collaborate closely to advance project requirements, including nuclear fuel procurement.” This marked the first time Washington has officially endorsed South Korea’s pursuit of nuclear-powered submarines. Historically, South Korea’s aspirations for SSNs, dating back to the Roh Moo-hyun administration in the early 2000s, were repeatedly thwarted by U.S. opposition. However, at this summit, President Lee Jae-myung formally and publicly requested the acquisition, and the U.S. president approved it for the first time. This milestone decision enjoys overwhelming domestic support, with a recent poll showing that 87 percent of the South Korean public favors the introduction of SSNs.2)
The Gyeongju summit provided the first-ever U.S. endorsement for South Korea’s SSN program, establishing the essential political and diplomatic groundwork for the project to proceed in earnest. Nevertheless, given the significant strategic and nonproliferation implications of nuclear-powered submarines, the road ahead is unlikely to be smooth. This article examines the three primary political and diplomatic debates that inevitably arise with the introduction of SSNs and presents appropriate policy positions for South Korea. First, considering South Korea’s formidable conventional military capabilities and the robust ROK–U.S. alliance, why are SSNs necessary? Second, would the acquisition of SSNs increase the risk of South Korea pursuing nuclear armament? Third, would the pursuit of SSNs, and the resulting increase in strategic autonomy, weaken the ROK–U.S. alliance or trigger a regional arms race, thereby undermining strategic stability in the region? -
Why does South Korea need SSNs, and what purpose do they serve? This has become the most frequently asked question since the ROK–U.S. agreement on the introduction of SSNs. In reality, the SSN program demands immense financial investment and an extended development timeline, and it faces opposition for various domestic and international reasons. Therefore, if South Korea is to move forward with the introduction of SSNs, it is imperative to establish a clear strategic rationale and secure broad public consensus as a prerequisite.
First, South Korea’s nuclear-powered submarines constitute a useful and effective non-nuclear deterrent against North Korea’s advanced nuclear and missile threats. North Korea is estimated to possess 50 to 100 nuclear weapons and is actively developing and deploying various delivery systems, including ICBMs, SLBMs, strategic cruise missiles, hypersonic missiles, and nuclear torpedoes. It is also in the process of developing nuclear-powered ballistic missile submarines (SSBNs). North Korea’s Law on the State Policy on Nuclear Forces (2022) codifies arbitrary, preemptive, and offensive nuclear use, posing an existential security threat to the South Korean public. Furthermore, Kim Jong Un has recently redefined inter-Korean relations not as relations between compatriots but as “hostile relations between two states,” thereby further intensifying the nuclear threat.
While some in South Korea have called for nuclear armament to counter North Korea’s nuclear threats, the Lee Jae-myung administration has clearly rejected such demands, emphasizing South Korea’s status as a model of international norm compliance and a “responsible global state.” Given that South Korea is a trading nation with a heavy reliance on external markets, nuclear armament, which would inevitably invite crippling economic sanctions, is not a viable option. Therefore, South Korea must maximize its non-nuclear deterrent capabilities. Currently, South Korea operates advanced diesel-electric and AIP (Air-Independent Propulsion) submarines. However, limitations in underwater endurance, operational range, and missile payload capacity constrain their effectiveness in deterring North Korea’s nuclear use or neutralizing its strategic submarines.
South Korea’s nuclear-powered submarines are virtually the only non-nuclear military asset capable of operating underwater for months without refueling while tracking and engaging North Korean submarines in the waters surrounding the Korean Peninsula. For instance, during the Gyeongju Summit, President Lee Jae-myung told President Trump, “Because the limited endurance of diesel submarines constrains our ability to track North Korean or Chinese submarines, we intend to construct nuclear-powered submarines to carry out maritime defense missions in the East and West Seas.”3)
Another role of nuclear-powered submarines is to serve as a platform for preemptive strikes when a North Korean nuclear attack against the South is imminent, and to act as a strategic asset for punitive retaliation against North Korea’s key targets by drawing on survivable forces in the aftermath of an actual attack. This would significantly bolster two of the three pillars of the South Korean military’s “Three-Axis System,” preemptive strike, missile defense, and massive retaliation, specifically the first and third pillars. In this regard, nuclear-powered submarines constitute a linchpin of South Korea’s “non-nuclear” strategic deterrence.
Next, South Korea’s nuclear-powered submarines, in coordination with the ROK-U.S. alliance, are expected to enhance strategic stability on the Korean Peninsula and in Northeast Asia by qualitatively bolstering regional defense and deterrence. While the nuclear, missile, and naval capabilities of North Korea, China, and Russia are expanding rapidly, U.S. forces in the region have remained relatively static. If this results in a weakened regional balance of power, the risk of reckless military provocations increases. By filling these capability gaps and imbalances within the framework of the ROK-U.S. alliance and ROK-U.S.-Japan security cooperation, South Korea’s nuclear-powered submarine force will contribute to restoring and maintaining the regional military balance and strategic stability.
Furthermore, as the U.S.-led global hegemonic order faces growing challenges and maritime security is increasingly threatened worldwide, South Korea, as a leading middle power, must fulfill the international security responsibilities expected by the international community. Moreover, as a trading nation and a resource-scarce country that is heavily dependent on maritime supply chains, South Korea has a vital need to participate actively in protecting Sea Lines of Communication (SLOCs). Nuclear-powered submarines, capable of long-range operations, would serve as a critical asset for carrying out these international maritime security missions.
In summary, South Korea’s nuclear-powered submarines represent a strategic choice to secure a “non-nuclear” deterrent against North Korea’s nuclear threats, pursue cooperative security through role-sharing with the United States, and contribute to global and maritime security as a leading middle power and trading nation. This constitutes a rational path for a responsible, non-nuclear middle power navigating complex security challenges.
While some U.S. strategists tend to view South Korea’s nuclear-powered submarines as part of the Western Pacific forces under the purview of the U.S. Indo-Pacific Command and emphasize their role in countering China, South Korea’s fundamental position is to focus their role on deterring North Korea and maintaining stability in the waters surrounding the Korean Peninsula. In this respect, a perceptual gap does exist between the two allies; however, such differences are likely to be sufficiently reconciled through consultations on the division of regional security roles. -
Some in the international community criticize South Korea’s acquisition of nuclear-powered submarines as a stepping stone toward nuclear armament, thereby increasing the risk of nuclear proliferation. These critics argue that while South Korea’s nuclear-powered submarines do not formally violate international nonproliferation norms, they would set a precedent for a Non-Nuclear-Weapon State (NNWS) to utilize nuclear energy for military purposes. They contend that this would further undermine the global nonproliferation governance, which has already been eroded by North Korea’s nuclear armament.
To dispel such concerns, President Lee Jae-myung and senior government officials have emphasized that South Korea will remain an exemplary non-nuclear member state of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) and will never possess nuclear weapons. They maintain that the country will strictly limit its use of nuclear energy to submarine propulsion in order to counter the existential threats posed by North Korea. In fact, the South Korean government intends to implement a dual-layer safeguard framework to fundamentally preclude any risk of nuclear proliferation stemming from the introduction of nuclear-powered submarines.
First, South Korea does not intend to produce nuclear fuel for its submarines domestically; instead, it seeks to procure it from the United States. Specifically, at the ROK–U.S. summit in Gyeongju on October 29, 2025, President Lee Jae-myung presented South Korea’s plan to construct nuclear-powered submarines to President Trump and requested the “provision of nuclear fuel.” Reflecting this, the “ROK–U.S. Summit Fact Sheet” released on November 14 stated that the United States approved the construction and would cooperate with South Korea on “fuel procurement arrangements.”
Meanwhile, some domestic voices are calling for an early revision of the ROK–U.S. Atomic Energy Agreement to restore full autonomy over uranium enrichment and to enable the domestic production of nuclear fuel for these submarines. Notably, Brazil, the only Non-Nuclear-Weapon State (NNWS) currently developing its own nuclear-powered submarines, produces the necessary fuel at its own enrichment facilities.
In South Korea’s case, unlike Brazil, the country does not possess its own uranium enrichment facilities, making domestic enrichment infeasible, with “military” enrichment for nuclear-powered submarines being an even less realistic option. Separately from the issue of fuel for nuclear-powered submarines, South Korea has been pursuing the introduction of enrichment capabilities to ensure a stable supply of nuclear fuel for its 26 operating nuclear reactors. Reflecting these requests, the United States stated in the aforementioned Fact Sheet that it “supports procedures that would lead to civilian uranium enrichment and spent fuel reprocessing for Korea’s peaceful use, consistent with the ROK–U.S. Atomic Energy Agreement and in compliance with U.S. legal requirements.” This makes clear that the United States supports the introduction of civilian enrichment facilities within the existing framework of the ROK–U.S. Atomic Energy Agreement. At the same time, by rejecting any revision of the agreement, the United States has foreclosed the possibility of South Korea producing nuclear fuel for its submarines domestically.
Following India’s nuclear test in 1974, the U.S. government moved to restrict the spread of additional enrichment and reprocessing (ENR) facilities in order to prevent nuclear proliferation through the peaceful use of nuclear technology. Accordingly, even if the United States were to permit uranium enrichment in South Korea in the future under the ROK–U.S. Atomic Energy Agreement, such permission would be strictly limited to civilian use, and the possibility of allowing “military enrichment” for nuclear-powered submarines would be virtually nonexistent.
Furthermore, arguments for revising the agreement to enable the domestic production of submarine fuel overlook the fact that the agreement is confined to “peaceful uses” only. In other words, even if domestic enrichment were to become possible through an amendment, producing fuel for nuclear-powered submarines at such facilities would remain prohibited. Ultimately, as President Lee Jae-myung has noted, the only realistic path for South Korea to acquire nuclear fuel for its nuclear-powered submarines is to obtain it from the United States through a separate ROK–U.S. bilateral military agreement that is entirely distinct from the existing Nuclear Cooperation Agreement.
Next, South Korea plans to utilize low-enriched uranium (LEU) fuel for its nuclear submarines rather than the more conventional highly enriched uranium (HEU). This stands in stark contrast to Australia’s AUKUS submarines, which employ weapons-grade HEU enriched to approximately 95 percent. As nuclear submarine fuel, HEU offers clear military-technical advantages over LEU (enriched to 20 percent or less): HEU-powered vessels do not require refueling throughout their 30–40-year service life. By contrast, LEU-fueled submarines typically require refueling roughly every ten years. This process involves cutting open the hull, replacing the fuel, and re-welding the structure, followed by a comprehensive re-certification of safety and operational readiness, a process that generally takes two to three years.
If South Korea were to acquire U.S.-built nuclear submarines, as initially proposed by President Trump, such submarines would likely be equipped with HEU fuel, similar to Australia’s AUKUS submarines. In that case, the United States would provide reactors that are completely sealed; consequently, Australia is denied access to the reactor systems, with the United States retaining exclusive responsibility for maintenance and emergency response. However, since South Korea prefers to indigenously manufacture both the submarine hull and the reactor, it is highly improbable that the United States would supply HEU, given its significant proliferation risks. Therefore, for South Korea, the choice of LEU fuel is both realistic and rational. Furthermore, because U.S.-supplied LEU fundamentally precludes the risk of nuclear proliferation, it would ultimately contribute to strengthening the global nonproliferation regime.
Meanwhile, the AUKUS nuclear submarines that the United States plans to provide to Australia, by utilizing HEU, run counter to the nonproliferation values espoused by both countries and risk undermining global nuclear nonproliferation governance. In this context, the option of integrating LEU-fueled reactors currently under development by South Korea into the AUKUS submarines could be considered a viable alternative. Furthermore, given that the United States faces significant shipbuilding capacity constraints, making it unlikely to meet the original delivery schedule for Australia, South Korea could leverage its advanced construction and reactor development capabilities to propose a partnership for the joint construction of submarine hulls and the co-development of LEU reactors. This would constitute a cooperative framework aligned with the strategic interests, nonproliferation objectives, and shared economic benefits of all three countries. -
There are two concerns regarding the potential negative strategic consequences of South Korea’s nuclear-powered submarines. First, it is argued that their introduction would fuel an arms race in Northeast Asia and undermine strategic stability. In a November 18 KCNA commentary, North Korea condemned the acquisition as “a grave development that destabilizes the security situation in the Asia-Pacific region beyond the Korean Peninsula and leads to a situation of global nuclear uncontrollability.”4)
It further claimed that the move is a “stepping stone toward indigenous nuclear armament” and would “trigger a nuclear domino effect and provoke a more intense arms race” in the region. Contrary to these claims, however, South Korea’s pursuit of nuclear-powered submarines is a reactive measure to the existential security threat posed by North Korea’s own nuclear armament. Had North Korea neither pursued nuclear weapons nor posed a nuclear threat, South Korea’s acquisition of nuclear-powered submarines would have been neither necessary nor politically feasible.
Additionally, China may oppose South Korea’s acquisition of nuclear-powered submarines and respond by further accelerating its naval buildup. However, South Korea intends to designate deterrence and retaliation against North Korean nuclear threats as the primary mission of these assets, while assigning the defense against unspecified security contingencies in the surrounding waters and the maintenance of regional stability as secondary objectives. Consequently, the argument that South Korea’s nuclear submarines would exacerbate regional strategic instability is untenable. On the contrary, this article maintains that these capabilities will serve to stabilize and restore regional security. South Korea has neither historically nor currently demonstrated aggressive intent toward its neighbors, nor has it pursued military expansion that undermines strategic equilibrium. In fact, historical precedents suggest that when South Korea possesses sufficient self-reliant defense capabilities, the likelihood of regional conflict is markedly diminished.
Moreover, as the military capabilities and activities of North Korea, China, and Russia continue to expand, and as the U.S. military presence and alliance system, long-standing guarantors of regional peace, face relative weakening, the likelihood of shifts in the balance of power and the erosion of strategic stability will increase. In this increasingly precarious environment, South Korea’s self-reliant defense capabilities, situated at the geographic and geopolitical center of Northeast Asia, are expected to serve as a crucial ballast for restoring and maintaining regional stability.
Next, some argue that South Korea’s introduction of nuclear-powered submarines could weaken the ROK-U.S. alliance over the long term.5) This perspective holds that, amid the recent divergence in strategic priorities between South Korea and the United States, the acquisition of such assets might embolden South Korea to seek greater “strategic autonomy,” potentially leading to a gradual decoupling from the alliance. Ironically, the Trump administration has sought to expand the strategic flexibility of U.S. Forces Korea (USFK) and assign them a larger role in countering China, reflecting a shift in focus toward broader U.S.-China strategic competition.
Meanwhile, the Lee Jae-myung administration seeks to expedite the transition of wartime operational control (OPCON), a goal that necessitates further strengthening national defense capabilities, including the acquisition of nuclear-powered submarines. This buildup is not driven by an intent to decouple from the ROK-U.S. alliance; rather, it is based on a mutual understanding of burden-sharing that allows the United States to focus more intensively on countering China. In practice, even with nuclear-powered submarines, South Korea’s role would remain limited to supplementing the deterrence of North Korea’s nuclear threats, and the country will continue to rely fundamentally on the U.S. nuclear umbrella and extended deterrence. Conversely, if the United States seeks to prevent South Korea’s indigenous nuclear armament, it must continue to provide a credible and robust nuclear guarantee. Therefore, as long as geopolitical competition in Northeast Asia persists and North Korea continues its nuclear buildup, South Korea and the United States are likely to maintain a close alliance driven by strategic mutual necessity.
As a non-nuclear-weapon state, South Korea inevitably relies on the U.S. nuclear umbrella and extended deterrence to counter North Korea’s nuclear threats. At the same time, as the country pursues the transition of wartime operational control (OPCON), it continues to bolster its defense capabilities, with nuclear-powered submarines emerging as a key pillar of its deterrence architecture. Leveraging their stealth and long-endurance submergence, these submarines will track and neutralize North Korean strategic vessels and, if necessary, provide powerful conventional retaliatory capabilities, thereby reinforcing South Korea’s Three-Axis System. To preclude any risk of proliferation, South Korea maintains a firm stance on utilizing U.S.-supplied LEU fuel rather than pursuing indigenous production or the use of HEU. Furthermore, these assets are not a means of decoupling from the alliance. Instead, they will enhance regional military balance and strategic stability through a more effective division of security responsibilities between South Korea and the United States. Ultimately, the acquisition of nuclear-powered submarines represents a rational security choice for South Korea, a responsible non-nuclear middle power that remains committed to both the ROK-U.S. alliance and the international nuclear nonproliferation regime.
| The Rationale and Roles of Nuclear-Powered Submarines
| Nuclear-Powered Submarines and Nuclear Proliferation Risks
| Concerns over the Strategic Consequences of Acquiring Nuclear-Powered Submarines
1) Nuclear-powered submarines, which use nuclear energy for propulsion, are generally classified into two categories: SSBNs, which primarily carry nuclear missiles and are tasked with strategic retaliatory missions, and SSNs, which do not carry nuclear missiles and instead focus on tracking and attacking adversary submarines. As a non-nuclear-weapon state under the NPT, South Korea can possess only SSNs. However, South Korea’s nuclear-powered submarines are expected not only to perform the traditional role of attack submarines by tracking and engaging enemy submarines, but also, when necessary, to carry a substantial number of powerful conventional missiles, such as the Hyunmoo-4, to conduct retaliatory strike missions against North Korea’s nuclear threats.
2) https://www.mindlenews.com/news/articleView.html?idxno=16302
3) The reference by President Lee Jae-myung to “tracking Chinese submarines” sparked controversy, prompting National Security Adviser Wi Sung-lac to clarify that the remark was not intended to single out any specific country, but rather to underscore the need to address submarine threats in waters adjacent to North Korea and China. He cautioned against further escalation of the controversy. For its part, China’s Ministry of Foreign Affairs limited its response to a general statement, expressing hope that both South Korea and China would faithfully adhere to their nonproliferation obligations.
4) https://www.yna.co.kr/view/AKR20251118010151504?section=search
5) https://www.yna.co.kr/view/AKR20251115001251071
※ The contents published on 'Sejong Focus' are personal opinions of the author and do not represent the official views of Sejong Institue
