South Korea’s SSN debate: toward an East-Asian AUKUS
South-Korea’s SSN Debate: toward an East Asian Aukus
WRITTEN BY SEUNGHWAN (SHANE) KIM AND JUN SUN YOO
21 January 2026
At the 2025 APEC Summit in Gyeongju, South Korea and the United States reached a landmark agreement authorising Seoul to acquire nuclear-powered submarines (SSNs) with US support — marking the first time Washington has extended such technology beyond the UK and Australia. Two weeks later, a jointly issued US–ROK fact sheet formally codified this approval, including Washington’s support for expanding Seoul’s authority over uranium enrichment and spent-fuel reprocessing, enabling South Korea to maintain its autonomous SSN fuel-cycle and reduce its heavy reliance on foreign uranium supplies.
Seoul’s strategic interest in SSNs stems from its long-standing efforts to deter North Korea’s rapidly improving second-strike capabilities, bolster its maritime denial posture, and secure persistent underwater surveillance beyond the capabilities of conventional diesel submarines. For Washington, extending SSN cooperation reflects a convergence of its strategic incentives: shoring up allied deterrence amid growing regional nuclear risks, emboldening greater allied burden-sharing, and reinforcing restraint against Chinese maritime incursions. However, South Korea faces a strategic dilemma: it presents significant military-strategic opportunities but also introduces challenges of binding with the international non-proliferation regime, overcoming US legal constraints, and addressing regional backlash.
Facing a myriad of challenges
Prior to the deployment of its SSN programme, Seoul confronts three diplomatic challenges within the US-ROK framework. First, the Non-Proliferation Treaty (NPT) legally constrains any military use of nuclear materials by non-nuclear-weapon states, requiring Seoul to maintain full transparency with the International Atomic Energy Agency (IAEA). Given South Korea’s advanced civilian nuclear capabilities but longstanding domestic debates and alliance sensitivities surrounding its nuclear ambitions, securing international trust and avoiding regional misperceptions is key as it seeks greater fuel-cycle autonomy.
Second, the US–ROK Civil Nuclear Agreement (123 Agreement) governs bilateral nuclear cooperation, and any deviation from its terms carries proliferation and alliance consequences such as termination of US-ROK nuclear cooperation or heightened scrutiny on South Korea. One of the major prerequisites listed in this agreement that could hinder South Korea’s SSN development is the prohibition on uranium enrichment and reprocessing without prior US consent. Although US President Donald Trump has given the green light to South Korea’s plan to build SSNs, any revision of this agreement would still require congressional approval, which remains difficult given the US Congress’ strict non-proliferation stance.
Washington increasingly expects allies not only to share regional defence burdens but also to expand industrial capacity, co-invest in critical technologies, and assume greater responsibility for deterring China. An East Asian AUKUS framework could align with these expectations by signalling Seoul’s commitment to Indo-Pacific security while adding industrial and operational depth to alliance deterrence.
Third, there is a clear divergence between Seoul and Washington over where the SSNs should be built. Trump stated in an earlier Truth Social post that South Korea’s SSNs would be constructed at the Philly Shipyard in the US. However, senior South Korean officials have expressed objections, explaining that the Philly facility lacks the technical capabilities to build SSNs and that Seoul’s goal is to construct them at domestic shipyards instead.
The recent US-ROK agreement has prompted regional reactions, with Japanese Prime Minister Sanae Takaichi stating that Japan would “not rule out all options” regarding SSNs, following similar earlier remarks by Defence Minister Shinjiro Koizumi. Meanwhile, following Trump’s announcement, China urged both Seoul and Washington to “earnestly fulfil their non-proliferation obligations and take actions that contribute to regional peace and stability”. These statements underscore how Japan’s interest in nuclear-naval options can be an example of a nuclear domino effect, while prompting China to frame it as destabilising for regional security. Thus, balancing deterrence credibility with regional stability will be the ultimate diplomatic test for Seoul.
Introducing an “East-Asian AUKUS”
To tackle these challenges, South Korea could position its SSN initiative as a cornerstone for building an “East-Asian AUKUS” — a hypothetical US-ROK-Japan trilateral framework modelled on AUKUS. An East Asian AUKUS framework provides a prescriptive roadmap for closer regional defence cooperation in the Indo-Pacific, with a particular emphasis on operational integration, collective deterrence, and maritime industrial cooperation.
Within the AUKUS framework, Pillar I centres on the trilateral transfer of nuclear-propulsion capabilities to Australia and Pillar II focuses on broader defence-technology cooperation. Within the hypothetical East-Asian AUKUS, Pillar I could instead focus on the US–ROK bilateral cooperation in SSN shipbuilding and nuclear material transfer, while Pillar II could centre on the US–ROK–Japan trilateral SSN interoperability, undersea information sharing, and joint undersea operations.
Within Pillar I, selecting a shipyard for South Korea’s SSNs could be framed as part of Seoul’s efforts to support this architecture. For instance, building US SSNs at the Philly Shipyard and ROK SSNs in South Korea would strengthen both countries’ submarine production capacity. Through continuous investment in submarine shipbuilding under the MASGA (Make American Shipbuilding Great Again) project — a large-scale US–South Korea shipbuilding and naval-industrial cooperation initiative — the US could rejuvenate its impoverished submarine shipbuilding infrastructure, while South Korea could eventually acquire SSN-building know-how from the US, which has more experience with SSNs. This synergy in Pillar I could demonstrate Seoul’s responsibility as an SSN operator, help shape an East Asian AUKUS, and contribute to regional security.
In Pillar II, Japan could contribute significantly through its world-class anti-submarine warfare (ASW) force and extensive chokepoint surveillance along the First Island Chain, thereby strengthening SSN operations. However, given constitutional constraints and the Japanese public’s concerns about nuclear power, Japan is reluctant to bring dual-use nuclear technology into its territory. Thus, Seoul could invite Tokyo to join a limited framework under Pillar II, balancing Japan’s domestic reluctance with the Takaichi administration’s emerging interest in SSNs, thereby strengthening trilateral ties, bolstering Seoul’s pragmatic foreign policy, and reinforcing collective security.
Simultaneously, it is crucial that South Korea portrays its SSN ambitions as fully consistent with non-proliferation norms to mitigate longstanding US concerns about potential precedent-setting for both nuclear and non-nuclear states. As a reassurance measure for Washington, Seoul could consider establishing a dual-track fuel and transparency framework modelled on AUKUS. The IAEA’s recent confirmation that Australia may invoke Article 14 of its Comprehensive Safeguards Agreement (CSA) for non-explosive military uses, including naval nuclear propulsion, establishes a precedent for other non-nuclear weapon states such as South Korea — demonstrating that nuclear propulsion ambitions do not automatically conflict with non-proliferation obligations. By situating its SSN ambitions within a high-assay low-enriched uranium (HALEU)-based framework, South Korea could align its programme with fuel types viewed as less proliferation-sensitive, helping to address long-standing US concerns about expanded enrichment capabilities.
Procedurally, South Korea could institutionalise SSN cooperation through existing bilateral mechanisms such as the Nuclear Consultative Group (NCG) or the Extended Deterrence Strategy and Consultation Group (EDSCG), expanding the remit to include nuclear propulsion and ensuring coherence across strategy, defence, and diplomatic channels. It would provide the alliance with a stable mechanism to manage sensitive discussions, reduce uncertainty from political shifts — given that the SSN project is expected to take more than ten years — and embed SSN issues within a broader alliance strategy rather than an ad hoc initiative.
Fortifying legal and diplomatic layers
From a legal standpoint, any pathway toward SSN cooperation rests on the US’ limited but real flexibility within its domestic nuclear governance framework. The sharing of Restricted Data (RD) and cooperation on uranium enrichment and reprocessing would normally require amendments to the US–ROK 123 Agreement and congressional approval. However, US law allows for narrowly tailored exceptions when strict compliance would undermine common defence and security objectives. This creates space for cooperation if the White House judges that Korean SSNs would strengthen alliance deterrence rather than exacerbate proliferation risks. By positioning its SSN programme as reinforcing US deterrence against China and North Korea, rather than signalling strategic autonomy, South Korea can supply the legal and political rationale Washington needs to exercise this flexibility — as exemplified in Section 91 and Section 144.c.(2) of the AEA.
On the regional front, Seoul should aim to reassure neighbouring states — especially China, which has shown reluctance toward South Korea’s recent moves. This reluctance signals Beijing’s concern over the expanding presence of an US ally, which could constrain China’s ability to project and consolidate influence in the region. In response, Seoul can emphasise that its SSNs will only be used for surveillance and defensive purposes, framing its SSN ambitions as a response to its proximity to multiple nuclear-armed states — notably North Korea’s growing nuclear arsenal — in a deteriorating regional security environment. South Korea, nevertheless, would have to balance these reassurance efforts against the risks of provoking Chinese countermeasures or inflaming lingering historical sensitivities — such as unresolved colonial-era disputes with Japan — which Beijing could strategically exploit to rekindle domestic opposition against Japan and undermine trilateral coordination.
Moreover, by coordinating operational roles with Japan and keeping activities within clearly defined channels, the framework could help reassure regional states of its non-escalatory purposes through transparency. Over time, an East Asian AUKUS model, combined with strategic communication with China, could demonstrate transparency and restraint toward both Japan and China while strengthening deterrence across the region.
The emerging debate over South Korea’s SSN operations reflects broader shifts outlined in the 2025 US National Security Strategy (NSS). Washington increasingly expects allies not only to share regional defence burdens but also to expand industrial capacity, co-invest in critical technologies, and assume greater responsibility for deterring China. An East Asian AUKUS framework could align with these expectations by signalling Seoul’s commitment to Indo-Pacific security while adding industrial and operational depth to alliance deterrence. In this sense, South Korea’s initiative serves as a test case for how like-minded states can build a sustainable regional security architecture amid growing US demands for tangible burden-sharing and strategic alignment.
DISCLAIMER: All views expressed are those of the writer and do not necessarily represent those of the 9DASHLINE.com platform.
Author biographies
SeungHwan (Shane) Kim is currently a nonresident fellow at the Indo-Pacific Studies Center. He was also an Emerging Leader in Diplomacy and National Security at the ROK National Diplomatic Academy. His works were published in numerous publications, including the National Interest, the Diplomat, the East Asia Forum, the Asia Times, the Pacific Forum, and more. The views and opinions expressed in this piece are those of the speakers and do not necessarily reflect the views or positions of any entities.
Jun Sun Yoo is currently a graduate student at the Johns Hopkins School of Advanced International Studies (SAIS), focusing on economic security and Indo-Pacific strategic dynamics. He previously served as a squad leader in the Republic of Korea Army’s 17th Mechanized Infantry Division. The views and opinions expressed in this piece are those of the author and do not necessarily reflect the views or positions of any entities. Image credit: US Navy/Wikimedia Commons (cropped).