Seoul’s Nuclear Submarine Breakthrough

US President Donald Trump ended the first Asia trip of his second term with a splash: an announcement via Truth Social that he had given South Korea approval to acquire nuclear submarine capability. Trump indicated that the submarines would be constructed in Philadelphia at the South-Korea-owned Hanwha Philly Shipyard, a commercial facility not designed to handle nuclear material or to construct sensitive military vessels.

The announcement and subsequent official comments from South Korean government and military leaders suggested limited coordination—and some confusion—around the announcement. Initial South Korean official reactions displayed uncertainty about the scope, timeframe, and location of the program. Navy Chief of Staff Kim Tae-hoon acknowledged to the South Korean National Assembly Defense Committee on 30 October that, “So far, we are still at the level of political declarations.”

While South Korea will be delighted by the long-sought US green light, it remains unclear how much engagement critical components of the US system, especially the Naval Nuclear Propulsion Program (the “Nuclear Navy”) and Department of Energy, had with the proposal before Trump’s announcement.

A Long Time Coming

South Korea has long desired nuclear-powered submarines as a means to elevate its sovereign deterrence capabilities and enable genuine blue-water naval operations. Former President Moon Jae-in reportedly asked Trump in 2017 if South Korea could purchase a nuclear submarine; Trump was apparently enthusiastic initially, but the proposal failed to gain traction.

Some in Seoul’s national security community were perturbed by the US decision in 2021 to share nuclear submarine technology with Australia under the AUKUS (Australia-UK-US) partnership. One Australian commentator has describedthis as “AUKUS envy”.

Seoul’s anxiety will only have risen with North Korea’s unveiling earlier this year of a developmental, nuclear-powered, and likely nuclear-armed, submarine. While analysts have suggested that Pyongyang is still years away from an operational nuclear submarine capability, such vessels would outstrip South Korea’s own conventionally-powered submarines and add an important dimension to North Korea’s capacity for nuclear coercion.

It was therefore unsurprising that incoming Foreign Minister Cho Hyun foreshadowed at his confirmation hearing in July that South Korea would seek US cooperation to develop or acquire nuclear submarines.

The Final Piece of the Puzzle?

South Korea has undoubted shipbuilding and nuclear capabilities, but the biggest barriers to its nuclear submarine aspirations have always been around reactor technology and fuel.

Speaking to the National Assembly Defense Committee on October 30, Defense Minister Ahn Kyu-back sought to place Trump’s announcement in the context of longstanding planning: “Rather than calling it ‘approval’, it’s more accurate to say that we had already prepared the conditions to build nuclear-powered submarines, and that fuel was the final piece.” He also noted, “We’ve made substantial advances in small modular reactors and we’re now at a significant level of development. Testing must occur on land before we can go underwater, and we’ve already completed much of that process.”

Australia’s AUKUS plans leapfrog these constraints by first procuring three to five US-made, Virginia-class submarines in the 2030s and then constructing a new UK-Australian vessel using largely established US-UK reactor technology. These reactors will use US-sourced highly enriched nuclear fuel that does not require replenishment during the operational life of each submarine.

South Korean Chief of Naval Operations Admiral Kang Dong-il has indicated that the South Korean submarines will use low-enriched uranium (20% or lower) fuel to power vessels of at least 5,000 tons displacement, making it as large or larger than the French Suffren-class nuclear submarine but likely smaller than the United Kingdom’s 7,400-ton Astute-class vessel.

Defense Minister Ahn has stated that the total number of submarines would need to be settled with the South Korean Navy, but he expected that at least four would be necessary. That number seems small given likely operational and maintenance requirements, the potential scale of North Korean (and Chinese) naval challenges, and the need for refuelling every seven to ten years.

Not So Fast

South Korean defense leaders have proposed a timeline for the program that seems optimistic. Even accounting for the country’s advanced shipbuilding capabilities and professed progress in reactor development, as well as the possibility that project planning is much more extensive than South Korean officials have disclosed publicly, this will be an enormously complex undertaking.

Defense Acquisition Program Administration Commissioner Seok Jong-geon told the National Assembly Defense Committee that, “Looking at advanced countries’ cases, it generally takes about ten years to build a nuclear-powered submarine.… If we combine our capabilities, it could be somewhat shortened.” Admiral Kang agreed with the ten-year timetable but acknowledged that the start date for the program had not been decided and that further bilateral consultations would be needed on whether the submarines would be constructed in the United States. Offshore construction in Philadelphia would obviously help sustain support within the Trump administration but will inevitably increase project management challenges.  The views of nearby communities in South Philadelphia and New Jersey about having a foreign nuclear shipbuilding project close to their neighborhoods will also be pertinent.

South Korean Democratic Party National Assembly member (and former first Deputy Director of the National Intelligence Service) Park Sun-won has suggested that the South Korean government had previously sought US approval for its enrichment and fuel-rod plans, presumably under the existing US-South Korea civil nuclear cooperation agreement (US Atomic Energy Act S123 agreement). However, Washington responded by proposing joint construction.

No matter how the project implementation plays out, it will clearly be a collaborative undertaking between the United States and South Korea. That will be a blessing for Seoul in terms of access to US expertise and experience, but it will also carry heavy US expectations. In particular: the Nuclear Navy and Department of Energy will insist on the highest technical, security, and safety stewardship and operational standards; additional treaty and US legislative action will likely be required to establish authorizations and frameworks for cooperation; construction, basing, and maintenance infrastructure will need to be developed jointly; and workforce and naval training arrangements will need to be mobilized and meet US expectations. None of this will be quickly resolved and all will come at immense cost.

AUKUS Versus SKUS

In reacting to Trump’s announcement, Australian Prime Minister Anthony Albanese expressed confidence that the US administration could deliver on both programs and reiterated Trump’s surprisingly unqualified endorsement of AUKUS during Albanese’s recent visit to the White House. Australian commentators have also noted that the two projects will run on separate tracks and that—even though the South Korean project might raise some complications for AUKUS, such as potential dilution or distraction of US inputs—this should not put AUKUS at risk. South Korean investment in the US naval industrial base could also ultimately benefit AUKUS.

Others are not so sanguine: Former US Secretary of Defense Lloyd Austin cautioned that the burden of Washington’s commitments under AUKUS would make it “very, very difficult” for the United States to support South Korea “on top of what we do right now”. Such concerns should be taken seriously: The US naval nuclear enterprise had plenty on its plate before AUKUS and will now have another nuclear propulsion novice to guide through to maturity under an inevitably long-term collaborative arrangement. Many in that enterprise are likely to greet this with resignation rather than enthusiasm.

Nonproliferation Dimensions

Nuclear nonproliferation advocates have long feared that the development of naval nuclear propulsion by non-nuclear states could represent a gateway to future proliferation.

In acknowledgement of this, AUKUS partners have accepted detailed and protracted engagement with the International Atomic Energy Agency (IAEA) to manage precedent risks and establish a robust nuclear material arrangement in line with Australia’s Safeguards Agreement with the IAEA and its obligations under the Non-Proliferation Treaty (NPT). These negotiations have attracted particularly close international attention as the AUKUS arrangement will see the transfer of ostensibly weapons-grade uranium from a nuclear-weapons state. China, clearly worried by AUKUS’s strategic implications and precedents, continues to push hard to influence and disrupt this process.

South Korea can expect similar scrutiny and pushback, especially if its nuclear fuel is US-sourced, even if derived from low-enriched uranium. It will, nonetheless, benefit significantly from the pathway being developed through IAEA-AUKUS engagement.

One notable complication for Seoul is likely to be its need—even greater than Australia’s—to convince the international community of its nonproliferation bona fides. While successive South Korean governments have reaffirmed ongoing commitments to the NPT, public and political debate on the desirability of acquiring nuclear weapons continues in the country and is more prominent than in any other US ally.

Public support in South Korea for a sovereign nuclear deterrent has been increasing as North Korea’s nuclear arsenal grows, developments in Ukraine raise alarm, and concerns about the reliability of US security assurances intensify. Former presidential and defense policy advisor and current Director of the Center for Korean Peninsula Strategy at the Sejong Institute Cheong Seong-chan has argued that nuclear weapons would allow South Korea greater agency in its security policies and reduce its exposure to fluctuations in US policy. Cheong has said, “In a world where the United States no longer wants to act as the global police, each country needs its own means of ensuring security."

Strategic Dualities

For Seoul, Canberra, and Washington, collaboration on acquisition of nuclear-powered submarines offers both strategic multipliers and dilemmas. If successful, the result will be allies more closely bound together in critical defense technology and industry. This in turn will yield both enormous programmatic benefits as well as fears of dependency and obligation. Australia and South Korea will be able to take part in more potent joint operations, and to take greater responsibility for regional security—and they can expect that the United States will want them to do so.

But just as these projects enable the greater levels of self-reliance that will be welcomed in Washington, they reflect an underlying desire on the part of both Seoul and Canberra for greater autonomy and agency within their US alliances in response to changing Indo-Pacific power dynamics.

As all these dimensions play out, the significance of these projects will inevitably transcend the capabilities themselves and help reshape the broader directions of the respective countries’ alliances.