From Diesel to Nuclear: South Korea’s Pursuit of Nuclear-Powered Submarines

Can South Korea change the stakes in the Asia-Pacific by harnessing the power of a nuclear-fuelled navy?

Following his tour to Asia in late October, President Donald Trump announced that the United States would assist South Korea in developing nuclear-powered submarines (SSN), would transfer US propulsion technology and would provide access to the required nuclear fuel for the reactors (Thomas, 2025).

In procuring SSNs, South Korea is set to boost their defence posture and will help offset recent security developments in the region.

Strategic and Security Environment

North Korea (DPRK) is naturally the primary concern for Seoul, and with the recent warming of relations between Pyongyang and Moscow, its capabilities have only been enhanced. Moscow has reportedly aided shipbuilding efforts (WSJ, 2025), transferred submarine technology (Reuters, 2025) and has helped boost missile and satellite capabilities (Chil, 2025). These developments compel Seoul to enhance its own defence posture in response, and acquiring SSNs would be one such enhancement.
Regarding China, Seoul sees a populous and wealthy nation across the Yellow Sea, with global ambitions and a modernizing military that could reset the regional and global order in their favour. Seoul will have to adapt to the ongoing geopolitical competition in the Indo-Pacific while maintaining its own robust defence posture to deter threats or attempts at coercion (MFA, 2023).

With regards to sea lines of communications (SLOC), South Korea imports 84-85% of their total energy use, primarily fossil fuels from Gulf Countries (Suzuki, 2024), and 54-70% of their food, primarily from the United States (Read, 2024). Maintaining open SLOC is a priority for Seoul and SSN patrols would help deter adversaries attempting to disturb these trade flows (MFA, 2023).

Developing and Expanding Capabilities

The current submarine fleet of South Korea consists of old KSS-I and KSS-II submarines with new KSS-III submarines slowly coming online. The KSS-III is a significant advancement from previous submarines, with a VLS for conventional ballistic missiles (Hyunmoo 4-4) and cruise missiles (Hyunmoo-3/ Haeseong-III) as well as a new PEM AIP module with large batteries which allow for weeks of low-speed submerged patrols.

However, the KSS-III remains a diesel-electric submarine (SSK), meaning that its range and the time between snorkels is limited. SSNs on the other hand, have virtually unlimited range—reactors have lifetimes of 10-30 years—and they can remain submerged for months compared to the days and weeks of SSKs.

Countering Threats and Enhancing Posture

South Korea acquiring SSNs would significantly boost their defence posture and would help address some of their security concerns.
Most importantly, SSNs would offer Seoul a reliable conventional second-strike capability. SSNs have much larger patrol boxes than the existing SSKs. SSNs would allow for larger patrol areas, beyond Japan into the wider Eastern and Southern Pacific and the Philippine Sea, thereby decreasing the ability of adversaries to locate, track and potentially target ROKN [Republic of Korea Navy] submarines.

For example, should South Korean missile launchers be eliminated or should its air force be grounded or destroyed, undetected SSNs armed with conventional SLBMs or cruise missiles, patrolling East of Japan would provide decision makers with a response.
Additionally, the ability to remain submerged for longer periods of time, patrolling a wider region would also serve as a deterrent to those who would attempt to disrupt trade flows to and from South Korea.

Finally, SSNs can also be used to locate and track North Korean and Chinese submarines in the region. The ability to remain submerged for months, with an unlimited range and very small acoustic signature would allow for a more effective way for South Korea to track these submarines. Additionally, as South Korean President Lee Jae Myung pointed out, this would also reduce the burden on the US navy in the region (Mcleary & Kine, 2025).

A more Sophisticated Defence Industry

South Korea already has an advanced defence industry and a sophisticated shipbuilding sector that exports submarines, frigates and LPDs (NNS, 2025a). More recently, Canada appears likely to procure KSS-III to replace its ageing submarine fleet (NNS, 2025b).

Designing, building and launching SSNs would not only raise Seoul’s prestige on the international stage, but it would also boost its shipbuilding industry and nuclear industry. While the details of how and where they would be built remain unclear (Singh, 2025), Korean companies will be deeply involved and have been preparing for this for years (Yeo, 2025). Additionally, this would allow for HD Hyundai Heavy Industries or Hanwha Ocean to potentially export these types of submarines, although this will be subject to US approval.

Consequences and Takeaways

DPRK

For North Korea, South Korean SSNs would mean that their submarines would be easier to track and target, thereby reducing their survivability (Ryall, 2025). The better endurance of these SSNs would make it difficult for North Korea to do the same, this is further exacerbated by North Korea’s weak ASW capabilities. Additionally, equipped with SLBMs and cruise missiles, these submarines provide Seoul with a conventional second-strike capability.

China

For China, South Korean SSNs would add to maritime ISR [Intelligence, Surveillance, and Reconnaissance] pressure on PLAN [People’s Liberation Army Navy] assets in the Yellow Sea and Eastern China Sea, making their naval assets easier to track. While not a serious or direct threat to China, South Korean SSNs would also further complicate the situation for the PLAN as it would allow for the USN [United States Navy] to focus its assets on other areas like the South China Sea or Taiwan.

US, Japan and Regional Allies

For the US, Japan and other regional South Korean allies, SSNs would enhance the capabilities of the ROKN and this would allow for greater burden sharing. This could include taskings for protecting regional SLOC or tracking submarines. JMSDF [Japan Maritime Self-Defense Force] and USN assets could be redeployed to other regions and for other taskings. Recently, Defence Secretary Hegseth pointed out the need for “flexibility” to meet regional threats (Lee, 2025).

Bibliography

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• Lee, J. (2025) “Hegseth says US-South Korea alliance focused on North Korea but ‘flexibility’ needed.” Reuters. Published 4th November 2025. Available at: https://www.reuters.com/business/aerospace-defense/hegseth-says-us-south-korea-alliance-focused-north-korea-flexibility-needed-2025-11-04/
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