Why does the Republic of Korea (ROK) need nuclear-powered submarines? Advocates have long argued for the necessity of adding nuclear-powered subs to Seoul's naval arsenal, and the volume of research on the subject has increased significantly since the APEC Summit in October 2025, when President Lee and Trump agreed to pursue the ROK's acquisition of nuclear-powered submarines. Nevertheless, controversy surrounding the decision has persisted. This policy memo examines the military rationale for the ROK's acquisition of nuclear-powered submarines, focusing specifically on the trade-off between speed and acoustic stealth. In doing so, it addresses skeptical views raised both within South Korea and abroad and assesses whether the operational advantages of nuclear propulsion outweigh its perceived drawbacks.

| A Velocity Profile

Conducting effective anti-submarine warfare (ASW) is the primary rationale behind the Republic of Korea's pursuit of nuclear-powered attack submarines (SSNs). To counter North Korea's emerging nuclear-powered submarine threat, the Republic of Korea Navy requires submarines capable of matching the speed, endurance, and operational reach of their North Korean counterparts. The principal objective of the acquisition program is to detect, track, and, if necessary, neutralize North Korea's future nuclear-powered submarine fleet, particularly its ballistic missile submarines (SSBNs). By providing sustained underwater endurance and high-speed transit capabilities, SSNs would significantly enhance South Korea's ability to monitor and counter North Korea's sea-based nuclear deterrent.

The most important characteristic of a nuclear-powered submarine is its high sustained speed. Although the exact maximum speeds of SSNs operated by different countries remain classified, they are generally estimated to range between 43 and 65 km/h. By comparison, conventional diesel-electric submarines typically have maximum speeds of approximately 37-39 km/h. As a result, the top speed of a nuclear-powered submarine is, on average, about 1.5 times greater than that of a conventional submarine. Moreover, an increase of one knot at high speeds requires significantly more energy than a one-knot increase at low speeds. Nuclear propulsion, therefore, provides substantial operational advantages by enabling sustained high-speed transit, rapid maneuvering during combat, and greater flexibility in evasive actions.

The East Sea covers approximately 978,000 square kilometers, and the maximum distance between the Korean Peninsula and the Japanese archipelago is roughly 1,000 kilometers. At maximum speed, a nuclear-powered submarine could traverse this distance in approximately 15.4 hours, whereas a conventional diesel-electric submarine would require about 25.6 hours. If the South Korean Navy fails to detect a North Korean nuclear-powered submarine during its initial departure from port, reacquiring and tracking it with South Korea's limited fleet of maritime patrol aircraft and rotary-wing anti-submarine assets would be extremely challenging. These airborne assets, albeit effective in ASW, cannot sustain continuous tracking over repeated sorties and extended periods. Submerged submarines are also relatively less exposed to space-based ISR systems. In such a scenario, Seoul would likely have to rely on U.S. and Japanese underwater surveillance networks, including the Sound Surveillance System (SOSUS), and other allied detection capabilities.

Even if a North Korean SSBN were detected, maintaining continuous contact would be difficult due to the speed disparity between nuclear-powered and conventional submarines. Should a North Korean nuclear submarine break through the Japanese archipelago and enter the Pacific Ocean, sustained pursuit by an AIP-equipped conventional submarine would become virtually impossible. Given that the submarine-launched ballistic missile (SLBM) currently under development by North Korea, the Pukguksong-3, is assessed to have a range exceeding 2,000 kilometers,¹⁾ long-range operations by a North Korean SSBN would create the possibility of a direct nuclear threat to South Korea from launch positions east of Japan.

North Korea is unlikely to stop after constructing a single SSBN. Rather, it is expected to pursue a broader naval nuclear propulsion program involving multiple SSNs, SSGNs, and SSBNs. In the early stages, North Korea will likely operate integrated submarine platforms carrying SLBMs, SLCMs, and nuclear torpedoes under a bastion strategy focused on waters near its coastline. As its nuclear-powered naval forces expand, however, North Korea's strategy and operational concepts may become increasingly offensive in nature, raising the possibility that the East Sea could evolve into a theater of high-intensity ASW.

Consequently, if high-intensity ASW, both strategic and tactical, operations were conducted in the East Sea, North Korean nuclear-powered submarines—possessing, at least in theory, greater diving depth and higher sustained speed—could enjoy significant operational advantages over South Korea's conventionally powered submarines. As North Korea's sea-based nuclear capabilities continue to expand, South Korea should prepare more effectively for ASW missions through the acquisition of nuclear-powered submarines, thereby strengthening its ability to protect maritime sovereignty and contribute to the deterrence of North Korea's nuclear threat.

| A Noise Profile

It is true that the relationship between speed and noise in nuclear-powered submarines—that is, the trade-off between velocity and stealth—remains an enduring challenge. During the early stages of U.S. nuclear submarine development, attack submarines were designed primarily with an emphasis on speed. The USS Nautilus, a 3,500-ton submarine equipped with the approximately 70 MW S2W reactor, relied on forced-circulation coolant pumps and generated substantial continuous noise, particularly in the low-frequency portion of the acoustic spectrum. Major sources of noise included the reactor coolant pumps, which had to operate continuously whenever the reactor was providing power, and the reduction gears required to lower the rotational speed of the steam turbine shafts. Mechanical vibrations also increased significantly at higher speeds.

As efforts to reduce acoustic signatures continued, the U.S. Navy sought to counter increasingly quieter Soviet submarines by improving the noise characteristics of newer reactor designs such as the S6G (Submarine, Sixth Generation, General Electric). However, achieving lower acoustic signatures often came at the expense of maneuverability and speed. Enhancing stealth generally requires reducing machinery noise and limiting operational speeds, making some sacrifice in performance unavoidable. The U.S. Navy continued to develop submarines capable of generating substantial thrust for fleet escort missions and anti-surface warfare operations. This led to the development of the third-generation Los Angeles-class attack submarine, equipped with the S6W (Submarine, Sixth Generation, Westinghouse) reactor. Although the design incorporated noise-reduction measures, its emphasis on high-speed performance inevitably generated additional acoustic signatures.

In general, ballistic missile submarines (SSBNs), whose primary mission is to ensure a credible second-strike capability, tend to prioritize stealth over speed to remain undetected. By contrast, attack submarines (SSNs), which are tasked with missions such as tracking adversary submarines, escorting naval forces, and conducting high-intensity combat operations, place greater emphasis on speed and maneuverability. In other words, the trade-off between speed and stealth cannot be resolved by prioritizing only one attribute. Effective submarine design and employment require balancing both considerations according to operational requirements. This tension reflects a long-standing debate within the U.S. submarine community. Throughout the design and construction of nuclear-powered submarines, there has been a persistent division between those who prioritized stealth and acoustic quieting and those who emphasized speed and maneuverability, with each camp advocating different approaches to submarine warfare and operational effectiveness.

Some argue that South Korea's Dosan Ahn Changho (SS-083), Ahn Mu (SS-085), and Shin Chae-ho (SS-086) class submarines, equipped with Air-Independent Propulsion (AIP) systems, are sufficient for coastal defense missions. Compared to nuclear-powered submarines, AIP submarines are generally quieter and less expensive to acquire and operate. Further, replacing the current fuel-cell-based AIP system with lithium-ion batteries, which offer higher power output, faster charging rates, and greater energy density, is expected to significantly improve underwater endurance and propulsion efficiency. Lithium-ion batteries may also reduce acoustic signatures by eliminating or reducing the operation of certain mechanical components, thereby enhancing stealth.

Nuclear-powered submarines generally have larger hulls and greater displacement than diesel-electric submarines, enabling them to carry larger payloads and a greater number of land-attack missiles. South Korea could therefore choose to build larger nuclear-powered submarines and employ them as guided-missile submarines (SSGNs). A useful example is the United States' decision in the 2000s to convert four ballistic missile submarines (SSBNs) into SSGNs. Whereas a standard Virginia-class SSN carries up to 12 Tomahawk cruise missiles (and up to 40 in the Block V configuration), the converted Ohio-class SSGNs—USS Ohio, USS Florida, USS Michigan, and USS Georgia—could carry as many as 154 Tomahawk missiles. At present, the Dosan Ahn Changho-class submarines are equipped with six to ten vertical launch system (VLS) cells and can carry only six to ten Hyunmoo-series ballistic missiles. Their range, generally estimated at around 500 kilometers, may be insufficient to strike strategic targets located deep within North Korean territory. Consequently, as South Korea advances its capabilities and expertise in nuclear submarine construction, it could consider developing an SSGN with a displacement exceeding 8,000 tons and equipped with many long-range land-attack missiles.

For missions involving sea-based retaliatory deterrence, stealth is often more important than speed. In this regard, future developments in submarine propulsion technology are particularly relevant. The U.S. Navy has discussed incorporating a full electric-drive propulsion system into the Columbia-class SSBN, primarily to further reduce acoustic signatures. South Korea is likewise likely to pursue continuous technological improvements aimed at enhancing the stealth of its future nuclear-powered submarines. Over the long term, this could include the development of a nuclear propulsion architecture incorporating electric-drive technologies designed to minimize noise and improve survivability in contested undersea environments.

However, there are circumstances in which operational requirements necessitate sacrificing a degree of stealth in favor of greater speed, diving performance, and firepower. South Korea may face situations in which the need to track and shadow North Korean SSBNs demands prioritizing speed and sustained underwater mobility over maximum stealth. In such scenarios, nuclear-powered submarines would possess significant advantages over conventionally powered submarines. Their ability to sustain high underwater speeds for extended periods without concerns about battery depletion or snorkeling would make them considerably more effective in conducting prolonged tracking and pursuit operations against North Korean strategic submarines. AIP submarines remain subject to inherent operational limitations. Their extended underwater endurance is generally achievable only at relatively low speeds, allowing operations for approximately two to four weeks under favorable conditions. Moreover, once the stored liquid oxygen required for the AIP system is depleted, the submarine must eventually snorkel to run its diesel generators and recharge its batteries.

In addition, although there is little publicly available information regarding the current state of North Korea's sonar technology, it is generally assessed to lag significantly behind that of South Korea. North Korean underwater detection systems are likely to rely primarily on passive sonar rather than sophisticated high-power active sonar systems. In addition, North Korea is believed to face substantial limitations in areas such as artificial intelligence (AI), digital signal processing (DSP), data fusion, and low-frequency acoustic analysis, all of which are increasingly important components of modern anti-submarine warfare.

Moreover, South Korea can optimize the trade-off between speed and stealth through a hybrid submarine force structure, employing AIP-equipped conventional submarines for missions that require maximum stealth and nuclear-powered submarines for missions that demand sustained speed, endurance, and operational reach. Even if South Korea gradually transitions toward a submarine force centered on nuclear-powered submarines, this does not imply abandoning its world-class AIP submarine technology, ceasing future development efforts, or retiring its existing conventional submarines prematurely. The Dosan Ahn Changho-class submarines began entering service in 2021 and are expected to remain operational for approximately 30 years. As a result, they are likely to continue serving into the mid-2050s. These submarines will remain valuable assets for a wide range of missions, particularly those emphasizing stealth, intelligence collection, and coastal defense.

Finally, efforts to reduce the acoustic signatures of nuclear-powered submarines have been ongoing for decades. Over time, navies have increasingly addressed noise issues through measures unrelated to the reactor itself, including the elimination of reduction gears, the application of advanced acoustic treatments to the hull, greater electrification of onboard systems, and improvements in propulsor design. For example, one of the principal arguments surrounding the development of electric-drive propulsion systems in the 1960s concerned their potential to reduce submarine noise. Electric-drive systems offered the possibility of eliminating or reducing reliance on mechanical reduction gears, which had long been a major source of machinery noise. They also had the potential to decrease structure-borne noise transmission, particularly during low-speed operations, thereby improving overall stealth. Although full electric-drive propulsion was not adopted across the U.S. nuclear submarine fleet, elements of electric-drive technology have been incorporated into modern designs. The Virginia-class attack submarine, for example, employs electrical systems in selected propulsion-related functions, reflecting the broader trend toward greater electrification as a means of improving acoustic performance and reducing detectability.

In conclusion, acoustic signature management is a critical consideration in submarine operations. However, stealth is only one of several operational objectives and must often be balanced against other requirements. In certain circumstances, commanders may choose to accept a higher acoustic signature in exchange for greater speed, endurance, mobility, or combat capability. Moreover, various operational concepts and technological measures can help mitigate the risks associated with increased noise. Consequently, the speed-stealth trade-off should not be viewed as an absolute constraint, but rather as a challenge that can be managed through force structure design, operational planning, and the complementary employment of different submarine platforms.

| Criticism 1. An Alternative Through Maritime Alliance with the U.S. and Japan

Should not Seoul rely upon Washington and Tokyo to neutralize North Korea's undersea threats? As of 2026, out of 49 SSNs, the U.S. assigned 27 to the Indo-Pacific Command,²⁾ and it is safe to assume that at least 1-2 SSNs might constantly patrol the East Sea. Japan possesses 24 conventional subs, the largest number among non-nuclear weapons states, and any transit through the La Perouse (Soya), Tsugaru, and Tsushima Straits should be detected by the Japanese Navy. The U.S. and Japan's ISR assets, including their space and airborne systems, would closely monitor North Korea's strategic assets and share key intelligence and information with South Korea. Therefore, a critic may argue that strengthening trilateral security cooperation among Seoul, Washington, and Tokyo is an option more realistic and effective than South Korea's acquisition of nuclear-powered submarines.

Although alliance cooperation is essential to deterrence against North Korea, the ROK cannot and should not abandon its responsibility to mitigate a direct threat posed by the DPRK's SSBNs, given uncertainty around the future security environment rapidly transforming in East Asia. There is no legal measure against DPRK's naval vessels transiting the La Perouse (Soya) Strait, controlled by both Russia and Japan. In the future, to everyone's surprise, Pyongyang may choose a route in the West Sea to launch its submarines for long-range deterrent operations. As its cargo vessel Puyun 6's illegal route in 2025 against international sanctions (Figure 1), North Korea's SSBNs may transit the West Sea close to China's coast until it passes through the Miyako Strait. Alliance cooperation is critical, but Seoul cannot leave to others its primary responsibility to protect its security. Alliance cooperation should complement, not substitute for, Seoul's own defense responsibilities.

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| Criticism 2. North Korea's Bastion Strategy

North Korea would likely employ a bastion strategy, leveraging shore-based and airborne defense assets to protect its submarine operating areas while concentrating on coastal defense missions, thus making a high-speed submarine in South Korea unnecessary. This argument is seemingly rational but weak. As addressed above, North Korea's maritime and deterrence strategy may evolve, as its nuclear-powered assets increase in volume. Against North Korea's bastion strategy, a nuclear-powered submarine is still an effective tool for South Korea. Detecting a South Korean SSN operating at standoff distances and at relatively deep depths using advanced sonar systems would be a difficult task. However, a modern South Korean nuclear-powered attack submarine with its larger hull size than that of conventional submarines, equipped with superior but voluminous sensors, processing capabilities, and sustained underwater endurance, could monitor North Korean submarine activities while maintaining a level of stealth that would make detection by North Korean forces considerably more challenging.

In contrast, in the West Sea, shallow waters, strong tidal currents, extensive mudflats, and numerous rocky outcrops generate significant seabed reverberation and acoustic clutter, creating large blind spots for sonar detection. These environmental characteristics make anti-submarine warfare conducted by submarines particularly challenging. From North Korea's perspective, deploying an SSBN force in the West Sea would also be highly inefficient given these operational constraints. The limited water depth restricts maneuvering space, reduces the survivability advantages normally associated with ballistic missile submarines, and complicates efforts to maintain a secure and concealed deterrent patrol area. As a result, if North Korea adopts a bastion-oriented maritime strategy, it is more likely to deploy conventionally powered missile submarines, such as SSBs, in the West Sea while reserving deeper waters in the East Sea for more capable strategic submarine operations. Consequently, ASW in the West Sea is likely to rely more heavily on maritime patrol aircraft, helicopters, unmanned systems, and surface combatants rather than submarines. Given the challenging acoustic environment, airborne and surface-based ASW assets may offer more effective means of detection, tracking, and engagement than submarine-on-submarine operations in this region. In sum, South Korea's nuclear-powered submarines would remain effective against North Korea's bastion strategy in the East Sea. At the same time, North Korea's bastion strategy in the West Sea would pose challenges to both conventional and nuclear-powered submarines alike.

| Policy Implications

Due to space limitations, this memo has not addressed several important issues related to nuclear-powered submarines, including endurance, strategic considerations (e.g., the trade-off between kill-chain capabilities and strategic stability), and regional security implications. Nor has it examined the political, industrial, and economic rationales for acquiring such capabilities. These topics will be addressed in future memoranda. Instead, by focusing on the relationship between speed and acoustic signature, this memo has argued for the military necessity of acquiring nuclear-powered submarines for South Korea. Specifically, it contends that such platforms would enhance South Korea's ability to deter and defend against North Korea's emerging undersea strategic threats.

The implications for the Republic of Korea Navy and the broader national security community are as follows. First, South Korea should prepare for the possibility of high-intensity ASW in the East Sea. As North Korea's nuclear-powered submarine force is likely to evolve into a more diversified fleet consisting of SSNs, SSBNs, and SSGNs, policymakers and military planners should explain to skeptics the feasibility and necessity of both strategic and tactical ASW operations. In particular, relying excessively on alliance support to address direct threats to South Korea's security may prove undesirable in an increasingly uncertain security environment. Seoul should therefore develop its own operational concepts and contingency plans for tracking, intercepting, and, if necessary, neutralizing North Korean submarines along their likely routes of deployment and breakout from the Korean Peninsula.

Second, South Korea should preserve both SSN and SSGN development options to employ future nuclear-powered submarines for both tactical and strategic missions. In the case of SSGNs in particular, stealth should be prioritized, and research into electric-drive propulsion systems should be pursued as a means of reducing acoustic signatures. In addition, the range of the Hyunmoo-4-4 submarine-launched ballistic missile should be extended to improve its ability to strike strategic targets at greater distances. At the same time, advocates of nuclear-powered submarines should engage critics by acknowledging that, in certain operational scenarios, speed, endurance, and maneuverability may be more important than maximum stealth.

Third, South Korea should develop a long-term operational concept for the hybrid employment of nuclear-powered and conventionally powered submarines through at least 2050, drawing lessons from the experiences of countries such as Russia, China, and France. AIP-equipped conventional submarines should continue to perform missions that place a premium on stealth, including area denial and coastal defense. Nuclear-powered submarines, by contrast, should be employed for more demanding missions requiring sustained speed, endurance, and operational reach, including the interdiction of maritime lines of communication, the surveillance and deterrence of North Korean strategic assets, and special operations missions. Such a hybrid force structure would allow South Korea to maximize the complementary strengths of both submarine types while mitigating their respective limitations.

1. Wikipedia, "Pukguksong-3," https://en.wikipedia.org/wiki/Pukguksong-3, accessed June 18, 2026.
2. Congressional Research Service, "Navy Virginia-Class Submarine Program and AUKUS Submarine (Pillar 1) Project: Background and Issues for Congress," January 26, 2026, https://www.congress.gov/crs-product/RL32418, accessed June 18, 2026; Submarine Force Pacific, "Attack Submarines," https://www.csp.navy.mil/SUBPAC-Commands/Submarines/Attack-Submarines/ accessed June 18, 2026.

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