North Korea’s Nuclear Submarine Program Could Seriously Complicate a Future US War Effort

On October 8 South Korean media outlets cited sources in the country’s armed forces and National Assembly to report that North Korea was believed to have begun construction of its first nuclear-powered submarine. Although further details regarding the ship’s estimated tonnage, role, and other capabilities remain elusive, the program has the potential to very significantly improve Pyongyang’s ability to project naval power away from its shores. While the North Korean submarine fleet has long been the world’s most numerous, it currently relies on some of the world’s smallest, shortest ranged, and most defensively focused ship classes primarily for sea denial operations.

Work toward the development of a nuclear-powered submarine has been reported by North Korean officials and media outlets for several years. At the Congress of the ruling Korean Workers’ Party in January 2021, top leader Kim Jong Un announced that “the design of a new nuclear-powered submarine was researched and was in the stage of final examination,” adding that design work on its weapons and sensors was already complete.

Subsequently on September 6, 2023, state media referred to work on a nuclear-powered submarine at the launch of the refurbished Soviet Romeo-class submarine the Hero Kim Kun Ok – a ship that, once modified, was the first in the North Korean fleet capable of carrying cruise missiles and multiple ballistic missiles.

In January 2024 Kim was reported by state media to have inspected work on the construction of a nuclear-powered submarine, although this may have referred to the construction of subsystems and not necessarily the vessel itself. The supreme leader was at the time reported to have been provided with a detailed understanding of the nuclear submarine construction, indicating that plans for proceeding with the program may have been finalized.

Six countries today field nuclear-powered submarines, all of which are nuclear weapons states. That leaves North Korea, alongside Israel and Pakistan, as one of three nuclear weapons states without such ships. Compared to their diesel-electric counterparts, nuclear-powered submarines have the advantage of being able to remain at sea several times longer, with modern vessels able to stay submerged for months at a time. This gives nuclear-powered subs an effectively unlimited range, allowing them to strike from almost anywhere in the world. They can also cover longer distances at much higher speeds.

To place the discrepancy in endurance in perspective, the Royal Australian Navy’s currently planned nuclear-powered attack submarines, set to be delivered by the United States, will be able to maintain deployments in the South China Sea for 77 days. Its previously planned French diesel electric submarines would only have been able to remain in the region for 11 days.

One of the most significant uncertainties surrounding the North Korean nuclear-powered submarine program remains whether its ships will be strategic ballistic missile submarines designed to deliver multiple megatons of nuclear force against enemy cities, or whether they will be attack submarines able to function in a much wider range of conventional and nuclear roles. The former submarine type would revolutionize the second arm of North Korea’s nuclear dyad, which may soon become a triad should an air launched variant of existing nuclear-tipped cruise missiles be developed. That would make it significantly more difficult for North Korea’s adversaries to neutralize its strategic nuclear forces on the ground.

Such ships could potentially be armed with intermediate range ballistic missiles, possibly with North Korea’s recently demonstrated hypersonic glide vehicle or multiple independent reentry vehicle technologies, to threaten cities across the U.S. mainland as well as Guam. Beyond survivability, a further benefit of submarine-launched arsenals is that they can be used to strike from unexpected directions, including launching an attack against the United States mainland from across its southern border, where missile defense is highly limited.

Although carrying small fractions of the firepower of ballistic missile submarines, a North Korean attack submarine program could potentially be a far more unfavorable development for the United States and its allies for a number of reasons. Ballistic missile submarines have only limited utility beyond deterring, and if necessary waging, a full scale nuclear war. North Korea’s ground-based strategic nuclear missile arsenal is already highly survivable, with missiles being road mobile, deploying from vast networks of underground bases and highways in the country’s complex mountainous terrain, and either using solid fuel composites or “ampulization” fuel sealing processes. It can be argued that provoking Pyongyang to utilize its strategic nuclear weapons is already sufficiently unthinkable that adding a second stage to this deterrent would have a less than revolutionary impact on the country’s strategic position. It would instead be an incremental gain much like the introduction of hypersonic glide vehicles and multiple warhead reentry vehicles have been.

In contrast to a ballistic missile submarine, an attack submarine would provide a fundamentally new capability to North Korea’s armed forces – namely the ability to project naval power across oceans. This includes deployment of both nuclear and conventionally armed cruise missiles, insertion of special forces, and anti-shipping duties using torpedoes and anti-ship cruise missiles. The implications of North Korea attaining such a capability with even a couple of modern attack submarines would be highly significant.

Having attained a strategic nuclear deterrent, North Korea has focused increasingly heavily on developing lower level assets that allow it to manage escalation with its adversaries. This has included the development of much more potent tactical missile and artillery assets, as well as compact low yield tactical nuclear warheads that are far more “usable” than its strategic arsenal – thus providing a means of responding to escalatory steps short of escalating to the stage of a full scale nuclear war. A nuclear-powered attack submarine armed with nuclear and conventional cruise missiles would be one of the most formidable assets in such an arsenal.

Although large and costly, such submarines allow smaller militaries to counter larger, more dominant ones asymmetrically due to their ability to operate in oceans dominated by enemy navies, strike multiple targets without warning at long range, and effectively disappear afterwards in ways that surface ships cannot.

To provide a hypothetical situation highlighting the utility of both submarine types, should a future administration in the United States launch a limited strike on a North Korean industrial target or military base, a ballistic missile submarine’s utility would be limited to reinforcing the top tier of Pyongyang’s escalatory ladder. While North Korea now retains the capability to launch proportional limited retaliatory strikes on targets across East Asia, an attack submarine would allow such strikes to be launched against faraway targets on U.S. soil, including Guam, Wake Island, and the United States mainland.

Such a submarine class would thus represent a major step toward rectifying the discrepancy in the reaches of the two countries’ armed forces – a gap that North Korea closed at the strategic level in 2017 with the fielding of its first intercontinental ballistic missiles (ICBMs) but could now begin to balance at the tactical level. The ability to launch limited cruise missile attacks against military bases and industrial targets on U.S. territory would be entirely new, and could help to deter the United States from considering limited strikes on the country in future.

Regarding the capabilities of North Korea’s future nuclear-powered submarine class, although it is possible that the country could field a small design with limited firepower and capabilities – as India has with the Arihant class – trends in North Korea’s defense sector indicate that the fielding of an internationally competitive, high performing design is likely. North Korea’s defense sector has a history in recent years of making technological leaps to field military equipment that appears fully on par with cutting edge foreign competitors, bridging technological gaps that were previously very wide. The Chonma 2 main battle tank, Pyongae-6 air defense system, and Bulsae-4 anti tank missile system are among the notable examples of systems demonstrating this, often introducing capabilities that the defense sectors of neighboring Russia and other major defense producers have not yet managed. Furthermore, unlike India, North Korea’s nuclear submarine fleet would not be directed against targets in the near abroad with limited anti-submarine warfare and anti-ballistic missile capabilities, but rather are intended for a war across an adversary-dominated ocean. For that purpose, a small and more basic nuclear submarine class would be far less cost effective.

Technology transfers and other support from Russia have been widely speculated to play an important role in supporting the North Korean nuclear submarine program, with Moscow’s increasingly heavy reliance on Pyongyang as an arms supplier and its interest in bolstering its neighbor against the United States making this highly possible. Although the standing of the Russian defense sector has diminished considerably since the end of the Cold War, the perceived asymmetric value of nuclear submarines has resulted in a concentration of investment in the area, allowing Russia to remain a world leader in the field. U.S. military leaders have highlighted that modern Russian nuclear-powered submarines such as the Yasen-M Class are considered on par with the most capable in the U.S. Navy. This makes Russian support potentially highly valuable for the North Korean program, and possibly key to facilitating a high performance.

While the future of the North Korean nuclear submarine fleet remains uncertain, fielding a fleet of two to four ships by the mid-late 2030s is a significant possibility. Another notable possibility is that a single submarine design could be built in separate strategic and attack variants, allowing the fleet to operate in both roles while maintaining commonality, which would mirror the U.S. Navy Ohio-class submarine’s development into two such variants.

North Korea’s defense sector has over the past decade moved from a technological laggard in most areas into a cutting edge player in many. The fielding of nuclear-powered submarines for roles beyond strategic nuclear deterrence potential represents a major landmark in the transformation of its armed forces’ capabilities. Recent advances in thermonuclear warhead miniaturization, long-range cruise missile engines, solid fuel ballistic missile engines, and hypersonic glide vehicles are among the notable areas that could serve to make North Korea’s future nuclear submarines highly potent.