North Korea's Underwater Nuclear Threat: Invisible Missiles Emerge

On October 14, in the East Sea, two types of cruise missiles were launched from North Korea's 3,000-ton Kim Gun-ok Hero submarine, which had been submerged. One missile was fired from a vertical launch system on the vessel, while the other was launched from the bow's torpedo tube. This successful simultaneous launch of two different missiles from a single submarine has raised concerns for South Korea and the United States, as they cannot predict what North Korea might fire next.

When missiles are launched from a submarine, it is difficult to immediately determine whether they are long-range or short-range, nuclear or conventional warheads, leading to confusion in response strategies. The Joint Chiefs of Staff did not publicly disclose this launch in real-time. North Korea's underwater nuclear missiles now pose a threat that is increasingly difficult to anticipate or intercept.

Surface ships are easily detected by satellites and radar, and can be visually identified. However, submarines submerged in the ocean are much harder to locate due to the water's inability to transmit radio waves effectively. Sonar, which detects sounds, is essentially the only means of detection, but its effectiveness varies based on water temperature, salinity, and seabed topography, making detection challenging. This is why military experts refer to submarines as 'the assassins of the sea.'

On the night of March 26, 2010, the South Korean naval patrol ship Cheonan, conducting a surveillance operation near Baengnyeong Island in the West Sea, was split in two and sank after being struck by a torpedo from a North Korean submarine. The 1,200-ton patrol vessel failed to detect the approaching threat from beneath the waves. An enemy hidden underwater is thus difficult to find and can be deadly.

Submarines also control maritime routes. Even a few submerged submarines near an enemy's port or key shipping lanes can prevent transport and oil tankers from moving freely. The mere threat of their presence can restrict an adversary's movements. Since most imports and exports are transported by sea, this can effectively block an opponent's economic lifelines.

Submarines are primarily classified as diesel or nuclear-powered. While both types are submarines, the differences between diesel and nuclear propulsion are significant. Diesel submarines must surface to recharge their batteries and take in air when their fuel runs low, increasing the risk of detection by satellites and radar. Attempting to evade or move quickly can also rapidly deplete their fuel supply, limiting operational time.

In contrast, nuclear submarines can remain submerged for months without worrying about fuel, thanks to their reactors. They can operate for extended periods, limited only by the biological constraints of the crew, and can travel long distances without energy supply issues, even at maximum speed.

Lee Jung-ik, a professor at KAIST's Department of Nuclear and Quantum Engineering, noted in a commentary for the Korea Maritime Strategy Research Institute that nuclear submarines can be built using low-enriched uranium, similar to that used in commercial nuclear power plants. He explained that even with low-enriched uranium, the refueling cycle for nuclear fuel can span several years. Thus, diesel and nuclear submarines differ significantly, akin to a diver who must frequently surface for air versus one equipped with an oxygen tank.

The concern is that North Korea is accelerating its nuclear submarine development. Although North Korea currently lacks nuclear submarines, it has been testing various delivery systems, including submarine-launched ballistic missiles (SLBMs) and submarine-launched cruise missiles (SLCMs), using diesel submarines to enhance its underwater nuclear strike capabilities. SLBMs are missiles that launch high into the air before descending rapidly on a parabolic trajectory, targeting distant enemy cities and critical facilities. SLCMs fly low over the water to evade radar and strike military bases or ports with precision. Both missile types can carry nuclear warheads, and North Korea claims that the cruise missiles launched on October 14 could also be equipped with tactical nuclear warheads.

Having already tested various delivery systems with diesel submarines, North Korea is in the process of constructing a nuclear submarine. Seoul Pyongyang News reported last month, citing a report from satellite imagery analysis firm SI Analytics, that a nuclear submarine is still under construction at the Sinpo shipyard. It is believed that the hull is nearing completion, as there has been little material input in the first half of the year. However, it remains unconfirmed whether the reactor is operational. Observers suggest that even after launching, it will take several years for equipment installation and test operations. The nuclear submarine revealed by North Korea last December is reported to weigh 8,700 tons, nearly three times the size of the 3,000-ton Kim Gun-ok Hero submarine that launched the cruise missiles. North Korea has referred to this submarine as a 'strategic guided missile submarine,' and it is believed to have ten SLBM launch tubes on its sail.

Yoo Yong-won, a member of the National Assembly from the People Power Party, remarked that having ten SLBMs mounted on the sail is an unprecedented occurrence, describing the structure as a 'deformed' design likely intended to accommodate large SLBMs. There are also ongoing suspicions regarding Russian technological support. The U.S. North Korea-focused media outlet 38 North has suggested that the size and specifications of North Korea's nuclear submarine closely resemble those of Russia's Akula-class submarines, raising the possibility that Russia may have provided reactor technology or key components.

If North Korea completes and deploys its nuclear submarine, the dynamics in the East Sea will change significantly. If it were to hide deep in the Pacific with SLBMs targeting the U.S. mainland, it would be much more challenging for South Korea and the U.S. to detect and track it. This raises concerns that South Korea's underwater dominance could be undermined.

Unlike North Korea, which is on the verge of launching its nuclear submarine, South Korea is still in the preparatory and negotiation stages for its nuclear submarine program. According to Voice of America (VOA), in a joint fact sheet released in November last year, the U.S. approved South Korea's nuclear submarine construction. However, discussions on how to procure fuel are still pending. In May, the Navy established a temporary organization called the 'Nuclear Submarine Propulsion Team,' and during the September summit between the U.S. and South Korea, they agreed to deepen cooperation on nuclear submarine fuel, enrichment, and reprocessing, but no specific progress has been announced. Under the U.S.-South Korea nuclear agreement, nuclear materials received from the U.S. can only be used for peaceful purposes, necessitating separate agreements.

Even after negotiations conclude, technological barriers remain. Nuclear submarine reactors are considered one of the most complex areas of nuclear technology. Professor Lee Jung-ik explained that in terms of technical difficulty, nuclear weapons are the easiest, followed by commercial nuclear power plants, with nuclear submarine reactors being the most challenging.

Nuclear power plants operate at relatively constant output over large areas, while nuclear submarine reactors must withstand rapid changes in output while protecting crew members from radiation exposure within a confined hull. They must also remain operational during combat and in harsh marine environments. Additionally, reducing noise is crucial, as nuclear submarines have more machinery operating at low speeds than diesel submarines.

The underwater competition is already in motion. North Korea is using South Korea's nuclear submarine development as justification for its own nuclear submarine program, while South Korea is requesting U.S. cooperation citing North Korea's nuclear submarine threat. The issue is the speed of development. North Korea is expected to launch its nuclear submarine as early as next year, while South Korea's goal for its first nuclear submarine launch is set for the mid-2030s. In the meantime, North Korea has demonstrated its capability to launch missiles from underwater and is accelerating its nuclear submarine construction, indicating that underwater nuclear threats are no longer a distant concern. Time is running out to counter the invisible threats emerging from beneath the waves.

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