South Korea's quiet march toward a nuclear-capable triad

A nuclear triad consists of land, sea, and air delivery systems for nuclear weapons. These include SLBM missiles carried by submarines, ballistic missiles stationed on land in silos or on mobile launchers, and aerial weapons in the form of bombs or cruise missiles carried by aircraft.

Only some nuclear powers, such as the United States, Russia, China, and India, possess a full triad. Some countries—like France or the United Kingdom—intentionally limit their nuclear arsenals. Others—like Pakistan or North Korea—are still attempting to build a complete range of nuclear weapons types.

A country that is consistently moving in this direction is South Korea. Although Seoul lacks its own nuclear weapons, it is developing delivery systems that—once equipped with nuclear warheads—will provide it with access to a full nuclear triad.

In August 2021, the first KSS-III class submarine (also known, due to the first ship, as Dosan Ahn Changho) entered service in the South Korean navy. It is an 84-metre unit displacing about 4,100 tonnes underwater. Alongside diesel propulsion, the submarine also has fuel cells, allowing it to operate for 20 days without access to atmospheric air.

The KSS-III type was initially designed as a carrier of SLBM missiles. The first batch of this type of submarine includes—besides torpedo launchers—six vertical launch pads located behind the sail. Currently, they house Hyunmoo 4-4 ballistic missiles with a range of about 500 kilometres. The second batch of slightly enlarged KSS-III submarines will have 10 launchers, and besides ballistic missiles, they will also be armed with cruise missiles.

South Korea plans to build 9 KSS-III type submarines. Seoul's plans also include developing another type of submarine, specifically designed to carry ballistic missiles. One of the options considered for this variant is nuclear propulsion.

Although South Korea's achievements in developing ballistic missiles are more modest than those of its northern neighbour, Seoul also possesses ballistic missiles with ranges increasing with each new development version of this weapon.

In the 1980s, the Hyunmoo-1 ballistic missile with a range of 180 kilometres (limited due to US pressure) entered service. Since 2008, the Hyunmoo-2 missile family has been developed, with different variants having ranges from 300 to 800 kilometres.

Various variants of Hyunmoo-3 missiles, carrying relatively light, 500-kilogram warheads, have ranges from 500 to 1,500 kilometres.

The Hyunmoo-4 missiles tested since 2020 have a lesser range of 800 kilometres but carry very heavy warheads weighing about 2,000 kilograms. They feature extremely high accuracy, with a circular error probable of 1-2 metres. These missiles are intended to destroy North Korean bunkers using conventional warheads.

In October 2024, South Korea unveiled another missile—Hyunmoo-5, capable of carrying a 7,300-kilogram warhead over a distance of 3,000 kilometres.

A range of 800 kilometres is enough to destroy any target within North Korea; hence, South Korea's ability to target increasingly distant objectives meets criticism, particularly from Tokyo and Beijing.

South Korea has aerial cruise missiles, but they are currently imported Taurus missiles. Seoul ultimately wants to have its own weapon of this type, as evidenced by the Cheonryong (Korean: heavenly dragon) missile displayed in September 2021.

Although during the disclosed test Cheonryong was carried by an old F-4 aircraft, the missile with a range of 1,000 kilometres was developed as part of the domestic aerial arsenal intended for the future KF-21 Boramae aircraft, significantly enhancing its strike capabilities.

Another long-range aerial weapon was unveiled by South Korea in October 2024 during the 2024 Sacheon Airshow. Among the displayed weapons was a mockup of an aerial missile currently under development with a ramjet engine, intended for KF-21 and FA-50 aircraft. The missile is supposed to move at a speed of Mach 3.5 and offer a range of 300 kilometres.

In the 1970s, South Korea conducted efforts aimed at acquiring nuclear weapons. These efforts were halted due to international pressure, mainly thanks to US intervention, and in 1975, Seoul ratified the Treaty on the Non-Proliferation of Nuclear Weapons.

Regardless—what was revealed years later—South Korean researchers successfully conducted experimental enrichment of small amounts of uranium to levels allowing for military applications, as part of a secret experiment.

Washington's pressure also limited the development of South Korean ballistic missiles. Until the mid-2010s, they could carry warheads weighing up to 500 kilograms over a distance of up to 800 kilometres. The lifting of these restrictions in 2017 resulted in the appearance—within just a few years—of a family of increasingly heavier ballistic missiles with increasing ranges.

The price for Seoul's abandonment of work on a nuclear arsenal wasn't only American guarantees connected with security, but also the stationing of American nuclear weapons on South Korean territory (at the turn of the 1980s and 1990s, there were even 950 bombs), which, however, were withdrawn from Korea.

In 2023, during a meeting with the heads of the defence and foreign affairs departments, President Yoon Suk announced that—if the threat from North Korea continues to grow—South Korea might consider inviting the US to redeploy nuclear warheads on its territory or developing its own nuclear weapons. An agreement made in the same year between Seoul and Washington permits the stationing of American submarines with nuclear weapons on board in Korea.

According to research conducted in 2023, over 70% of South Korean residents support the development of their own nuclear weapons. Given that South Korea is a world leader in nuclear energy, boasting extensive industrial and research facilities, it is not a lack of capability that restrains it from building a nuclear arsenal, but the political consequences of such a move.