Update on South Korean progress on their SSN plans.
ISW:
https://understandingwar.org/researc...-april-7-2026/
South Korea is pre-emptively addressing potential nonproliferation concerns stemming from its plans to build a nuclear-powered attack submarine (SSN).
South Korea’s Ministry of Foreign Affairs (MOFA) plans to invite International Atomic Energy Agency (IAEA) Director General Rafael Grossi to Seoul to discuss the IAEA verification procedures required for the construction of SSNs.
US President Donald Trump approved South Korea’s plan to build SSNs during his October 29 summit with South Korean President Lee Jae Myung. A November 13 US-South Korea joint factsheet indicated that Washington would work with Seoul to achieve construction requirements, including exploring “avenues to source fuel.”
SSNs use nuclear reactors fueled by enriched uranium, enabling extended endurance without refueling for 30 to 40 years. South Korea’s diesel-electric submarines must surface to charge their batteries every 20 days, however.
US and UK SSNs use highly enriched uranium (HEU), and Australia is acquiring United States-designed HEU-powered SSNs under AUKUS. HEU of over 60 percent concentration is used to make nuclear weapons. Australia’s program has therefore involved IAEA safeguards.
South Korea has not yet determined the source of its fuel supply or whether its SSNs will use HEU or low-enriched uranium (LEU). LEU is not sufficiently enriched to be used for nuclear weapons, so using LEU may help to alleviate some proliferation concerns. South Korea would still require IAEA verification to produce the SSNs regardless of whether it chooses HEU or LEU.
Trump suggested in October 2025 that the submarines could be built at the United States-based Hanwha Philly Shipyard. South Korean National Security Council Advisor Wi Sung-lac said in November 2025 that the submarines would be built domestically, however.
South Korea currently relies on US nuclear technology and fuel to operate its commercial nuclear energy reactors under the “123 agreement,” which restricts South Korea’s use of transferred nuclear material for any military purpose. The United States would have to revise Article 13 of the 123 agreement to supply fuel for South Korean SSNs.
The US Congress would not have to approve this revision, but the revision must undergo a 90-day congressional review period before taking effect. Several US senators raised concerns regarding South Korean nuclear proliferation in a January 30 letter to Trump.
South Korea is likely pursuing SSNs due to perceived capability gaps in countering North Korea’s submarine-based nuclear threat. The Lee administration is also advancing the transition of wartime operational control, which prepares Seoul to acquire independent defense capabilities.
Lee has argued that South Korea’s diesel-electric submarines lack the endurance to track North Korean and People’s Republic of China (PRC) submarines, aligning with broader US efforts to encourage allied burden-sharing in the Indo-Pacific.
North Korea denounced South Korea’s SSN development plan in a state media commentary in November 2025, accusing South Korea of pursuing “independent nuclear armaments,” and triggering “a regional nuclear domino effect.” The PRC has not formally protested the initiative.
The PRC opposed AUKUS’s nuclear submarine cooperation and may respond with economic retaliation—as it did following South Korea’s 2017 deployment of its Terminal High Altitude Area Defense system—to deter Seoul from pursuing the program.