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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 Korean President Lee Jae Myung expressed regret to North Korea on April 6 over recent South Korean drone incursions, which elicited a positive response from Workers’ Party of Korea (WPK) General Secretary Kim Jong Un’s sister Kim Yo Jong. The Lee administration will likely interpret Kim’s statement positively, though ISW-CDOT does not assess that inter-Korean dialogue is likely. Lee expressed his regret over the drone incursion for the first time on April 6, describing the incident as an act by reckless individuals that prompted unnecessary military tension. South Korean Minister of Unification Chung Dong-young expressed the Ministry of Unification’s (MOU) regret over the incident multiple times in February, to which Kim Yo Jong also responded favorably. Three South Korean civilians, a National Intelligence Service employee, and two active-duty South Korean military officers allegedly sent drones into North Korean airspace four times between September 2025 and January 2026. Kim Yo Jong responded positively to Lee’s statement on the same day it was made, conveying that Kim Jong Un assessed Lee’s stance as that of a “frank and broad-minded man’s attitude.”
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