U.S. President Donald Trump said on the 30th that Korea will build a nuclear-powered submarine at the Philadelphia Naval Shipyard in the United States, accelerating discussions between the two countries on a nuclear-powered submarine program and a revision of the nuclear cooperation agreement. It was a response just one day after President Lee Jae-myung asked to allow the supply of fuel for nuclear-powered submarines.
Korea has effectively secured the core technologies needed to build a nuclear-powered submarine, including small reactor design, high-pressure hull materials, and low-noise propulsion. What remains is fuel. Even if the United States allows the supply of commercial, non-weapons-grade low-enriched uranium (LEU) only, related industries, including small modular reactors (SMRs), could grow substantially.
Jeong Beom-jin, a professor of nuclear engineering at Kyunghee University, said, "Holding enrichment authority and importing already enriched fuel are completely different matters," adding, "If you directly demand enrichment authority, it could be misunderstood as weapons development and become a heavy political burden."
Both nuclear-powered submarines and commercial power reactors use energy from the fission of uranium in a reactor. A reactor can only fission uranium-235, the uranium isotope with an atomic mass of 235. Since uranium-235 accounts for only 0.7% of natural uranium, enrichment to raise this ratio is essential. In general, less than 20% uranium-235 is classified as low-enriched, and 20% or more is classified as highly enriched (HEU).
Under the U.S.-Korea nuclear cooperation agreement revised in June 2015, Korea can reprocess spent nuclear fuel and enrich uranium below 20% only in research fields. However, it has never actually conducted enrichment. Even 3% to 5% low-enriched uranium used in commercial reactors is imported from the United Kingdom, France, Russia, and China, then processed domestically by adding protective cladding.
Reactors for submarines require uranium-235 at a higher enrichment level than power reactors. The higher the enrichment of uranium-235, the more the amount of fuel and the size of the reactor core can be reduced, while extending submerged endurance and maintenance intervals. Conversely, low enrichment shortens replacement cycles and enlarges the reactor, but it is relatively safer from a nonproliferation standpoint.
The United States and the United Kingdom have traditionally used highly enriched uranium in the 90% range to implement small, long-life reactors and have made it a core strategy of naval power. But highly enriched uranium is not strictly necessary. That is because operational environments and design standards differ by country. In practice, France has designed and operates with 7% enriched uranium, and China with enrichment in the 4% range.
Seo Kyun-ryeol, an emeritus professor of nuclear engineering at Seoul National University, said, "Like France and China, low-enriched fuel is sufficient to meet operational requirements," adding, "Because Korea conducts operations around the Korean Peninsula, it can secure sustained submerged endurance with low-enriched fuel alone." He added, "With technical preparations in place as they are, if the U.S. Congress approves construction, Korea could complete a nuclear-powered submarine within five years."
Hwang Il-soon, an emeritus professor of nuclear engineering at Seoul National University, said, "Our Navy has so far pursued the development of nuclear-powered submarines using 19.9% enriched uranium." Nuclear-powered submarines use enriched uranium as a power source, not for weapons, so building nuclear-powered submarines does not violate the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).
The Korean government has regarded nuclear-powered submarines as a key means to counter the North Korean nuclear threat and has pushed forward with construction in strict secrecy since the 1990s. At the time, the Kim Young-sam administration drew up a plan to build and deploy nine 3,000-ton vessels for operational service in cooperation with the Korea Atomic Energy Research Institute and the Agency for Defense Development (ADD), but at first it was only barely capable of building based on foreign design blueprints, so hull design was a major obstacle.
Korea is currently developing the next-generation submarine of 4,000 tons or more (KSS-III Batch-III). This submarine is designed to introduce an SMR-based hybrid propulsion system, enabling operational endurance and low-noise navigation equivalent to a nuclear-powered submarine. President Trump's latest approval is seen as a signal that KSS-III Batch-III can be developed to an actual operational level.