S. Korea Already Deep Into Nuclear Submarine Design
A recent media report reveals South Korea's nuclear submarine design is nearly complete — years ahead of expectations. However, the designated US shipyard isn't close to being ready.
Back in 2021, the Korea Atomic Energy Research Institute (KAERI) revealed a new organization called the Munmu Advanced Research Institute for Nuclear Science (MARINS). While its mission has been propagated as the research hub for small modular reactors (SMR), a recent South Korean media report has claimed MARINS is also where the design work for the nuclear submarine has been taking place for some time.
Did You Know? MARINS is located in Gyeongju, the city from which President Trump gave a thumbs up to the South Korean nuclear submarine program in late October 2025.
According to the report, the design process has been quietly progressing at MARINS under the leadership of the Agency for Defense Development (ADD), the government organization that leads the country’s defense tech R&D programs. Staffed with around 80 personnel drawn from its own organization, as well as local naval shipyards and the Republic of Korea Navy (ROKN), the team at MARINS has already moved through concept design and is deep into basic design — the critical phase that establishes hull dimensions, reactor integration, weapons systems, and overall architecture. The basic design is expected to be finished in H1 2026, which will likely be followed by reactor selection and the detailed design phase.
In other words, South Korea has already progressed well beyond the early-stage exploratory work that many assumed was now happening. But, does that mean these boats will be launched sooner than expected?
The Philadelphia Paradox
Here’s where things get thorny. While South Korea’s design work has been underway, the US shipyard Trump designated for construction faces daunting challenges. Defense Minister Ahn Gyu-back didn’t mince words in a recent testimony: “Philly Shipyard currently lacks significant technical capability, workforce, and facilities [for nuclear submarine construction].”
The gap between current capability and requirements is vast. Hanwha Philly Shipyard, acquired by South Korea’s Hanwha in 2024, currently builds commercial training vessels and is preparing to produce modules for naval auxiliary ships. Nuclear submarine construction demands an entirely different (and notably higher) level of capabilities: radiation shielding facilities, nuclear safety protocols, security infrastructure meeting U.S. Navy standards, and a workforce trained in nuclear-qualified procedures.
Some in South Korea estimate that establishing an adequate nuclear submarine production line in Philadelphia would require at least three to five years of preparation, possibly longer. That’s before cutting the first piece of steel.
The pressure this creates for Hanwha is immense. The company has pledged USD 5 billion for infrastructure and workforce development at its Philadelphia shipyard, part of the broader USD 150 billion South Korean investment in US shipbuilding that accompanied Trump’s approval. However, transforming a commercial shipyard that has never built any submarine — let alone a nuclear one — into a facility capable of producing arguably the most complex type of crewed naval vessel represents an extraordinary industrial challenge.
Alternative Approaches: Splitting the Work?
The emerging debate in Seoul has revolved around the following two approaches:
Split construction between Geoje and Philadelphia. Major hull sections and conventional systems would be fabricated in Geoje, with final assembly, reactor installation, and nuclear systems integration occurring in Philadelphia under US supervision. This could address fuel security concerns, while fully leveraging South Korea’s proven submarine construction capability.
Construction in Geoje with US fuel supply. Defense Minister Ahn clearly prefers this route, telling lawmakers it would be “more practical to build them in South Korea because of the country’s advanced shipbuilding technology and decades of experience.” Under this model, Seoul would construct submarines domestically using low enriched uranium (LEU) supplied by the US.
Defense Minister Ahn recently referenced South Korea’s progress in SMR development, suggesting submarines could be built with domestically-produced reactors using US-supplied fuel. As noted in a previous post, the country’s SMR development started years ago.
As mentioned by an unnamed South Korean government official, a somewhat appropriate yardstick may be found in history. Britain began designing a nuclear submarine during the 1950’s, while the 1958 US–UK Mutual Defense Agreement (MDA) soon unlocked US reactor technology and fuel. With that help, the UK Royal Navy moved from concept to commission by 1963, when it launched HMS Dreadnought. Meanwhile, China pursued an entirely indigenous path after the Sino-Soviet split and needed nearly 30 years to achieve the same milestone — a reminder that support from an experienced ally could shrink timelines.
What Comes Next?
For the time-being, the design work continues in Gyeongju while diplomatic and industrial questions remain unresolved. The disconnect between South Korea’s advanced design progress and the unpreparedness of the designated construction site creates a timeline paradox: MARINS could complete detailed design over the next few years, but Hanwha’s Philadelphia shipyard might not be ready to begin construction until 2030 or later.
This gap highlights a fundamental tension in the arrangement. South Korea faces an urgent operational requirement — North Korea’s nuclear submarine development, likely with Russian assistance, demands a capability response measured in years, not decades. American shipbuilding revitalization is a worthy goal, but operates on a different clock. Whether these competing timelines can be reconciled, or whether South Korea will ultimately insist on domestic construction using US-supplied fuel, remains the program’s central unresolved question. The submarines are being designed now. Where they’ll actually be built might still be anyone’s guess.