October 31, 2025
GYEONGJU – US President Donald Trump’s “approval” for South Korea to build a nuclear-powered submarine marks a watershed moment in the decadeslong debate over Seoul’s pursuit of nuclear propulsion at sea — a move that could redefine the future of the alliance and reshape the strategic balance in Northeast Asia.
Trump announced Thursday on his Truth Social account that South Korea has his “approval” to build a nuclear-powered submarine, adding that construction would take place at the Philadelphia Shipyard, now operated by South Korean conglomerate Hanwha Group.
The statement came just a day after President Lee Jae Myung urged Trump to “make a resolute decision” to enable South Korea to receive nuclear fuel for submarine propulsion during their bilateral summit held on the sidelines of the Asia-Pacific Economic Cooperation forum in Gyeongju.
“I have given them approval to build a nuclear-powered submarine instead of the old-fashioned, far less nimble diesel submarines they currently use,” Trump wrote.
He then added in a separate upload, “South Korea will build its nuclear submarines right here, at the great Philadelphia Shipyard. Shipbuilding in our country will soon be making a big comeback.”
The approval — though politically declarative and pending congressional and Pentagon reviews — symbolizes a major shift in Washington’s stance toward Seoul’s long-standing ambition for a nuclear submarine program, which has been constrained by the ROK-US Atomic Energy Agreement. ROK stands for the Republic of Korea, the official name for South Korea.
The introduction of a nuclear-powered submarine would represent a milestone in the modernization of the South Korea-US alliance, according to an analyst. It signals Washington’s growing trust in Seoul’s defense capability and its expectation that South Korea will shoulder a larger strategic role across the Indo-Pacific region, including in its deterrence of China and monitoring of North Korea’s missile-capable submarines.
“Supporting South Korea’s acquisition of nuclear-powered submarines aligns with the strategic interests of the US and will ultimately strengthen the combined readiness posture of the alliance,” said Yu Ji-hoon, a research fellow at the Korea Institute for Defense Analyses.
“In doing so, it will contribute to establishing and maintaining a more stable regional security environment.”
Yu noted that such a capability would help offset the instability caused by China’s growing naval presence and Pyongyang’s advancing missile threats, contributing to a more stable regional security order.
He explained that nuclear-powered submarines possess significant advantages over conventional diesel-electric vessels, including longer underwater endurance, faster cruising speeds and superior stealth. Such qualities could markedly enhance Seoul’s overall deterrence posture.
North Korea’s recent advances in submarine-launched ballistic missile capability have heightened the need for around-the-clock underwater surveillance. Diesel submarines, such as South Korea’s 3,600-ton Jang Young-sil, launched earlier this month, can only remain submerged for about three weeks. Nuclear propulsion would enable continuous patrols lasting several months.
Trump’s declaration that the submarines will be built at the Philly Shipyard intertwines strategic cooperation with political theater, noted Choi Il, a retired South Korean Navy captain and now head of South Korea’s Submarine Research Institute.
This is “a double-edged sword for Seoul,” he explained.
“On the one hand, building at Philadelphia opens an extraordinary channel for South Korea to tap directly into American experience with nuclear-propulsion integration,” Choi said. “It could accelerate our learning curve by years.”
But the defense expert warned that the cooperation would come with a steep domestic price tag.
“Even if the hull is assembled in the US, Korea cannot outsource the backbone of its program — training, refueling and maintenance infrastructure must still be built at home,” he noted.
“That means we face a dual-investment structure: overseas construction and parallel domestic buildup — it will be costly, but it’s the price of gaining strategic autonomy.”
Yu stressed that while the project aligns with US strategic interests and the modernization of the alliance, South Korea must ensure transparency and compliance with nonproliferation norms to avoid regional tension.
“South Korea must ensure full transparency throughout the process,” he said, “and put in place robust safeguards and procedures to prevent any risk of nuclear proliferation.”
He warned that the sensitive and symbolic nature of a nuclear submarine program could provoke political and diplomatic friction with neighboring countries.
“To mitigate such concerns,” Yu added, “Seoul should engage in proactive diplomacy to clearly communicate that its pursuit of nuclear-powered submarines is purely defensive in nature and intended to reinforce regional stability, not to unsettle it.”
Beijing reacted cautiously to Trump’s latest approval. Rather than issuing a strong rebuke, Chinese Foreign Ministry spokesperson Guo Jiakun urged both Seoul and Washington to fulfill their nonproliferation obligations and to act in ways that promote regional peace and stability.
Guo’s comments, delivered at a regular press briefing in Beijing on Thursday, were widely interpreted as a measured response designed to avoid diplomatic friction ahead of Chinese President Xi Jinping’s state visit to South Korea and the South Korea-China summit set for Saturday.
Meanwhile, Seoul officials acknowledged that building the country’s first nuclear-powered submarine will be a long-term project. Chief of Naval Operations Adm. Kang Dong-gil told lawmakers Thursday that construction of the Jangbogo-III Batch-III, believed to be the model slated for nuclear propulsion, would take at least a decade once it begins.
“The start date (for the submarine project) has yet to be decided,” Kang said during a National Assembly hearing, “but even if a decision is made soon, completion would likely come in the mid-2030s or later.” He estimated the vessel’s displacement would exceed 5,000 tons, with uranium enrichment levels “around or below 20 percent, suitable for peaceful use.”
Defense Minister Ahn Gyu-back, also during a Thursday parliamentary hearing, projected that South Korea would eventually require at least four nuclear-powered submarines to maintain an effective defense posture. He stressed that Seoul had already developed most of the technical conditions needed for construction, adding that “the final piece was the fuel — and that has now been completed through US cooperation.”
When asked about Trump’s announcement that South Korea’s submarines would be built at the Philadelphia Shipyard, Ahn said further bilateral discussions would be needed to clarify details. He confirmed, however, that the current plan remains for South Korea to build the hull and reactor domestically while receiving enriched uranium fuel from the US.
Currently, only a handful of nations possess nuclear-powered submarines. The US and Russia operate the largest fleets, comprising both ballistic missile and attack submarines. China, France, and the UK maintain their own nuclear-propelled vessels, while India joined the group in the 2010s with its domestically built Arihant-class submarines. Brazil is developing its first nuclear-powered submarine under a program with French assistance, aiming for launch later this decade. The US, together with the UK and Australia, is also advancing plans under the AUKUS security partnership to provide Canberra with nuclear-powered submarines in the 2030s — a landmark initiative that underscores the growing strategic importance of undersea deterrence across the Indo-Pacific.