U.S. President Donald Trump speaks to members of the media on board Air Force One en route to South Korea, October 29, 2025. REUTERS/Evelyn Hockstein

By Josh Smith, David Brunnstrom and Costas Pitas

SEOUL/WASHINGTON (Reuters) -U.S. President Donald Trump said on Thursday he has given South Korea approval to build a nuclear-powered submarine, a dramatic move that would admit Seoul to a small club of nations possessing such vessels. The submarine will be built in a Philadelphia shipyard, where South Korean firms have increased investment, Trump wrote on social media.

"I have given them approval to build a Nuclear Powered Submarine, rather than the old fashioned, and far less nimble, diesel powered Submarines that they have now," Trump wrote on his Truth Social platform. The U.S. president, who met with South Korean President Lee Jae Myung and other regional leaders during his visit, also said Seoul had agreed to buy vast quantities of U.S. oil and gas. Trump and Lee finalized details of a fraught trade deal at a summit in South Korea on Wednesday. Lee had also been seeking U.S. permission for South Korea to reprocess nuclear fuel. NUCLEAR RESTRICTIONS EASING? South Korea's Industry Ministry said its officials had not been involved in any detailed discussions about building the submarines in Philadelphia. While South Korea has a sophisticated shipbuilding industry, Trump did not spell out where the propulsion technology would come from for a nuclear-powered submarine, which only a handful of countries currently possess.

One opposition lawmaker said on Thursday the Philadelphia shipyard does not have facilities to build submarines.

Asked about Trump's submarine announcement, Hanwha Ocean, which owns the shipyard with another Hanwha affiliate, said it was ready to cooperate with both countries and provide support with advanced technology, but did not mention specifics.

Defence Minister Ahn Gyu-back told lawmakers that plans called for South Korea to build its own submarins and modular reactors, and receive a supply of enriched uranium fuel from the United States.

Seok Jong-gun, the minister for the defense acquisition program administration told the same hearing that South Korea had been developing small nuclear reactors for some time and would be able to build one for a submarine in less than the decade usually needed to develop such nuclear-powered vessels.

"We believe if we use the technologies we have been preparing for the future ... we’ll be able to achieve this within a short period of time," he said. The United States has been working with Australia and Britain on a project for Australia to acquire nuclear-powered submarines involving technology transfers from the United States. The United States has so far only shared that technology with Britain, back in the 1950s. Lee said when he met Trump on Wednesday that allowing South Korea to build several nuclear-powered submarines equipped with conventional weapons would significantly reduce the burden on the U.S. military. He also asked for Trump's support to make substantial progress on South Korea being allowed to reprocess spent nuclear fuel, or on uranium enrichment, something currently not allowed under the nuclear agreement between the two countries, even though South Korea possesses nuclear reactors to generate power.

Chinese foreign ministry spokesperson Guo Jiakun said Beijing "hopes that South Korea and the United States will earnestly fulfill their nuclear non-proliferation obligations and do things to promote regional peace and stability, and not the other way around". APPROVAL RAISES QUESTIONS Lee's predecessors had wanted to build nuclear-powered submarines, but the United States had opposed this idea for decades. Daryl Kimball, executive director of the Washington-based Arms Control Association, said the issue of South Korea acquiring such submarines "raises all sorts of questions". "As with the AUKUS deal, (South Korea) is probably looking for nuclear propulsion services suitable for subs, including the fuel, from the U.S," he said. Kimball said such submarines usually involved the use of highly-enriched uranium and would "require a very complex new regime of safeguards" by the International Atomic Energy Agency, which has a key role in implementing the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). "It remains technically and militarily unnecessary for South Korea to acquire the technology to extract weapons-usable plutonium from spent fuel or to acquire uranium enrichment capabilities, which can also be used to produce nuclear weapons," he said. "If the United States seeks to prevent the proliferation of nuclear weapons worldwide, the Trump administration should resist such overtures from allies as strongly as it works to deny adversary access to these dual-use technologies." Kim Dong-yup, a North Korea studies professor at Kyungnam University, said the Lee-Trump summit had formalised a "transaction scheme of security guarantees and economic contributions" for maintaining the extended deterrence and alliance in exchange for South Korea's increased defense spending and nuclear-powered subs and U.S. investments. "In the end, this South Korea-U.S. summit can be summarized in one word: the commercialization of the alliance and the commodification of peace," he said. "The problem is that the balance of that deal was to maximize American interests rather than the autonomy of the Korean Peninsula."

(Reporting by Costas Pitas and David Brunnstrom; Additional reporting by Joyce Lee, Josh Smith and Ju-min Park in Seoul and Colleen Howe in Beijing; Editing by Leslie Adler and Lincoln Feast.)