AnalysisSeoul scores diplomatic victory as Trump gives nod to nuclear sub plan

President Lee Jae Myung shakes hands with U.S. President Donald Trump as the U.S. leader receives a gold crown as a gift, along with the Grand Order of Mugunghwa, during a ceremony at Gyeongju National Museum in Gyeongju, North Gyeongsang Province, Wednesday. AP-Yonhap

GYEONGJU, North Gyeongsang Province — South Korea has received Washington's approval to build its first nuclear-powered submarine, in what analysts describe as a diplomatic win for Seoul in its long-sought bid to strengthen maritime power amid growing nuclear threats from North Korea.

Observers say the move also underscores Seoul's deeper alignment with Washington's strategy to contain China in the Indo-Pacific region, which could provoke a backlash from Beijing.

The agreement was reached following a summit on Wednesday between President Lee Jae Myung and U.S. President Donald Trump in Gyeongju, North Gyeongsang Province, held on the sidelines of the Asia-Pacific Economic Cooperation summit.

"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 social media Thursday, referring to his meeting with Lee.

In a separate post, Trump added that Seoul will build its first nuclear submarine at Hanwha Philly Shipyard, which is now owned by Hanwha Ocean, a major Korean defense company.

President Lee Jae Myung delivers a congratulatory speech at the christening ceremony for the State of Maine, a ship under the National Security Multi-mission Vessel program of the U.S. Department of Transportation, at Hanwha Philly Shipyard in Philadelphia, Aug. 26 (local time). Newsis

The U.S. decision came swiftly after Lee publicly urged Trump during the summit's livestreamed opening remarks to allow South Korea to obtain nuclear submarine fuel. Lee also said that constructing several conventionally armed submarines and deploying them to defend waters around the Korean Peninsula would help to lessen the burden on U.S. forces.

South Korea currently relies solely on diesel-electric submarines, including those equipped with air-independent propulsion, which must surface or snorkel to recharge. A nuclear submarine, capable of remaining submerged indefinitely, would represent a significant leap in the country’s naval capabilities.

Washington's approval was critical, as under the South Korea-U.S. nuclear energy agreement, Seoul is prohibited from reprocessing spent nuclear fuel or using it for military purposes without U.S. consent.

Doo Jin-ho, head of the Eurasia Research Center at the Korea Research Institute for National Strategy, described the approval as a major diplomatic victory for Lee.

Although previous U.S. administrations shied away from the idea due to concerns over the Nuclear Non-Proliferation Treaty (NPT), Doo said Trump's more business-minded approach helped to make the deal possible.

"For Trump, whose top priority is deterring China’s influence in the Indo-Pacific, Korea’s submarine capabilities would be a significant asset," he said. "Together with Japan's efforts to enhance its submarine fleet, the U.S. would have two Asian allies projecting strength underwater against Beijing. It's a scenario that would appeal greatly to Trump."

He added, "There must have been behind-the-scenes discussions leading up to the meeting, but by publicly raising the issue during the summit's livestreamed session, Lee made a bold diplomatic gamble."

Seoul's pursuit of a nuclear submarine dates back decades. Previous administrations, including those of liberal presidents Roh Moo-hyun and Moon Jae-in, explored the idea, but each effort stalled due to U.S. opposition.

Defense Minister Ahn Gyu-back welcomed the agreement, calling it a meaningful step toward acquiring the strategic naval asset.

"The necessary conditions to build such a submarine were already in place, and what we needed at the final stage was fuel. With U.S. cooperation on that front, we can now bring the project to completion,” Ahn said during a National Assembly audit, when asked about U.S. approval for the nuclear submarine.

South Korean and U.S. officials are expected to hold follow-up discussions on revising the nuclear agreement. The Republic of Korea Navy estimates that building the submarine will take about a decade, or possibly less, given the nation's advanced reactor technology and growing expertise in small modular reactors.

A 3,600-ton-class naval submarine, the first of the three Changbogo-III Batch-II class submarines, is seen at shipbuilder Hanwha Ocean's dockyard on Island Geoje in South Gyeongsang Province, Oct. 21, a day before its launch ceremony. Courtesy of ROK Navy

Still, experts caution that the political implications should not be overlooked.

While building a nuclear submarine does not technically violate the NPT, some view it as a potential stepping stone toward Seoul acquiring nuclear weapons.

Another concern is China's reaction. During the summit, Lee noted that South Korea's current diesel-powered submarines have limited endurance underwater, making it difficult to track submarines from North Korea or China.

That remark, analysts say, could be interpreted as suggesting that a nuclear submarine would bolster Seoul's naval strength not only against Pyongyang but also against Beijing.

“It was a mistake for the president to mention China publicly. That should have been handled behind closed doors. Beijing could see this as a major provocation amid Seoul's growing military and shipbuilding ties with Washington," said Park Won-gon, a professor of North Korean studies at Ewha Womans University.

Later on Thursday, China called on Seoul and Washington to uphold their nuclear nonproliferation obligations.

Foreign Minister Cho Hyun speaks during a press conference at the Asia-Pacific Economic Cooperation summit in Gyeongju, North Gyeongsang Province, Wednesday. Korea Times photo by Shim Hyun-chul

"China hopes that South Korea and the United States will fully uphold their nuclear nonproliferation obligations and work to promote regional peace and stability, rather than the opposite," Chinese foreign ministry spokesperson Guo Yaxun said during a briefing.

Hours later, Foreign Minister Cho Hyun reaffirmed South Korea's commitment to the NPT.

“(Nuclear submarines) are unrelated to nuclear latency or potential nuclear weapons capability. South Korea's pursuit of such submarines is entirely separate from weaponization and is not intended to develop latent nuclear capability," Cho said during a press conference at the APEC meeting.

"Our country remains under the U.S. extended deterrence, or nuclear umbrella, and has no plan of developing independent nuclear weapons," he added.