US submarines are outnumbered in the Pacific. South Korea has a plan to help

By Yoonjung Seo, Gawon Bae, Brad Lendon, CNN

Seoul, South Korea (CNN) — South Korea wants to join the undersea naval elite – and has received the blessing of US President Donald Trump to do just that.

Having Seoul become the seventh country in the world operating nuclear-powered submarines – joining the United States, Russia, China, France, the United Kingdom and India – could be a win-win.

South Korea could more effectively counter North Korean or Chinese moves in the waters around the Korean Peninsula; and that would free up the US Navy’s nuclear-powered attack subs to concentrate on patrols in hot spots like the South China Sea and the waters around Taiwan.

On both sides of the Pacific, building the vessels could mean thousands of high-paying manufacturing jobs, helping the US and South Korean economies.

“For South Korea, this would be a game-changer countering North Korea’s undersea threat,” said Yu Jihoon, a research fellow at the Korea Institute for Defense Analyses and former South Korean submarine officer.

Nuclear-powered subs “could transform South Korea’s role within the alliance to be a more capable security provider, so the strategic implications for the South Korea-US alliance are even more significant,” he added.

But, as is often the case in naval shipbuilding, the devil is in the details.

Nuclear-powered submarines (SSNs) come with many advantages. They can stay submerged for long periods of time – essentially for years, if they can carry enough provisions for the crew – whereas most conventionally powered subs must surface for air to run diesel engines, which in turn charge their batteries for running at depth.

They are also generally faster than conventionally powered subs and are in many cases quieter.

Acquiring them has been a decades-long wish of the South Korean government.

But Seoul has faced a key roadblock: under a decades-old nuclear agreement with the US, it is not allowed to reprocess spent nuclear fuel, despite having the technology to do so.

Korean officials have discussed the issue with previous US administrations, but always behind closed doors.

So, when South Korean President Lee Jae Myung mentioned his government’s decades-long wish for the US to lift the ban, during his open talks with Trump in late October, many were surprised.

Then the day after Lee’s request, Trump appeared to approve it. “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,” he wrote on Truth Social.

In his open request to Trump, Lee said that he “wanted to better track submarines in the seas near North Korea and China.”

He said the shorter submergence time of diesel-powered subs limits how long Seoul’s subs can track those of China, which has nuclear-powered attack subs, and North Korea, which is pursuing them.

Lee told Trump this plan, if approved, could help Washington as it would “significantly reduce the burden on US forces” around the Korean Peninsula.

The US Navy could certainly use help beneath the waves.

Speaking at a US House subcommittee hearing in 2019, the then head of US Indo-Pacific Command, Adm. Philip Davidson, gave a stark assessment of the challenges Washington faced.

“There are four-hundred foreign submarines in the world, of which roughly 75% reside in the Indo-Pacific region. One hundred and sixty of these submarines belong to China, Russia, and North Korea,” Davidson told the panel.

“While these three countries increase their capacity, the United States retires attack submarines (SSNs) faster than they are replaced,” Davidson said.

As of July 1, 2025, a US Navy website shows it had 49 attack subs in its fleet, which must cover all the world’s oceans. About two-thirds of that fleet is available to “surge” in an emergency, the acting chief of naval operations said last April, but fewer subs are out on patrol during routine operations.

Many in South Korea say it’s capable of building the SSNs. The nation’s Defense Minister Ahn Kyu-back told a parliamentary audit on October 30 that South Korea has “already secured various conditions needed to build nuclear-powered submarines.”

Choi Il, a retired South Korean Navy submarine captain who now runs a private research institute, agrees with Ahn. “South Korea already possesses the capability to build submarines of 3,000 tons or larger,” he said.

Choi further noted that South Korea’s existing Jangbogo-III submarines, with diesel-electric propulsion, are “structurally designed to allow a nuclear propulsion system.” But South Korean Chief of Naval Operations Adm. Kang Dong-gil told the October audit that it could “take more than 10 years” to convert the Jangbogo to nuclear propulsion.

Experts say the hinge of any deal will be location: where is the best, most-efficient place to build South Korean, nuclear-powered submarines?

In a subsequent Truth Social post, Trump added that the SSNs would be built at the Philadelphia Shipyard, recently acquired by South Korean shipbuilding and defense conglomerate Hanwha.

Later, when the fact sheet between the US and South Korea was released, South Korea’s national security adviser Wi Sung-lac told reporters that the discussions on the submarines had been conducted on the premise that they “would be built in South Korea,” though the fact sheet had no specific mention of the production location.

“The core issue in Trump’s post was the mention of the Philly shipyard,” said Kim Dong-yeob, South Korean military expert and a professor at the Institute for Far Eastern Studies at Kyungman University.

“What we wanted was not simply to possess nuclear-powered submarines but to secure related technologies and achieve industrial effects through domestic construction,” Kim told CNN.

“Building at the Philly shipyard means losing technology transfer. It is essentially no different from buying weapons built in the US.”

Hanwha Ocean, part owner of the Philly shipyard, disagreed with the concern and welcomed the decision. In a statement, the company said that it “is ready to provide support with its state-of-the art shipbuilding technology,” and added “investments and partnerships in facilities such as the Philly shipyard will contribute to the prosperity and shared security of both nations.”

A shipbuilding industry official who is familiar with the matter told CNN that Hanwha Ocean plans to introduce advanced workforce training programs at the Philly shipyard. Hanwha Ocean’s technical capabilities designing and building five of the six existing 3,000-ton diesel submarines (Jangbogo-III class) would mean there’d be no issue building nuclear-powered submarines at the Philly shipyard.

But no one doubts that such work would require a long time. Even if the plan clears congressional hurdles without delay, experts still project at least 10 years until South Korea could acquire a nuclear-powered submarine.

The Philly shipyard is being modernized by Hanwha Ocean, yet it is geared for commercial shipbuilding. Building subs would require additional investments for a roof-covered facility and dry dock.

Considering the lack of infrastructure, especially for sourcing the onboard nuclear reactor, US shipping expert Sal Mercogliano said the Philly yard would most likely only handle part of the build.

“We may see large modules and sections come across from Korea over to the US and then it’s really the nuclear power plant and the associated propulsion systems that are going to be done domestically here in the US because the US has a track record for the nuclear plants,” Mercogliano, a professor at Campbell University in North Carolina, said.

US Navy submarines are built at two shipyards: General Dynamics Electric Boat in Connecticut, and Huntington Ingalls Industries’ Newport News Shipbuilding in Virginia. Both yards have full construction schedules as the US modernizes its own submarine fleet, so fitting new South Korean construction into those facilities seems problematic.

Meanwhile, the US congressional approval and US Defense Department technical review for actual implementation could also take time. US Secretary of Defense Pete Hegseth said his department will “work closely” with the departments of State and Energy to fulfil the president’s commitment, as he visited Seoul days after Trump.

However, whether the scope of US permission will be limited to fuel supply or include nuclear propulsion technology transfer remains unclear.

While South Korea does not have a nuclear-powered submarine, in October it launched the 3,600-ton Jang Yeongsil, “the world’s best diesel submarine,” according to the country’s Defense Acquisition Program Administration (DAPA).

Designed and built in South Korea, it was made by Hanwha Ocean.

Although it is a conventional submarine, the Jang Yeongsil is equipped with lithium-ion batteries, which enable it to stay submerged and sustain maximum-speed maneuverability longer than others with traditional lead-acid batteries.

“Lithium-ion batteries, in general, compared to lead-acid batteries can be charged much faster… the other thing is that the lithium-ion batteries can contain a lot more energy for the same weight or same volume,” Thomas Shugart, a retired US Navy captain and adjunct senior fellow at the Center for a New American Security, told CNN.

“So, maybe a submarine that before could only keep submerged for three or four days, now maybe it’s more like 10 or 12 days.”

If South Korea can domestically build such an advanced sub, why does it still want a nuclear-powered one?

Retired South Korean Adm. Kim Duk-ki believes that owning a nuclear-powered submarine would allow South Korea to “more efficiently” contain Chinese and Russian submarines and surface ships in case of a war on the Korean Peninsula.

“South Korea needs to operate across long distances to block Chinese submarines that would come to Korean waters to support North Korea in case of war, and there could be limitations to doing that only with conventional submarines,” Kim told CNN, emphasizing the fast speed of nuclear-powered submarines.

Shugart, however, questions South Korea’s wish: “It doesn’t make a lot of sense to me from an operational perspective.”

“The primary benefit to nuclear-powered (submarines) is mainly speed, being able to go fast for a long period of time and cross long distances at a rapid clip,” Shugart said.

But “South Korea and Japan are right there where the action is likely to be,” he continued, adding that SSNs could make sense if Seoul’s intention was to carry out aggressive anti-submarine warfare.

A potential South Korean nuclear-powered sub could also exacerbate an arms race in the region. North Korea called South Korea’s pursuit of SSNs “a strategic move for its own nuclear weaponization,” despite Seoul clarifying that the submarine would not carry nuclear weapons.

“This is bound to cause a nuclear domino phenomenon in the region and spark a hot arms race,” North Korean state media KCNA said.

Earlier this year, Pyongyang revealed its own nuclear-powered submarine. It’s under construction with the aim to finish it by the end of 2025, in accordance with the five-year weapons development plan Kim Jong Un announced in 2021.

For China’s part, Foreign Ministry spokesperson Guo Jiakun urged caution and restraint.

“China hopes that both South Korea and the United States will earnestly fulfil their nuclear non-proliferation obligations and do things that promote regional peace and stability, not the opposite,” Guo said.

Geopolitical repercussions, including Beijing’s backlash, are among the most concerning factors according to professor Kim of Kyungman University.

“It is essentially declaring that South Korea would participate as the frontline spear and shield in the US-led China containment strategy,” Kim added. “Whether we can withstand the potential economic retaliation from China needs to be analyzed.”

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