South Korea Has Long Wanted Nuclear Subs. A New Reactor Could Open a Door.

The meeting was low-key, a presentation last month by South Korean officials to about 50 villagers gathered in a community hall on the country’s southeastern coast.

The government, the audience was told, planned to build a test version of a small nuclear reactor at a new atomic research complex — the country’s largest ever — that is under construction in the village of Gampo. The modular reactor, to be completed by 2027, would be similar to those that power seagoing vessels like icebreakers and container ships.

But that may not be the only ambition for this advanced technology. The project, nuclear experts say, could potentially allow South Korea to fulfill a long-held dream of developing a nuclear-powered submarine. It’s something that its most powerful ally, the United States, has opposed for decades.

In September, Australia announced that it would build nuclear-powered submarines with American and British help as the allies seek to balance out China’s growing military power. For South Korea, however, any such partnership has been off-limits for nearly 50 years under the terms of a treaty with Washington that blocks it from using nuclear materials for military purposes.

President Moon Jae-in’s government has been arguing for removing the prohibition, saying building nuclear submarines is crucial to countering North Korea’s ambitions to do the same. The sense of urgency has grown as the North’s progress has deepened concerns about South Korea’s preparedness. The North has tested a series of submarine-launched ballistic missiles in recent years and announced in January that it was working on a nuclear submarine design.

“There will be no better way of chasing, monitoring and deterring North Korean nuclear submarines than by deploying our own nuclear submarines,” said Moon Keun-sik, a retired navy captain who headed an earlier attempt by South Korea to build nuclear-powered subs. “We cannot depend on the United States to do it for us.”

The South Korean reactor project comes amid growing fears of an arms race in the Indo-Pacific region, driven by the superpower conflict between China and the United States. On Monday, Australia announced a military deal with South Korea that was called the largest ever between Australia and an Asian nation.

On the nuclear power front, South Korea is not the only country developing what are known as small modular reactors, or SMRs, as a carbon-free source of power. But its project, the Advanced Reactor for Multiple Applications, has drawn special attention.

Lim Chae-young, who headed the reactor project at the Korea Atomic Energy Research Institute, or KAERI, said that “we are not building it with a submarine in mind.” Still, the reactor’s 70-megawatt output is similar to that of early U.S. submarine reactors and would be enough to power South Korea’s next-generation 4,000-ton submarines, said Bryan Clark, a submarine expert at the Hudson Institute, a Washington-based think tank.

The country operates 24 nuclear reactors, which produce 29 percent of its electricity. It has also built 21 submarines since the early 1990s. But all these vessels are propelled by batteries charged with diesel engines and must surface frequently to get fuel or air for their engines. Nuclear-powered subs can stay underwater for months at a stretch and can move much faster.

South Korea’s first attempt to develop a nuclear-powered submarine, under a covert task force known as 362 that was launched in 2003, was cut short amid controversy.

Mr. Moon, the retired navy captain, headed the task force. By 2004, it had finished a basic design of a submarine reactor with Russian help, according to Kim Si-hwan, who worked on the project as a researcher at the Korean atomic energy institute.

The institute’s technical cooperation with Russia on small reactors goes as far back as 1995. In its 2017 annual report, OKBM Afrikantov, a Russian company that makes reactors for submarines, icebreakers and floating power plants, reported “continued discussion with KAERI on cooperation under the integral reactor project.”

The covert project was abandoned in 2004 after the discovery that the institute’s scientists had secretly enriched uranium in 2000, dabbling in a technology used to make nuclear weapons.

But South Korea has never abandoned its hopes, with the hurdles long being diplomatic, not technological. In 2016, the Washington-based Nuclear Threat Initiative said that, if an arms race broke out in Asia, “both Japan and South Korea are capable of building nuclear-powered submarines or surface vessels.”

When President Moon was campaigning for office a year later, he declared, “It’s time for us to acquire nuclear-powered submarines.”

Shortly after his inauguration in 2017, he asked Washington to help solve the problem of the 1972 treaty, which South Korea had agreed to in exchange for U.S. help in building a nuclear power industry.

According to Moon Chung-in, a former special adviser to Mr. Moon, President Donald J. Trump made a surprising suggestion: Why didn’t South Korea just buy American nuclear submarines? But Washington never followed up on this, nor did it help South Korea secure nuclear fuel for submarines, because of proliferation concerns.

“Without enriched uranium fuel, South Korea’s nuclear-powered submarine, even if it was built, would be nothing but an empty shell,” said Lee Byong-chul, a professor at the Institute for Far Eastern Studies at Kyungnam University in South Korea.

Mr. Moon’s office declined to comment.

Last year, the South Korean Defense Ministry said it would build six more submarines, the first three powered by lithium-ion batteries. It didn’t clarify the power source for the other three 4,000-ton submarines. But Kim Hyun-chong, who at the time was a deputy national security adviser for Mr. Moon, said that South Korea’s next generation of submarines would be nuclear-powered.

The advanced reactor that South Korea is developing uses 19.75 percent enriched uranium for fuel, while commercial nuclear power plants use uranium enriched at less than 5 percent. Although 19.75 percent enriched fuel is not unusual for small modular reactors under development, uranium with a similar level of enrichment has also been used to fuel some of the world’s nuclear submarines.

“It could be for commercial or other marine purposes, but it is a very plausible basis for developing a nuclear-powered submarine, and the higher level of enriched fuel is a fairly strong indicator of that possibility,” said Toby Dalton, a co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.

Not everyone thinks that South Korea needs nuclear-powered submarines.

Mr. Clark, the Hudson Institute expert, said diesel-electric submarines were generally smaller, quieter and less costly than nuclear ones, making them suitable for short-range regional operations, such as patrolling littoral waters around the Korean Peninsula. “Seoul has more important capabilities to spend its money on,” he said.

Lee Jae-myung, the candidate for Mr. Moon’s governing party in the presidential election in March, has yet to announce his stance on the matter. Yoon Suk-yeol, the main opposition candidate, said he would give priority to improving South Korea’s satellite and airborne surveillance against North Korea, rather than investing in a nuclear submarine.

“I don’t think we need it right now,” Mr. Yoon said.

But calls for nuclear subs persist.

“If North Korea builds nuclear submarines, it will be a game-changer,” said Yoon Suk-joon, a researcher at the Korea Institute for Military Affairs. “The best way to deal with them is to have our own nuclear subs lurk near a North Korean submarine base, for months if necessary, and follow them when they come out.”