A US ally is getting nuclear-powered submarines with no AUKUS. Here's how

South Korea has diesel-electric submarines expected to last for decades, but it plans to acquire nuclear-powered vessels. (AFP: Seung-il Ryu/NurPhoto)

The South Korean city of Gyeongju is famous for its uncanny, grass-covered burial mounds bearing the tombs of ancient kings.

It will also go down in history as the place where the United States finally agreed to South Korea's long-held aspirations to acquire nuclear-powered submarines, ahead of the Asia-Pacific Economic Cooperation leaders' meeting last year.

Months later, South Korea's government has announced its plan to build the submarines by the mid-2030s, but it did not reveal how many, nor the expected cost.

The ancient city of Gyeongju is where South Korea had its breakthrough in getting nuclear-powered submarines. (AFP: Yao Qilin/Xinhua)

As with the AUKUS agreement, the United States will help a close ally gain a fleet of nuclear-powered submarines.

But beyond that, South Korea and Australia are taking different paths to building their new vessels, and they're acquiring them for different reasons.

So what is Seoul's plan, and how does it compare to Australia's AUKUS submarine endeavour?

South Korea's ambitions for nuclear-powered submarines go back 20 years, but it had been unable to secure approval from the US, which was concerned about nuclear proliferation.

However, US President Donald Trump broke with previous administrations and in October agreed to South Korea having nuclear-powered submarines, framing it as a win for American industry.

"South Korea will be building its Nuclear Powered Submarine in the Philadelphia Shipyards, right here in the good ol' USA," he posted on Truth Social.

Donald Trump with Lee Jae Myung as he receives a gift of a gold crown in Gyeongju last year. (AP: Mark Schiefelbein)

Plans have changed since then, with South Korea's Defence Minister Ahn Gyu-back announcing the submarines will be developed and built by his country.

The submarines would use low-enriched uranium fuel and the first would be launched in about a decade, he said.

Other than that, experts say the details are scant, maybe intentionally so.

"Most importantly, they haven't put a dollar figure on it," said Euan Graham, senior fellow at the Australian Strategic Policy Institute (ASPI).

In contrast, the AUKUS submarine program comes with a $368 billion price tag, one that Dr Graham expects won't reflect the final cost.

Observers agree the cost is one of the major risks in Seoul's plan to build nuclear-powered submarines.

The vessels are expensive, not only to build, but also to operate, m