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 is something that its most powerful ally, the United States, has opposed for decades.