Nuclear power plants have not exactly sprouted like daisies across the United States. The country’s first commercial nuclear reactor of the twenty-first century didn’t come online until 2016: Unit 2 of the Watts Bar Nuclear Plant in Tennessee, which arrived two decades after Unit 1. But that drought, and a similar one in Europe, might be ending. Two reactors began operating near Baxley in Georgia in 2023 and 2024, with governments and private parties around the world keen to invest in nuclear-plant developers.

Some are eyeing conventional light-water reactors, but a lot of effort and funding is now going towards developing a new breed of smaller reactors that rely on different fuels and coolants. Known as small modular reactors (SMRs), they can produce up to 300 megawatts of electrical power — enough to run about 300,000 homes. This is much less than the 1,000 MWe typically produced by conventional light-water reactors, but SMRs should be cheaper and easier to construct.

Existing commercial reactors are “very efficient, very good for the grid, but also very expensive to build,” says Jacopo Buongiorno, a nuclear engineer who directs the Center for Advanced Nuclear Energy Systems at MIT in Cambridge. Part of the advantage of SMRs lies in their modularity. Instead of constructing an entire power plant from the ground up, large portions of the system will be built in a factory and shipped to the site for assembly. Think Lego, says Buongiorno: “I have my prefabricated bricks and I connect them to make my reactor.”

In March 2025, the US Department of Energy announced US$900 million in grants to support SMR deployment. One year later, the European Commission said it would invest up to €200 million in SMR construction. Critics argue that SMRs are not necessarily more economical than larger plants. Edward Lyman, a physicist and director of nuclear power safety for the Union of Concerned Scientists, has said that the smaller power output means factories would have to produce dozens of modular reactors to become cost-effective. He has also argued that some SMR designs might be more dangerous due to different fuels and coolants.

Despite these concerns, the lure of faster and cheaper construction — and growing demand for clean energy, not least from data centres running artificial-intelligence systems — has made developing modular reactors commercially attractive. Kairos Power in Alameda, California, is building a test reactor called Hermes 1 in Oak Ridge, Tennessee, and began construction on a 50-MWe demonstration plant, Hermes 2, in April. The company expects to begin commercial operations in 2030 and has a deal to sell its power to Google.

Hermes 1 is the first non-light-water reactor approved by the US Nuclear Regulatory Commission (NRC) in more than 50 years. Light-water reactors are cooled by water and usually powered by ceramic uranium dioxide pellets packed into zirconium alloy tubes. Hermes 1, by contrast, is fuelled by tristructural isotopic (TRISO) pebbles — poppy-seed-sized uranium particles encased in a carbon-ceramic shell — and cooled by a molten salt mixture of lithium fluoride and beryllium fluoride known as FLiBe. TRISO fuel is the oldest and best understood example of accident-tolerant fuels, says Nichola...