GoKawiilShadowed by past disasters, nuclear engineers are developing different types of reactor, fuel, coolant and material that could help to make fission the zero-carbon energy technology of choice.
Companies are in the process of building test units for small modular reactors, which could be used to power rural areas. Credit: Kairos Power
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. That was 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 up conventional light-water reactors, which use normal water as a coolant — the only type currently used commercially in the United States. 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, and which will require the development of new structural materials.
Nature Outlook: Nuclear power
Known as small modular reactors (SMRs), they can produce up to 300 megawatts of electrical power (MWe) — 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 Massachusetts Institute of Technology 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 (DoE) announced US$900 million in grants to support the deployment of SMRs. One year later, the European Commission said it would invest up to €200 million (US$228 million) in the construction of SMRs.
Critics argue that SMRs are not necessarily more economical than are larger nuclear power plants. Edward Lyman, a physicist and director of nuclear power safety for the Union of Concerned Scientists, a non-profit organization in Cambridge, Massachusetts, has said that the smaller power output of SMRs means that factories would have to produce dozens of modular reactors for them to become more cost-effective than conventional reactors. He has argued that some SMR designs might be more dangerous than are existing reactors, owing to their 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 that run 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.
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