Presidential candidates are in the business of making big promises, and few of the Democratic contenders for the 2020 nomination have promised more than Andrew Yang. An entrepreneur turned politico, Yang has styled himself as the techie’s candidate. His platform is defined by its embrace of high-tech solutions for a variety of social problems and earned him endorsements from Silicon Valley heavyweights like Elon Musk, Sam Altman, and Jack Dorsey. He advocates for returning ownership of digital data to users, a universal basic income as a salve for automation-fueled unemployment, and geoengineering to reverse climate change.

Yet of all Yang’s futuristic policies, one in particular stands out for its uniqueness and specificity. To transition the United States from fossil fuels to green energy, Yang wants the government to invest $50 billion in the development of thorium molten-salt nuclear reactors—and he wants them on the grid by 2027.

“Nuclear isn’t a perfect solution, but it’s a solid solution for now,” Yang’s climate policy page reads. It calls out thorium molten-salt reactors in particular as “a technology we should invest in as a stopgap for any shortfalls we have in our renewable energy sources as we move to a future powered by renewable energy.”

Thorium molten-salt reactors were first invented 60 years ago, but Yang appears to be the first presidential candidate to campaign on their promise to make nuclear energy safer, cleaner, and cheaper. Like all molten-salt reactors, they eschew solid rods of uranium-235 in favor of a liquid fuel made of thorium and a small amount of uranium dissolved in a molten salt. This approach to nuclear energy reduces proliferation risk, produces minimal amounts of short-lived toxic waste, and resists nuclear meltdowns.

As in a conventional nuclear reactor, splitting the nuclei of a nuclear fuel—a process known as fission—produces heat, which gets used to turn a turbine to generate electricity. But the Cold War arms race meant the US was already in the business of enriching uranium for weapons, so nuclear reactors based on solid uranium took off while liquid reactors stalled. No country has built a commercial molten-salt reactor.

As a result, many practical questions remain about the best way to design a thorium liquid-fuel reactor. Foremost among them, says Lin-Wen Hu, director of research and irradiation services at MIT’s Nuclear Reactor Laboratory, is finding materials that can contain the corrosive molten salts. Furthermore, figuring out how to extract unwanted elements produced as thorium decays—such as protactinium-233—from the fuel remains a major technical challenge.

“There is still a lot of work to be done in terms of demonstrating molten-salt reactor technology, even for uranium-based reactors,” Hu says. “Molten-salt reactors need to be demonstrated with a uranium fuel cycle before that system can be used for a thorium fuel cycle. Moving toward a thorium fuel cycle has a lot of unknowns."

Plenty of countries, most notably China, are investing heavily in molten-salt reactor research in general and thorium reactors in particular. Unlike the United States, China doesn’t have to overcome the inertia of a robust and entrenched nuclear industry with a 70-year history. The country is also believed to have large deposits of thorium within its borders, but comparatively little uranium. The incentives are aligned for China to aggressively pursue thorium molten-salt reactors, but experts say this isn’t the case in the US.

“The nuclear industry is conservative, and there's a lot of momentum behind uranium,” says Leslie Dewan, a nuclear engineer and a founding principal at Nucleation Capital, which invests in advanced nuclear energy companies. “That makes it more difficult to shift course into thorium.”