Some AI companies are turning to nuclear power to meet demand for electricity. But traditional nuclear plants can take decades to bring online.

Now some tech companies are partnering with startups trying to build small, modular nuclear reactors, designed with speed in mind. One such company, Kairos, has a deal with Google to build a fleet of modular reactors. To do so, it’s relying on a technique first developed in the mid-20th century: molten-salt cooling.