How does a nuclear power plant turn splitting atoms into electricity?

A nuclear power plant is, at heart, a very unusual way to boil water. Splitting uranium atoms releases heat, the heat makes steam, and the steam spins a turbine connected to a generator. What is unusual is the furnace.

The trick is the chain reaction. When a uranium-235 atom splits, it releases energy and a few neutrons. If those neutrons split more atoms, which release more neutrons, the reaction sustains itself. The scale is startling: splitting one uranium-235 atom frees about 100 million times the energy of burning one carbon atom. One kilogram of uranium-235 releases as much energy as burning about 2.7 million kilograms of coal.

Left alone, a chain reaction would grow exponentially, so a reactor is built to keep it balanced at exactly one new split for each split. Two ingredients do the balancing. A moderator, usually ordinary water, slows the speedy neutrons so they are likely to split the next atom. Control rods, made of neutron-absorbing elements such as boron, slide in and out of the core to soak up neutrons and turn the reaction up or down.

Fission was only understood in 1938 and 1939, when Otto Hahn and Fritz Strassmann found it and Lise Meitner and her nephew Otto Frisch explained it. Just four years later, on 2 December 1942, Enrico Fermi's team made the first self-sustaining chain reaction under the stands of a university stadium in Chicago. In 2023, nuclear plants supplied about 9 percent of the world's electricity.