How nuclear energy works
and what it’s used for

When uranium atoms are split, the heat produced is called nuclear energy. This energy is converted to electricity by a nuclear power plant.

Nuclear energy results from fission—or the splitting of atoms. An atom consists of a center, or nucleus, with particles called electrons orbiting the nucleus. There are several kinds of uranium atoms, each called an isotope of uranium. Most of the uranium atoms in uranium ore are the uranium-238 isotope; less than 1 percent is the uranium-235 isotope. The difference is that U-235 fissions easily and U-238 does not.

In a U-235 atom, the nucleus—which is composed of protons and neutrons—is unstable, and the nuclei break up, releasing neutrons. When the neutrons hit other uranium atoms, those atoms split—or fission—releasing neutrons of their own, along with heat. These neutrons strike other atoms, splitting them. One fission triggers others, which trigger still more until there is a chain reaction. When that happens, fission becomes self-sustaining.

Fission releases large amounts of energy (heat), which is called nuclear energy because it comes from a reaction in the nucleus of the uranium-235 atom. For uranium to be used as a fuel, its U-235 content must be increased from less than 1 percent to between 3 and 5 percent.

The fuel that produces energy in a nuclear power plant comes in the form of ceramic pellets about the size of the end of your finger. The pellets are inserted into long, vertical tubes, which are bundled together and placed inside the plant’s reactor—where the fission takes place.

A huge number of fission events can take place every second in the reactor—as many as 10,000,000,000,000,000,000 or 10 quintillion. And the neutrons are traveling extremely fast—about 40 million miles an hour. In fact, they’re traveling too fast for fission to be efficient. So they are slowed down—to about 7,500 miles an hour—by immersing the uranium fuel bundles in water. The fuel rods are then separated from each other by about one-eighth inch of water. The water serves another purpose, too. It carries away the heat produced by the fissioning atoms, and is turned into steam. As the water turns to steam, its volume expands about 1,000 times.

The steam makes electricity by spinning the turbine that drives an electric generator. Inside the generator, coils of wire and magnetic fields interact—and electricity is created.

Nuclear Energy Institute—Washington, DC
August 2000