In the U.S., pressurized-water reactors account for more than 65 percent of commercial reactors.

FREMONT, CA: A nuclear power plant's heart is made up of nuclear reactors. They contain and regulate nuclear chain reactions, which produce heat via a physical process known as fission. This heat is utilized to create steam, which is then used to turn a turbine to generate power. Nuclear power is the most reliable carbon-free electricity sources, with over 440 operational reactors globally, including 94 in the U.S.

Nuclear Fission Creates Heat

A reactor's primary function is to contain and control nuclear fission, which is the process by which atoms break and release energy. Uranium is utilized as a nuclear fuel in reactors, and the uranium is separated into little ceramic pellets and piled into fuel rods, which are sealed metal tubes. To make a fuel assembly, more than 200 of these rods are usually grouped. Depending on the power level, a reactor core can be made up of several hundred assemblies.

The fuel rods are immersed in water inside the reactor vessel, which serves as a coolant and a moderator. To keep the chain reaction going, the moderator helps slow down the neutrons produced by fission. The reaction rate can then be reduced or increased by inserting or removing control rods into the reactor core. Fission heat transforms water into steam, which powers a turbine to generate carbon-free electricity.

Light-Water Reactors in the U.S.

Light-water reactors are used in all commercial nuclear reactors in the U.S. This means that they employ ordinary water as a cooler as well as a neutron moderator. In the U.S. there are two kinds of light-water reactors in use.

Pressurized Water Reactors

In the U.S., pressurized-water reactors, or PWRs, account for more than 65 percent of commercial reactors. To keep the water from boiling, these reactors pump high-pressure water into the reactor core. Nuclear fission heats the water in the core, which is subsequently fed into tubes inside a heat exchanger. Further, those tubes heat a separate water source to make steam. The steam is then used to power an electric generator, which generates electricity. The procedure is repeated when the core water returns to the reactor to be warmed.

Boiling water reactors (BWRs) account for almost a third of all reactors in operation in the U.S. Inside the reactor vessel, BWRs heat water and produce steam. Fission heats water when it is forced up through the reactor core. The steam is then fed directly into a turbine, which generates power. The steam that is not consumed is condensed back into water and reused in the heating process.