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Nuclear explained Nuclear power plants

Nuclear reactors are machines that contain and control nuclear chain reactions while releasing heat at a controlled rate.

A nuclear power plant uses the heat that a nuclear reactor produces to turn water into steam, which then drives turbine generators that generate electricity.

U.S. nuclear power plants use two types of nuclear reactors

Nuclear power plants in the United States have either a boiling-water reactor or a pressurized-water reactor.

Boiling-water nuclear reactors

In a boiling-water reactor, the reactor core heats water, which turns directly into steam in the reactor vessel. The steam is used to power a turbine generator.

Diagram of a boiling water nuclear reactor.

Diagram of a boiling-water nuclear reactor

Source: U.S. Nuclear Regulatory Commission (public domain)

Diagram of a pressurized nuclear water reactor.

Diagram of a pressurized-water nuclear reactor

Source: U.S. Nuclear Regulatory Commission (public domain)

Pressurized-water nuclear reactors

In a pressurized-water reactor, the reactor core heats water and keeps it under pressure to prevent the water from turning into steam. This hot radioactive water flows through tubes in a steam generator.

A steam generator is a giant cylinder filled with nonradioactive water (or clean water). Inside the giant water-filled cylinder are thousands of tubes filled with the hot radioactive water from the reactor core that eventually bring the clean water to a boil and turn it into steam.

The radioactive water flows back to the reactor core to be reheated, and once reheated, returns to the steam generator. The clean water may come from one of several sources such as oceans, lakes, or rivers.

did youknow


In the United States, 62 of the 93 operating nuclear power reactors (as of July 31, 2023) were pressurized-water reactors.

Small modular reactors

The U.S. Department of Energy is supporting the design, certification, and commercialization of small modular reactors (SMRs). SMRs are about one-third the size of the reactors that are operating and under construction in the United States. SMRs have simple, compact designs that can be assembled in a factory and transported by train or truck to the power plant site. The size and simplicity of the SMRs could reduce the time it takes to construct a new nuclear power plant.

Last updated: August 21, 2023.