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How Do Nuclear Plants Work?

In a nuclear-fueled power plant – much like a fossil-fueled power plant – water is turned into steam, which in turn drives turbine generators to produce electricity. The difference is the source of heat. At nuclear power plants, the heat to make the steam is created when uranium atoms split – called fission. There is no combustion in a nuclear reactor. Here’s how the process works.

There are two types of nuclear reactors in the United States:

Pressurized Water Reactor

Pressurized Water Reactors (also known as PWRs) keep water under pressure so that it heats, but does not boil. This heated water is circulated through tubes in steam generators, allowing the water in the steam generators to turn to steam, which then turns the turbine generator. Water from the reactor and the water that is turned into steam are in separate systems and do not mix.

View animated image of a Pressurized Water Reactor

Source: Nuclear Regulatory Commission

View a detailed description of the process.

1. The Fuel
The fuel used in nuclear generation is primarily uranium 235. It is manufactured as small round fuel pellets. A single pellet is less than an inch long, but produces the energy equivalent to a ton of coal. The pellets are placed end-to-end into fuel rods that are 12 feet long. Over 200 of these rods are grouped into what is known as a fuel assembly.

2. Reactor
The process of producing electricity begins when uranium atoms are split (i.e., fission) by particles known as neutrons. Uranium 235 has a unique quality that causes it to break apart when it collides with a neutron. Once an atom of uranium 235 is split, neutrons from the uranium atom are free to collide with other uranium 235 atoms. A chain reaction begins, producing heat. This reaction is controlled in several ways, including by control rods which absorb neutrons.
Control rods are inserted among the fuel assembly rods that hold the uranium pellets. When they are in place, they absorb the atomic particles that would normally initiate the chain reaction. When they are withdrawn from the fuel assembly, fission is allowed to occur.

3. Pressurizer
The heat produced in the reactor is transferred to the first of three water systems: the primary coolant. The primary coolant is heated to over 600 degrees Fahrenheit. In a pressurized water reactor, a pressurizer keeps the water under pressure to prevent it from boiling.

4. Steam Generator
The hot, pressurized water passes through thousands of tubes in nearby steam generators. These tubes are surrounded by another water system called the secondary coolant. The heat from the primary coolant is transferred to the secondary coolant, which then turns into steam.
The primary and secondary systems are closed systems. This means that the water flowing through the reactor remains separate and does not mix with the water from the other system or the lake.

5. Turbine
The steam is piped from the containment building into the turbine building to push the giant blades of a turbine. The turbine is connected to an electric generator by a rotating shaft. As the turbine blades begin to spin, a magnet inside the generator also turns to produce electricity.

6. Condenser Coolant
After turning the turbines, the steam is cooled by passing it over tubes carrying a third water system, called the condenser coolant or lake water. The steam is cooled so it condenses back into water and is returned to the steam generator to be used again and again.

7. Lake or Cooling Towers
At some nuclear stations, lake water flows through thousands of condenser tubes to condense steam back to water. It is then discharged down a long canal (for cooling) and eventually enters the main part of the lake.

At other plants, the condenser cooling water is circulated through cooling towers to remove the extra heat it has gained. The water is pumped to the top of the cooling towers and is allowed to pour down through the structure. At the same time, a set of fans at the top of each tower pulls air up through the condenser water. This lowers the temperature of the water. After it is cooled, the condenser water flows back into the turbine building to begin its work of condensing steam again.

Boiling Water Reactor

In Boiling Water Reactors (also known as BWRs), the water heated by fission actually boils and turns into steam to turn the turbine generator. In both PWRs and BWRs, the steam is turned back into water and can be used again in the process.

View animated image of a Boiling Water Reactor

Source: Nuclear Regulatory Commission