How Nuclear Plants Work
What is nuclear energy, and how does it work?
TVA Nuclear operates two different types of nuclear power plants, one using pressurized water and the other boiling water. Sequoyah and Watts Bar Nuclear Plants are based on a pressurized water reactor, and Browns Ferry Nuclear Plant has a boiling water reactor.
A nuclear plant works in much the same way that a TVA dam or fossil-fuel plant does, in that large turbine blades are used to operate a generator to produce electricity. At a hydroelectric dam, the force of the falling water spins the turbine blades, while at a coal-fired or nuclear plant, the force of steam spins the blades. A nuclear plant, however, uses uranium instead of coal as a fuel to make steam.
The uranium is formed into ceramic pellets and placed in metal tubes called fuel rods. In TVAs pressurized water reactors, about 51,000 fuel rods are placed in the reactor vessel to make up the corethe part of the plant that produces heat.
When a uranium atom splits in the process called nuclear fission, it gives off energy in the form of heat. To regulate the heat-producing process, control rods and borated water are used. The borated water speeds up or slows down the fission process, and the control rods shut down the reaction when theyre inserted between the fuel rods.
The water is heated to about 304 degrees Celsius (580 degrees Fahrenheit). Its kept under high pressure to prevent it from boiling as it travels to tubes inside four steam generators.
A secondary source of water passes around the outside of the tubes in the steam generators. The heat from the water inside the tubes is transferred to the secondary source of water, which boils and turns to steam. The steam formed in the generators is piped into the main turbine, where the force of the steam turns the turbine blades. 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, and that produces electricity.
Once the steam has been used to drive the main turbine, the low-energy steam is converted back to water by circulating around tubes (which carry cool water from an adjacent lake) in a large boxlike structure called a condenser. The condensed steam, now water, is pumped to the steam generators to repeat the cycle. The water in the condenser tubes picks up heat from the steam passing around the outside of the tubes. This heated water may be passed through a 140-meter-high (459-foot) cooling tower before being returned to the lake or reused in the plant. The three water systems are separated from each other to ensure that radioactive water does not mix with nonradioactive.
In the boiling water reactor design used at the Browns Ferry Nuclear Plant, steam is produced in the reactor core instead of in the steam generators. The turbine and generator work to make electricity just as they do in a pressurized-water nuclear plant or in a coal-fired plant.
The safety and health of the public and TVA employees are the first consideration and top priority in the operation of TVA nuclear plants. Not only are the plants designed to be safe and reliable, but TVA employees maintain them to ensure they will operate as designed.
Maintaining a reliable and efficient plant and a highly trained work force makes it unlikely that a radiological incident will happen. Nevertheless, emergency preparedness is an important part of TVAs nuclear program. Emergency planners work closely with federal, state, and local agencies to develop, maintain, and practice emergency preparedness procedures. Drills are held throughout the year to practice and coordinate the activities that allow TVA to ensure the protection of the public and its employees.
TVA has installed sirens within a 10-mile radius of nuclear plants to notify residents promptly in the event of an incident, and alert them to tune to their emergency broadcast stations (see information, above right). The sirens may also be used to warn area residents of tornadoes and other natural disasters.
Sources of radiation
Radiation is energy emitted as invisible particles, waves, or rays. Radioactive atoms produce radiation as they disintegrate. Everyone is exposed to small amounts of radiation each day. Air, water, food, and sunshine are a few sources of natural background radiation. Radiation also comes from other sources, such as color televisions and medical x-rays.
People are concerned about radiation exposure because it can alter or damage the human cell structure. That is why nuclear power plants are carefully monitored and employees are trained to limit their exposure to a level that is as low as reasonably achievable.
The containment building, the reactor vessel, the fuel assemblies, and several other barriers are designed to contain radiation and protect plant workers and the public living near a nuclear plant from any exposure to elevated levels of radiation.
Repeated surveys around TVAs operating nuclear plants have shown no detectable increase in radiation levels over normal background levels.