Nuclear power plants contain reactors that create controlled chain-reaction fission, a process that continuously splits the nuclei of uranium atoms. This process produces a lot of energy, radiation, and very high heat.
Nuclear power plants harness the energy released by fission and put it to use to drive generators that produce electricity. Although nuclear power contributes only about 20 percent of the electricity generated in the United States, the nation’s nuclear capacity is the highest of any other country – 101 gigawatts in 2010.
Nuclear reactors have these components in common:
Fuel – Uranium, a radioactive, heavy metal ore, is the most common fuel for nuclear reactors. Following the enrichment process, uranium becomes a very concentrated fuel.
A commercial nuclear reactor requires thousands of pounds of enriched uranium fuel in order to operate. Civilian nuclear power plants in the U.S. purchase approximately 50 million pounds of uranium (U3O8 equivalent) fuel annually, the majority of which comes from overseas.
Uranium is mined in locations worldwide, primarily in Kazakhstan, Canada, Australia and Africa. The United States is among the top 10 producers of uranium.
Control Rods – Made from neutron-absorbing material such as cadmium, hafnium, or boron, control rods are inserted or withdrawn from the core to control the rate of reaction or to stop it if necessary.
Moderator – Material in the reactor core which slows down the neutrons released from fission so they cause more fission. The moderator is usually ordinary (light) water, but may be heavy water (D20) or graphite.
Coolant – Liquid or gas that circulates through the core to transfer the heat from it. In light water reactors the water moderator also functions as primary coolant.
Containment – Nuclear reactors are encased in heavily reinforced concrete structures to prevent radioactivity from escaping into the atmosphere.
Basic Process of Nuclear Energy
Nuclear physics is very technical, but the basic process for producing electricity with nuclear power is as follows:
The reactor core produces heat and radioactivity in a process called fission, commonly known as atom-splitting. Inside the reactor core is uranium nuclear fuel. As the nuclei of the uranium split they release neutrons. When the neutrons hit other uranium atoms, those nuclei also split, releasing their neutrons to strike other atoms, causing more fission. This continuous atom-splitting is a chain reaction.
The heat from controlled fission reactions is used to produce steam from water, either directly as in the boiling water reactor (BWR), or indirectly as in the pressurized water reactor (PWR), which contains a steam generator.
The steam drives a turbine that powers a generator.
The generator produces electricity that is distributed to the power grid.
Nuclear Reactor Types
Worldwide, various types of nuclear power reactors are used. However, the most common types are pressurized water reactors (PWR) and boiling water reactors (BWR), which are classified as light water reactors. In the United States, PWR and BWR are the only two types of commercial nuclear power plants in operation.
Boiling water reactor (BWR) – In this type of reactor, fission produces heat that boils water in the reactor core. Steam from the boiling water powers a turbine that drives a generator to produce electricity. The reactors at northeastern Japan’s Fukushima Naiishi plant damaged in the March 2011 earthquake and tsunami are BWRs.
Pressurized water reactor (PWR) – This type of reactor is the most common for producing energy. It uses water as coolant and moderator, an agent that helps control the speed of fission. In the closed primary coolant system the water, heated by thermal energy from fission while passing through the core, is kept under high pressure and therefore it doesn’t boil. Steam is produced in a secondary coolant loop and is used to power a turbine that drives an electric generator.
CANDU and heavy water moderated reactors – These designs use heavy water as moderator. The heavy water – with deuterium replacing the two hydrogen atoms – as moderator slows down neutrons in the fission process and allows use of natural uranium, rather than enriched uranium as a fuel.
Pebble bed modular reactor – A high temperature reactor that uses helium coolant and fuel encased in spheres of graphite and silicon carbide to ensure fission product containment and resistance to meltdown.