Nuclear reactors produce power through fission. In the United States, nuclear power plants use either pressurized water reactors or boiling water reactors.
This section describes boiling water nuclear reactors.
Boiling water nuclear reactors are a type of light water reactor. In boiling water nuclear reactors (BWRs), the heat generated by the splitting of uranium atoms is used to boil water within the reactor core. The steam produced passes through a turbine, generating power. For more information see the Turbines section of the encyclopedia.
In nuclear power plants electricity is produced through fission instead of by burning fuel. The fission process starts with uranium in the form of ceramic pellets, such as those shown below. These solid pellets contain two isotopes of uranium. U-235 makes up less than 1 percent of natural uranium, but it fissions easily. U-238 makes up the other 99 percent of natural uranium, but fissions very little. The concentration of U-235 is enriched to produce more fissionable uranium in the power plant, but is kept low enough to prevent powerful explosions.
The fission reaction is restricted by control rods, inserted among the rods that hold the uranium fuel. These control rods absorb neutrons, preventing them from splitting more atoms. The number of control rods dictates the speed of the reaction: the more control rods, the slower the reaction.
The heat created in the fission process boils the cooling water surrounding the reactor core, producing steam. The steam is fed directly to a turbine, generating energy. The steam is then condensed and recycled, as shown below. This is an example of a direct cycle, in which the same water is used to both cool the reactor and operate the turbine. This configuration simplifies the heat exchange system.
Boiling water nuclear reactors are used in power plants around the world. 38 of the 117 nuclear power plants operating in the United States in 1996 were BWRs.
This section describes pressurized water nuclear reactors.
Pressurized water nuclear reactors, or PWRs, are another type of light water reactors, using ordinary water as its coolant. In PWRs, such as the one shown below, the heat generated by splitting uranium atoms is transferred to the water coolant in the core of the reactor. The reactor is kept at a high enough pressure to prevent boiling. The heat is transferred to a second water cycle to produce steam, which then generates power in a turbine.
The core of the reactor is immersed in water, and is contained within a cylindrical pressure vessel. A pressurizer keeps the heated water exiting the reactor at a constant high pressure. This water is circulated through heat exchangers. There it transfers the heat to steam in a secondary water circuit, which is maintained at a much lower pressure. The steam from the generators then passes through a turbine, producing power. Afterwards, the steam is changed back to water in a condensor.
Pressurized water reactors are used in power plants around the world. Below is a picture of the pressurized water reactor located in Middletown, PA.
The gas-cooled graphite-moderated nuclear reactor uses natural uranium fuel and carbon dioxide as a coolant. Graphite is typically used to moderate reactions involving natural uranium fuel.
Carbon dioxide coolant removes heat from the reactor core, then travels to a heat exchanger. Water in the secondary circuit picks up this heat, and evaporates to create steam, which can then generate power in a turbine. Carbon dioxide is used as a coolant because it is relatively inexpensive and works well at high-pressures.
Gas-cooled reactors have been developed for the most part in the United Kingdom and have been used in several other countries. The only gas cooled-reactor that ever existed in the United States was at Fort St. Vrain Nuclear Station, which has been shut down.
Heavy water reactors use D2O as a moderator of the nuclear fission. The deuterated water increases the neutron lifetime in comparison with a light water reactor. The reactor is cooled with heavy or light water, fueled with natural uranium dioxide pellets. The heat from the reaction is used to produce steam, which turns a turbine to generate power.
These reactors use heavy water as a moderator, as opposed to light water, because heavy water absorbs fewer neutrons and the uranium is used more efficiently. This also allows natural uranium to be used, which is less expensive than enriched uranium. Although the uranium is less expensive, deuterated water is costly and makes up 20% of the operating cost for each reactor.
It is important to be able to shut the reactor down if there is an emergency. As illustrated in the diagram below, there are two independent shut down systems. The first system consists of cadmium rods that are inserted vertically into reactor. These cadmium rods will absorb the neutrons and stop the nuclear reaction. The second system involves the injection of gadolinium nitrate dissolved in heavy water, which also acts as an absorbant for the neutrons.
The heavy water reactor pictured below is used in Canada in CANDU (Canada Deuterium Uranium) type power plant. The pressure tubes of the fuel element are horizontal and permit refueling while the reactor is in operation.
Canadian Nuclear Association, Ottawa, Ontario
Nuclear Energy Institute, Washington, DC
NuScale Power, Corvallis, OR
United States Nuclear Regulatory Commision, Washington, DC
World Nuclear Association, London, UK
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