Power plant design. Most nuclear power plants cover 200 to 300 acres (80 to 120 hectares). The majority are built near a large river or lake because nuclear plants require enormous quantities of water for cooling purposes.
A nuclear plant consists of several main buildings, one of which houses the reactor and its related parts. Another main building houses the plant’s turbines and electric generators. Every plant also has facilities for storing unused and used fuel. Many plants are largely automated. Each of these plants has a main control room, which may be in a separate building or in one of the main buildings.
The reactor building, or containment building, has a thick concrete floor and thick walls of steel or of concrete lined with steel. The concrete and steel guard against the escape of radioactive material from an accidental leak in the nuclear reactor.
Power reactors that are used in nuclear power plants in the United States consist of three main parts: (1) a reactor, or pressure, vessel; (2) a core; and (3) a set of control rods. In addition, reactor operations depend upon two substances–moderators and coolants.
The reactor, or pressure, vessel is a tanklike structure that encloses the other main parts of the reactor. The vessel has steel walls that are typically up to least 6 inches (15 centimeters) thick and capable of containing the high pressure exerted in a reactor.
The core contains the nuclear fuel, in which the fission chain reaction occurs. The core sits in the lower half of the reactor vessel. A great many fuel assemblies stand upright in the core between an upper and lower support plate. Each fuel assembly contains a bundle of fuel rods. A fuel rod consists of pellets of fuel inside a metal tube. The pellet material is usually a powder called uranium dioxide. The tubing material is typically zircalloy, a mixture of the metal zirconium and one or more other metals. Neutrons can pass from the fuel through the tube walls, but most other nuclear particles cannot.
The control rods are long metal rods that are used to regulate fission in the fuel. The control rods contain such neutron-absorbing materials as boron or cadmium. A mechanism outside the reactor vessel is attached to the rods. This mechanism inserts the rods into the core and withdraws them when necessary. When inserted fully into the core, the control rods absorb many neutrons and so prevent a fission chain reaction from occurring. To begin operation of the reactor, the control rods are partially withdrawn until a chain reaction occurs at a constant rate. To increase power in the reactor, the rods are withdrawn slightly more. Thus, fewer neutrons are absorbed, and more are available to cause fission. To stop the chain reaction, the rods are inserted all the way into the core to absorb most of the neutrons.
The moderator is a substance that slows down neutrons as they pass through it. Slow neutrons are needed for fission. The moderator fills the space between the fuel rods in the fuel assemblies. It slows down neutrons as they pass from one fuel rod to another.
The coolant is a liquid or gas that carries off the heat created by the fission chain reaction. The coolant circulates throughout the core. It carries the heat from the reactor to an energy conversion system. Thus, the coolant keeps the fuel and cladding from getting too hot, and it transfers energy to a place where electricity can be generated.
All commercial power reactors in the United States are light water reactors. In these devices, light (ordinary) water serves as the moderator and the coolant. Canadian reactors are heavy water reactors. They use heavy water as the moderator and the coolant. Heavy water contains deuterium in place of ordinary hydrogen. For more information on reactors, see the section Research on new types of reactors in this article.
Fuel preparation. After uranium ore has been mined, it goes through a long milling and refining process to separate the uranium from other elements in the ore. Light water absorbs more neutrons than do other types of moderators. The uranium used in light water reactors must therefore be enriched–that is, the percentage of U-235 must be increased. Neutrons then have a better chance of striking a U-235 nucleus. In the United States, uranium that has been separated from the ore is sent to an enrichment plant.
Enrichment plants increase the proportion of U-235 in the uranium, depending on the intended use of the uranium. Most light water reactors use fuel with about 2 to 4 percent U-235. Each tube measures about 1/2 inch (13 millimeters) in diameter and 10 to 14 feet (3 to 5 meters) long.
After a tube has been filled with uranium dioxide pellets, its ends are welded shut. These fuel rods are then fastened together into bundles of 30 to 300 each. Each bundle, or fuel assembly, weighs 300 to 1,500 pounds (140 to 680 kilograms). Commercial power reactors need 50 to 150 short tons (45 to 136 metric tons) of uranium dioxide. The amount depends on the size of the reactor.
Chain reactions. A reactor requires a certain minimum amount of fuel to keep up a chain reaction. This amount, called the critical mass, varies according to the design and size of the reactor. Reactors are designed to hold more than a critical mass of fuel to allow for fuel use during operation. The position of the control rods determines the effective mass of the fuel, the amount of fuel taking part in the chain reaction. If the effective mass is decreased below the critical mass, the chain reaction will die out and reactor power will decrease. If the effective mass is increased above the critical mass, the chain reaction will become more rapid and reactor power will increase. In an emergency, if the chain reaction became too rapid, the reactor could overheat. However, the control rods are available to slow down the chain reaction if it becomes too rapid.
To prepare a reactor for operation, the fuel assemblies are loaded into the core with the control rods completely inserted. In a light water reactor, the water used as a moderator to slow down the neutrons fills the spaces between the fuel assemblies. The control rods are then slowly withdrawn, and a chain reaction begins. The farther the rods are withdrawn, the greater the rate of the reaction because fewer neutrons are absorbed. More neutrons thus are available to cause fission. When the desired power is reached, the control rods are positioned so that the effective mass is equal to the critical mass. The water in the core carries off the heat created by the chain reaction. To stop the reaction, the rods are again inserted all the way into the core to absorb most neutrons.
Steam production. The light water reactors used by almost all U.S. nuclear plants are of two main types. One type, the pressurized water reactor, produces steam outside the reactor vessel. The other type, the boiling water reactor, makes steam inside the vessel.
Most nuclear plants in the United States use pressurized water reactors. These reactors heat the moderator water in the core under extremely high pressure. The pressure allows the water to heat past its normal boiling point of 212 ?F (100 ?C) without actually boiling. The chain reaction heats the water to about 600 ?F (316 ?C). Pipes carry this extremely hot, though not boiling, water to steam generators outside the reactor. The steam generators transfer heat from the pressurized water to a separate supply of water that boils and so produces steam.
In a boiling water reactor, the chain reaction boils the moderator-water in the core. Steam is therefore produced inside the reactor vessel. Pipes carry the steam from the reactor to the plant’s turbines.
In producing electric energy, a nuclear plant’s steam turbines and electric generators work like those in a fossil-fuel plant. The steam produced by a reactor spins the blades of the plant’s turbines, which drive the generators. Many plants have combination turbines and generators called turbo generators.
After steam has passed through a plant’s turbines, it is piped to a condenser. The condenser changes the steam back into water. A reactor can thus use the same water over and over. But a condenser requires a constant supply of fresh water to cool the steam. Most plants pump this water from a nearby river or lake. The water, which becomes warm as it passes through the condenser, is then pumped back into the river or lake. This warm wastewater may heat the water in the river or lake enough to endanger plants and animals that live there. For this reason, the discharge of the wastewater is sometimes called thermal pollution.
To help solve the problem of thermal pollution, most new nuclear plants have cooling towers. Hot water from the steam condensers is moved through the towers in such a way that the heat passes into the atmosphere. The cooled water is returned to the steam condenser for reuse.
Hazards and safeguards. An ordinary power reactor cannot explode like an atomic bomb. Only a greatly supercritical mass of plutonium 239 or of highly enriched uranium 235 can explode in this way. A supercritical mass contains more than the amount of plutonium or uranium required to sustain a chain reaction.
The chief hazards of nuclear power production result from the great quantities of radioactive material that a reactor produces. These materials give off radiation in the form of alpha and beta particles and gamma rays. The reactor vessel is surrounded by thick concrete blocks called a shield, which normally prevents almost all radiation from escaping.
Federal regulations limit the amount of radiation allowed from U.S. nuclear plants. Every plant has instruments that continually measure the radioactivity in and around the plant. They automatically set off an alarm if the radioactivity rises above a predetermined level. If necessary, the reactor is shut down.
A plant’s routine safety measures greatly reduce the possibility of a serious accident. Nevertheless, every plant has emergency safety systems. Possible emergencies range from a break in a reactor water pipe to a leak of radiation from the reactor vessel. Any such emergency automatically activates a system that instantly shuts down the reactor, a process called scramming. The usual method of scramming is to insert the control rods rapidly into the core.
A leak or break in a reactor water pipe could have dangerous consequences if it results in a loss of coolant. Even after a reactor has been shut down, the radioactive materials remaining in the reactor core can become so hot without sufficient coolant that the core melts. This condition, called a meltdown, could result in the release of dangerous amounts of radiation. In most cases, the large containment structure that houses a reactor would prevent radioactive material from escaping into the atmosphere. To prevent such an accident from occurring, all reactors are equipped with an emergency core cooling system, which automatically floods the core with water in case of a loss of coolant.
Wastes and waste disposal. The fissioning of U-235 produces more neutrons than are needed to continue a chain reaction. Some of them combine with U-238 nuclei, which far outnumber U-235 nuclei in the reactor fuel. When U-238 captures a neutron, it is changed into U-239. The U-239 then decays into neptunium 239 (Np-239), which decays into plutonium 239 (Pu-239). This same process forms Pu-239 in a breeder reactor. Slow neutrons can fission Pu-239, as well as U-235. Some of the newly formed Pu-239 is thus fissioned during the fissioning of U-235. Even in small amounts, plutonium can cause cancer or genetic damage in human beings. Larger amounts can cause radiation sickness and death. Safe disposal of these wastes is one of the most difficult problems involved in nuclear power.
Most nuclear plants need to replace their fuel assemblies only about once a year. The radioactive wastes generate heat, and so used fuel assemblies must be cooled after removal from a reactor. Nuclear plants cool the assemblies by storing them underwater in specially designed storage pools.
In the United States, the federal government is working on guidelines for the safe and permanent disposal of nuclear wastes. The current U.S. plan calls for isolating long-lived radioactive waste from the environment in underground storage sites. A law passed by Congress in 1982 required the federal government to build two sites for nuclear wastes from commercial power plants. In 1987, the law was changed to require a single site.
A storage site for nuclear waste must lie in a highly stable area that is free of earthquakes, faulting, and other geologic activity. The site must be dry so that the waste containers cannot be corroded and water supplies cannot be contaminated. The site also must be constructed so that future generations do not dig into it and release radioactivity. The government is studying the suitability of a location in Nevada. In the meantime, commercial nuclear power plants in the United States continue to store used fuel assemblies and other wastes in pools of water on the plant grounds.
Other countries, including Japan, Russia, and the United Kingdom, are pursuing a reprocessing plan. Under this plan, nuclear plants would ship their used fuel assemblies to the reprocessing plants for removal of Pu-239 and unused U-235. These radioactive isotopes would then be recycled into fuel for nuclear reactors. However, this method would leave radioactive isotopes in the chemical solutions used for reprocessing. These solutions would have to be changed into a solid form that could be safely stored.
In every country that has a nuclear energy industry, the government plays a role in the industry. But the government’s role varies greatly among countries. This section deals mainly with the U.S. and Canadian nuclear energy industries.
Organization of the industry. Private utility companies own most of the nuclear power plants in the United States. The rest are publicly owned. Private companies also manufacture reactors, mine uranium, and handle most other aspects of U.S. nuclear power production.
Canada’s nuclear power plants are all publicly owned. Atomic Energy of Canada Limited (AECL), a government corporation, has overall responsibility for the country’s nuclear research and development program. AECL also designs the CANDU (CANada Deuterium oxide-Uranium) heavy water reactors used by all Canadian nuclear plants. Private companies make the various reactor parts and mine and process the country’s uranium. Canada has no uranium enrichment plants because CANDU reactors operate with unenriched uranium fuel.
The industry and the economy. The main economic advantage of nuclear power plants is that this fuel is less expensive than fossil fuels. But nuclear plants cost somewhat more to build than do fossil-fuel plants.
Under normal economic conditions, a nuclear plant’s savings in fuel eventually make up for its higher construction expenses. At first, these expenses add to the cost of producing electricity. But after some years, a plant will have paid off its construction costs. It can then produce electricity more cheaply than a fossil-fuel plant can. But two main problems–sharply higher costs and equipment failures–have somewhat lessened this long-run economic advantage of nuclear power plants. Many nuclear plants in the United States have had to shut down for months at a time because of equipment failures. Such losses of operating time further add to the cost of producing electricity.
The industry and the environment. Unlike fossil-fuel plants, nuclear plants do not release solid or chemical pollutants into the atmosphere. A nuclear plant releases small amounts of radioactive gas into the air. In addition, the cooling water used in pressurized water plants picks up a small amount of radioactive tritium in the steam condenser. The tritium remains in this water when it is returned to a river or lake. But these small amounts of radiation released into the environment are not believed to be harmful. Thermal pollution remains a problem at some nuclear plants. But cooling towers help correct this problem.
In a small number of nuclear accidents, hazardous amounts of radiation have been released into the atmosphere. Accidental releases of radioactive substances have occurred in Russia, the United States, and the United Kingdom; and an especially serious accident occurred in 1986 at the Chernobyl nuclear power plant in Ukraine (then part of the Soviet Union). The subsection Hazards and safeguards that appears earlier in this article discusses the main methods of guarding against accidents.
Critics of nuclear power also fear another danger to the environment. As power production increases, the creation of high-level radioactive wastes also increases. The United States has no permanent storage place for such wastes. The problem of storing radioactive wastes is discussed in the subsection Wastes and waste disposal.
Government regulation. The Nuclear Regulatory Commission (NRC), an agency of the federal government, regulates nonmilitary nuclear power production in the United States. One of the NRC’s main duties is to ensure that nuclear power plants operate safely, and it makes and enforces a variety of safety standards. Every nuclear reactor and power plant must be inspected and licensed by the NRC before it may begin operations. The NRC also supervises the manufacture and distribution of nuclear fuels, and controls the disposal of radioactive wastes from commercial production.