Nuclear Reactors Use As Fuel example essay topic

Nuclear energy 28 page term paper Nuclear energy, also called atomic energy, is the powerful energy released by changes in the nucleus (core) of atoms. The heat and light of the sun result from nuclear energy. Scientists and engineers have found many uses for this energy, including the production of electric energy and the explosion of nuclear weapons. Scientists knew nothing about nuclear energy until the early 1900's, though they knew that all matter consists of atoms. Scientists then further learned that a nucleus makes up most of the mass of every atom and that this nucleus is held together by an extremely strong force. A huge amount of energy is concentrated in the nucleus because of this force.

The next step was to make nuclei let go of much of that energy. Scientists first released nuclear energy on a large scale at the University of Chicago in 1942, three years after World War II began. This achievement led to the development of the atomic bomb. The first atomic bomb was exploded in the desert near Alamogordo, New Mexico, on July 16, 1945. In August, United States planes dropped bombs on Hiroshima and Nagasaki, Japan. The bombs largely destroyed both cities and helped end World War II.

Since 1945, peaceful uses of nuclear energy have been developed. The energy released by nuclei creates large amounts of heat. This heat can be used to make steam, and the steam can be used to generate electric energy. Engineers have built devices called nuclear reactors to produce and control nuclear energy. A nuclear reactor operates somewhat like a furnace. But instead of using such fuels as coal or oil, almost all reactors use uranium.

And instead of burning in the reactor, the uranium fish power production is by far the most important peaceful use of nuclear energy. Nuclear energy also powers some submarines and other ships. In addition, the fission that produces nuclear energy is valuable because it releases particles and rays called nuclear radiation that have uses in medicine, industry, and science. However, nuclear radiation can be extremely dangerous. Exposure to too much radiation can result in a condition called radiation sickness. Almost all the world's electric energy is produced by hydroelectric and thermal power plants.

Hydroelectric plants use the force of rushing water from a dam or waterfall to generate electricity. Thermal plants use the force of steam from boiling water. The great majority of thermal plants burn fossil fuels-coal, oil, and natural gas-to produce heat to boil water. The remaining thermal plants fission uranium. Few countries have enough water power to generate large amounts of hydroelectricity. Most countries depend mainly on fossil fuels.

But fossil fuels are a non-renewable resource. Therefore, many experts predict that nuclear power will become increasingly important. Worldwide distribution of nuclear energy. In the mid-1990's, about 425 nuclear power reactors operated in about 30 countries.

Nuclear power plants produced less than 20 percent of the world's electric energy. The United States had about 110 nuclear reactors and was the world's largest producer of nuclear energy. Reactors produced about 20 percent of the country's electricity. Canada had 22 reactors, which produced about 15 percent of Canada's electricity. Other countries, notably France and Japan, have a large nuclear power generating capacity. Advantages and disadvantages of nuclear energy.

Nuclear power plants have two main advantages over fossil-fuel plants. (1) Once built, a nuclear plant can be less expensive to operate than a fossil-fuel plant, mainly because a nuclear plant uses a much smaller volume of fuel. (2) Uranium, unlike fossil fuels, releases no chemical or solid pollutants into the air during use. However, nuclear power plants have three major disadvantages.

These drawbacks have slowed the development of nuclear energy in the United States. (1) Nuclear plants cost more to build than fossil-fuel plants. (2) Because of the need to assure that hazardous amounts of radioactive materials are not released, nuclear plants must meet certain government regulations that fossil-fuel plants do not have to meet. For example, a nuclear plant must satisfy the government that it can quickly and automatically deal with any kind of emergency. (3) Used nuclear fuel produces dangerous radiation long after it has been removed from the reactor. The full development of nuclear energy.

Many experts believe that the benefits of nuclear energy outweigh any problems involved in its production. According to these experts, oil may be so scarce by the mid-2000's that it will be too expensive to drill. Canada, Germany, Russia, the United States, and some other countries have enough coal to meet their energy requirements for hundreds of years at present rates of use. However, coal releases large amounts of sulfur and other pollutants into the air when it is burned. If nuclear energy were fully developed, it could completely replace oil and coal as a source of electric power. But a number of problems must be solved before nuclear energy can be fully developed.

For example, almost all today's power reactors use a scarce type of uranium known as U-235. If U-235 continues to be used at its present rate, the world's supply of it will become so small that it will be too expensive to mine and process by about 2050. Therefore, for nuclear energy to replace other energy sources, it must be based on fuel that is much more plentiful than U-235. NUCLEAR ENERGY / The science of nuclear energy The process by which a nucleus releases energy is called a nuclear reaction. To understand the various types of nuclear reactions, a person must know something about the nature of matter. The composition of matter All the matter that makes up all solids, liquids, and gases is composed of chemical elements.

The chemical elements, in turn, are composed of atoms. A chemical element consists of a substance that cannot be broken down chemically into simpler substances. There are 112 known chemical elements. Ninety-one of them are found on or in the earth. The other 21 elements are artificially created.

Scientists rank the elements according to mass, a measure of the quantity of matter in an object. An object's mass is proportional to its weight. Hydrogen is the lightest natural element, and uranium is the heaviest. Most of the artificially created elements are heavier than uranium. Atoms and nuclei. An atom consists of a positively charged nucleus and one or more electrons, which are negatively charged.

The nucleus makes up almost all of an atom's mass. The electrons, which are almost massless, revolve about the nucleus. Electrons determine the various chemical combinations that an atom enters into with other kinds of atoms. However, electrons do not play an active part in nuclear reactions. The nuclei of every chemical element except hydrogen consist of particles called protons and neutrons.

An ordinary nucleus of hydrogen, the lightest element, has one proton and no neutrons. The heaviest elements, such as uranium and thorium, have the largest number of protons and neutrons. Protons carry a positive charge. Neutrons have no net charge. Extremely strong forces, called nuclear forces, hold the protons and neutrons together in the nucleus.

The nuclear forces of each type of nucleus determine the amount of energy that would be required to release its neutrons and protons. Isotopes. Most chemical elements have more than one form. These different forms are called the isotopes of an element. The atoms that make up each of the different forms have different masses and are also called isotopes.

Scientists identify an isotope by its mass number-that is, the total number of protons and neutrons in each of its nuclei. All the isotopes of a given element have the same number of protons in every nucleus. Every hydrogen nucleus, for example, has just 1 proton. Every uranium nucleus has 92 protons.

However, each isotope of an element has a different number of neutrons in its nuclei and so has a different mass number. For example, the most plentiful isotope of uranium has 146 neutrons. Its mass number is therefore 238 (the sum of 92 and 146). Scientists call this isotope uranium 238 or U-238. The uranium isotope that almost all nuclear reactors use as fuel has 143 neutrons, and so its mass number is 235. This isotope is called uranium 235 or U-235.

No two elements have the same number of protons in their atoms. However, if an atom gains or loses one or more protons, it becomes an atom of a different element. However, if an atom gains or loses one or more neutrons, it becomes another isotope of the same element. Nuclear reactions A nuclear reaction changes the structure of a nucleus. The nucleus gains or loses one or more neutrons or protons.

It thus changes into the nucleus of a different isotope or element. If the nucleus changes into the nucleus of a different element, the change is called a transmutation. Three types of nuclear reactions release useful amounts of energy. These reactions are (1) radioactive decay, (2) nuclear fission, and (3) nuclear fusion.

During each reaction, the matter involved loses mass. The mass is lost because it changes into energy. Radioactive decay, or radioactivity, is the process by which a nucleus changes into the nucleus of another isotope or element. The process releases energy chiefly in the form of particles and rays called nuclear radiation. Uranium, thorium, and several other elements decay naturally and so contribute to the natural, or background, radiation that is always present on the earth.

Nuclear reactors produce radioactive isotopes artificially. Nuclear radiation accounts for about 10 percent of the energy produced in a reactor. Nuclear radiation consists largely of alpha and beta particles and gamma rays. An alpha particle, which is made up of two protons and two neutrons, is identical with a helium nucleus. A beta particle is identical with an electron. It results from the breakdown of a neutron in a radioactive nucleus.

The breakdown also produces a proton, which remains in the nucleus. Gamma rays are electromagnetic waves similar to X rays. Scientists measure the rate of radioactive decay in units of time called half-lives. A half-life equals the time required for half the atoms of a particular radioactive element or isotope to decay. Half-lives range from a fraction of a second to billions of years. Nuclear fission is the splitting of heavy nuclei to release energy.

All commercial nuclear reactors produce energy in this way. To produce fission, a reactor requires a bombarding particle, such as a neutron, and a target material, such as U-235. Nuclear fission occurs when the bombarding particle splits a nucleus in the target material into two parts called fission fragments. Each fragment consists of a nucleus with about half the neutrons and protons of the original nucleus.

The energy is released in many forms. But most of the energy released by fission eventually takes the form of heat. The bombarding particle must first be captured by a nucleus for fission to occur. Reactors use neutrons as bombarding particles because they are the only atomic particles that are both easily captured and able to cause fission. Neutrons can also pass through most kinds of matter, including uranium. The target material.

Commercial power reactors use uranium as their target material, or fuel. A uranium nucleus is the easiest of all natural nuclei to split because it has a large number of protons. Protons naturally repel one another, and so a nucleus with many protons has a tendency to "fly apart" and can be split with little difficulty. Uranium also makes a good nuclear reactor fuel because it can sustain a continuous series of fission reactions. As a result, uranium can produce a steady supply of energy. To create a series of reactions, each fissioned nucleus must give off neutrons.

Each of these neutrons can split still another uranium nucleus, thus releasing still more neutrons. As this process is repeated over and over, it becomes a self-sustaining chain reaction. Chain reactions can produce an enormous amount of energy. Only nuclei that have many more neutrons than protons, such as uranium nuclei, can produce a nuclear chain reaction. The scarce uranium isotope U-235 is the only natural material that nuclear reactors can use to produce a chain reaction.

Nuclei of the much more abundant U-238 isotope usually absorb neutrons without fissioning. An absorbed neutron simply becomes part of the U-238 nucleus. Neutrons released in fission travel too rapidly to be absorbed by U-235 nuclei in numbers large enough to sustain a chain reaction. Reactors can use U-235 as a fuel because they utilize other materials called moderators to slow the neutrons down. Some reactors use water as a moderator, while others use graphite. The slowed neutrons travel at a velocity of about 2.2 kilometers per second and are known as thermal neutrons.

Reactors that use moderators are called thermal reactors. Most of today's reactors are thermal reactors. Thermal neutrons are highly effective in causing fission in U-235. Therefore, the uranium in a thermal reactor can have a low percentage of U-235 content. Depending on their design, today's power reactors use a U-235 content ranging from 0.71 percent-the percentage in natural uranium-to about 4 percent. Special purpose reactors may use fuel with a higher percentage of U-235.

Scientists have also developed fast reactors, in which high-velocity neutrons cause the fissions. These reactors use plutonium or uranium 233 fuel. Fast breeder reactors produce more fuel material than they consume. A fast breeder reactor that converts U-238 to plutonium can greatly extend the use of uranium as an energy resource. In addition, a fast reactor can be designed to consume certain radioactive elements that have long-lives and are present in used fuel.

Such a reactor would reduce the amount of certain radioactive wastes that must be disposed of. The section Research on new types of reactors in this article discusses fast reactors in more detail. Nuclear fusion occurs when two lightweight nuclei fuse (combine) and form a nucleus of a heavier element. The products of the fusion have less mass than the original nuclei had. The lost mass has therefore been changed into energy.

Fusion reactions that produce large amounts of energy can be created by means of extremely intense heat. Such reactions are called thermonuclear reactions. Thermonuclear reactions produce the energy of both the sun and the hydrogen bomb. A thermonuclear reaction can occur in only a form of matter called plasma. Plasma is a gas like substance made up of free electrons and free nuclei (nuclei that have no electrons revolving about them). Normally, nuclei repel one another because of the positive charges of their protons.

However, if a plasma containing lightweight atomic nuclei is heated many millions of degrees, the nuclei begin moving so fast that they overcome the force of repulsion and fuse. Problems of controlling fusion. Scientists have not yet succeeded in harnessing the energy of fusion to produce electric energy. In their fusion experiments, scientists generally work with plasmas that are made from isotopes of hydrogen. Hydrogen has three isotopes. A mixture of deuterium and tritium is an excellent thermonuclear fuel because ordinary seawater contains plentiful stocks of deuterium and lithium.

One barrel of seawater contains enough of these substances to produce as much energy as the burning of about one-fifth of a barrel of oil. To produce a controlled thermonuclear reaction, a plasma of one or more hydrogen isotopes must be heated many millions of degrees. But scientists have yet to develop a container that can hold plasma this hot. The plasma expands quickly.

In addition, the walls of the container must be kept at low temperatures to prevent them from melting. But if the plasma touches the walls, it becomes too cool to produce fusion. The plasma must therefore be kept away from the walls of the container long enough for its nuclei to fuse and produce usable amounts of energy. Fusion devices. Most experimental fusion reactors are designed to contain hot plasma in "magnetic bottles" twisted into various shapes. The walls of the bottles are made of copper or some other metal and are surrounded by electromagnets.

An electric current is passed through the electromagnets, creating a magnetic field on the inside of the walls. The magnetism pushes the plasma away from the walls. All the fusion devices thus far developed, however, use much more energy than they create. The section Research on new types of reactors discusses experimental fusion reactors in greater detail.

NUCLEAR ENERGY / How nuclear energy is produced All large commercial nuclear power plants produce energy by fissioning U-235. But U-235 makes up about 0.71 percent of the uranium found in nature. About 99.28 percent of all natural uranium is U-238. The two types occur together in uranium ores, such as carnotite and pitchblende.

Separating the U-235 from the U-238 in these ores is difficult and costly. For this reason, the fuel used in reactors consists largely of U-238. But the fuel has enough U-235 to produce a chain reaction. Nuclear fuel requires special processing before and after it is used. The processing begins with the mining of uranium ore and ends with the disposal of fuel wastes. This section deals chiefly with the methods used in the U.S. nuclear power industry.

These methods resemble those used in other countries. 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 tank like 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. The Atomic Energy Control Board, a Canadian government agency, regulates Canada's nuclear energy industry. The board's duties resemble those of the Nuclear Regulatory Commission. Careers in nuclear energy cover a wide range of occupations and require widely varying amounts of training.

A high percentage of the jobs require a college degree or extensive technical education. Many of these jobs are in large research laboratories, which work to improve nuclear processes and to lessen their hazards. Other careers requiring advanced training are in such areas as uranium mining and processing, reactor manufacturing and inspection, power plant operation, and government regulation. In 1972, scientists discovered that a natural chain reaction had released nuclear energy nearly 2 billion years ago in a uranium deposit in west-central Africa.

Two billion years ago, there had been so little radioactive decay that the ore contained enough U-235 for a chain reaction. An accumulation of ground water acted as a moderator to begin the reaction. As heat from the reaction changed the water into steam, less and less water was available to serve as a moderator and the reaction died out. Except for such rare natural occurrences, nuclear energy was not released on a large scale on the earth until 1942. That year, scientists produced the first artificially created chain reaction. Scientific discoveries that took place within the last 100 years led to the large-scale release of nuclear energy.

Early developments Before the late 1800's, scientists did not suspect that atoms could release nuclear energy. Then in 1896, the French physicist Antoine Henri Becquerel found that uranium constantly gives off energy in the form of invisible rays. He thus became the discoverer of radioactivity. Other scientists soon began experiments to learn more about this mysterious phenomenon. The beginning of nuclear physics. In 1898, the great British physicist Ernest Rutherford identified two kinds of radioactive "rays", which he called alpha rays and beta rays.

He and other researchers later showed that these rays are actually high-energy particles, which became known as alpha and beta particles. Experiments with these particles then led Rutherford to discover the atom's nucleus. This achievement, which Rutherford announced in 1911, marked the beginning of a new science-nuclear physics. About 1914, scientists began doing experiments to see what happens when nuclear particles collide. The experimenters used alpha particles from naturally radioactive materials to bombard the nuclei of light atoms. Light nuclei do not repel positively charged particles, such as alpha particles, as strongly as heavy nuclei do.

Rutherford used this method to produce the first artificial transmutations in a series of experiments from 1917 to 1919. He bombarded nitrogen atoms with alpha particles. In rare collisions, a nitrogen 14 nucleus absorbed an alpha particle (a helium 4 nucleus). At the same time, the alpha particle pushed a proton out of the nitrogen nucleus. The nucleus thereby became an oxygen 17 nucleus.

Artificial fission. To produce nuclear reactions in heavy nuclei, scientists needed a particle that heavy nuclei would not repel. In 1932, the British physicist James Chadwick discovered such a particle-the neutron. In 1938, two German radio chemists, Otto Hahn and Fritz Strassmann, reported they had produced the element barium by bombarding uranium with neutrons. At first, scientists could not explain how uranium had produced barium, which is much lighter than uranium.

All previous transmutations had resulted in an element about as heavy as the original one. Then in 1939, the Austrian physicist Lise Meitner and her nephew Otto Frisch showed that Hahn and Strassman had in fact produced the first known artificial fission reaction. A uranium nucleus had split into two nearly equal fragments, one of which consisted of a barium nucleus. Two neutrons were also emitted. The other fragment consisted of a nucleus of krypton, a somewhat lighter element than barium. These two nuclei, together with the emitted neutrons, are lighter than a uranium nucleus and a neutron.

The reaction had therefore produced more energy than it consumed. Scientists soon realized that if many uranium nuclei could be made to fission, a tremendous amount of energy would be released. The amount of energy could be calculated from a theory developed by the great German-born physicist Albert Einstein in 1905. The theory shows that matter can change into energy and that matter and energy are related by the equation E equals m times c-squared.

This equation states that the energy (E) into which a given amount of matter can change equals the mass (m) of that matter multiplied by the speed of light squared (c-squared). The speed of light squared is obtained by multiplying the speed of light by itself. Using this equation, scientists determined that the fissioning of 1 pound (0.45 kilogram) of uranium would release as much energy as 8,000 short tons (7,300 metric tons) of TNT. Uranium could therefore be used to make a powerful bomb. The beginning of the nuclear age The development of nuclear weapons.

World War II broke out in Europe in September 1939. The month before, Einstein had written to U.S. President Franklin D. Roosevelt urging him to commit the United States to developing an atomic bomb. Einstein had fled to the United States from Germany to escape Nazi persecution. He warned Roosevelt that German scientists might already be working on a nuclear bomb. Roosevelt acted on Einstein's urging, and early in 1940 scientists received the first funds for uranium research in the United States. The United States entered World War II in 1941.

The government then ordered an all-out effort to build an atomic bomb and in 1942 established the top-secret Manhattan Project to achieve this goal. A group of scientists at the University of Chicago had charge of producing plutonium for the Manhattan Project. The group included such noted physicists as Enrico Fermi, Leo Szilard, and Eugene Wigner, all of whom had been born in Europe and had settled in the United States. Fermi headed the group. Under the scientists' direction, workers built an atomic pile, or reactor, beneath the stands of the university athletic field. The pile consisted of 50 short tons (45 metric tons) of natural uranium oxide and uranium embedded in 500 short tons (450 metric tons) of graphite.

The graphite served as a moderator. The pile was designed to demonstrate a controlled nuclear chain reaction in the uranium. Cadmium rods controlled the reaction. On Dec. 2, 1942, this reactor produced the first artificial chain reaction. The success of the University of Chicago project led the U.S. government to build a plutonium-producing plant in Hanford, Wash. The government also built a uranium enrichment plant in Oak Ridge, Tenn.

Plutonium and greatly enriched uranium from these plants were used in the two atomic bombs that the United States dropped on Japan in August 1945. After World War II, scientists began work on developing a hydrogen bomb. The United States exploded the first hydrogen bomb in 1952 and so achieved the world's first large-scale thermonuclear reaction But the AEC became responsible for regulating the nuclear energy industry. It also kept control in such areas as uranium enrichment and waste disposal. The United States made the world's first full-scale use of controlled nuclear energy in 1954.

That year, the U.S. Navy launched the first nuclear-powered vessel, the submarine Nautilus. The world's first full-scale nuclear power plant began operations in 1956 at Calder Hall in northwestern England. In 1957, the first large-scale nuclear plant in the United States opened in Shipping port, Pa. It supplied electricity to the Pittsburgh area until 1982, when the plant was closed. Canada opened its first full-scale plant in 1962 at Rolph ton, Ont.

The successful start of the nuclear power industry convinced world leaders of the need for international cooperation in the field. In 1957, the United Nations (UN) established the International Atomic Energy Agency to promote the peaceful uses of nuclear energy. Also in 1957, Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany formed the European Atomic Energy Community (Euratom). The organization encourages the development of nuclear power among its member countries.

Denmark, the United Kingdom, and Ireland joined Euratom in 1973. The spread of nuclear capability During the 1960's and early 1970's, a number of countries acquired reactors and used them to start nuclear power development. Progress was also made during this period toward limiting nuclear weapons tests and stopping the spread of nuclear weapons. In 1970, for example, a nuclear nonproliferation treaty went into effect.

The treaty prohibits the nuclear powers that have agreed to abide by the document from giving nuclear weapons to nations that do not already have them. The nonproliferation treaty also prohibits nations without nuclear weapons from acquiring them. But the nonproliferation treaty does not prohibit nations from selling or buying nuclear reactors. A reactor can be used not only for peaceful purposes but also to produce plutonium for nuclear weapons. India used a research reactor for this purpose and in 1974 exploded its first atomic bomb. Canada had supplied the reactor to India with the understanding it would be used for peaceful purposes only.

Canada has signed the nonproliferation treaty, but India has not. Critics of India's action question the wisdom of supplying reactors to countries that do not already have them. Meanwhile, the United States had been greatly increasing its nuclear power capacity. But opposition to nuclear power development also increased in the United States during the late 1960's and early 1970's.

Critics began to question nearly every aspect of nuclear power production, from the cost of uranium enrichment to the problems of waste disposal. Many critics of the United States nuclear program charged that the government overlooked various safety risks at nuclear plants to promote nuclear power development. Partly as a result of such criticism, Congress disbanded the Atomic Energy Commission (AEC) in 1974 and divided its functions between two newly formed agencies. The Energy Research and Development Administration (ERDA) took over the AEC's development programs. The Nuclear Regulatory Commission (NRC) took over its regulatory duties.

The NRC, it was believed, could better regulate the industry if it was not also responsible for the industry's growth and development. In 1977, Congress abolished ERDA and transferred its responsibilities to the newly created Department of Energy. Safety concerns There have been a number of accidents at nuclear power plants. Most of them have not been serious. However, in 1957, a fire at the Wind scale plutonium production plant in northern England resulted in the release of a large quantity of radioactivity. The British government banned the sale of milk from cows in that part of England for more than a month after the fire.

In the United States, concerns about the safety of nuclear reactors increased after a serious accident in 1979 at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania. Mechanical and human failures resulted in a breakdown of the reactor's cooling system and the destruction of the reactor core. Scientists and technicians prevented a failure of the reactor vessel that might have released large amounts of radioactive isotopes into the reactor containment building. Cleanup of the plant was completed in the early 1990's.

The worst nuclear accident in history occurred in 1986 at the Chernobyl nuclear power plant near Kiev in Ukraine, which was then part of the Soviet Union. An explosion and fire ripped apart the reactor and released large amounts of radioactive isotopes into the atmosphere. Unlike most Western reactors, the Chernobyl reactors lacked an enclosure to prevent radioactive isotopes from escaping. Soviet officials reported that 31 people died from radiation sickness or burns and more than 200 others were seriously injured. The radioactive substances spread over parts of what are now Ukraine, Russia, and Belarus, and were carried by wind into northern and central Europe. Experts expected a significant increase in the number of cancer deaths among those near the reactor.

But they predicted that the health effects outside the Chernobyl area would be slight. As a result of the accidents at Three Mile Island and Chernobyl, opposition to nuclear power increased in many countries during the late 1980's. In the United States, the NRC tightened its control of nuclear plants. Experts have expressed particular concern over the safety of older Soviet-designed reactors now operating in Russia, Ukraine, and several countries of the former Soviet bloc. Western scientists and engineers are helping to remedy some of the most urgent safety problems in these reactors. As the nuclear power industry has continued to develop, many improvements in plant equipment and operation have increased safety.

Nonetheless, some experts insist that the next generation of reactors should take greater advantage of design features that rely less-or not at all-on mechanical equipment such as pumps and valves to remove heat if an accident occurs. Some of these reactors are known as passively safe reactors.