Nuclear Energy Nuclear energy is the energy contained in the center, or nucleus, of an atom. The nucleus is the most powerful source of energy that exists. Until the 1930's, scientists did not know how to release this energy. Since then, people have put nuclear energy to many constructive uses. Nuclear energy is used in scientific research and in medical treatments. It powers satellites and submarines, and it is used to produce electricity. People have also put nuclear energy to destructive uses, through the creation of nuclear weapons. Nuclear energy has great promise. It also poses great risks. Understanding the risks and benefits requires some knowledge of the source of nuclear energy: atoms and the particles that make them up. Atomic Structure All substances are made up of atoms. The atom is the basic unit of a chemical element, and the atoms that make up one element are different from the atoms that make up another element. Atoms themselves are made up of smaller subatomic particles. The nucleus (plural: nuclei) consists of two kinds of particles, protons and neutrons, bound closely together. Clouds of smaller particles called electrons surround the nucleus. Each electron carries a negative electrical charge. Each proton carries a positive charge. (Neutrons have no charge.) Opposite electrical charges attract each other, and this attraction holds the electrons and the nucleus together as an atom.
Atoms of different elements may join in clusters to form molecules of various substances. The atoms join together by exchanging or sharing electrons. Their nuclei remain unchanged. Thus it is the electrons that determine how an atom behaves chemically--that is, in what ways it may combine. The forms of energy we are most familiar with result from reactions that involve electrons. For example, when wood burns, molecules in the wood are pulled apart. Their atoms are combined in new ways with oxygen from the air. In the process, chemical energy is released. In contrast, nuclear energy involves changes in the nucleus of the atom. To understand how this energy is released, we need to take a closer look at the nucleus. Inside the Nucleus The nucleus contains most of the atom's mass. (Mass is the total amount of matter in a substance.) Atoms of the same element all have the same number of protons--for example, all oxygen atoms have eight protons, and all hydrogen atoms have one proton. But atoms of the same element may contain different numbers of neutrons. A hydrogen nucleus may have one proton and no neutrons, one proton and one neutron (a form called deuterium, or heavy hydrogen), or one proton and two neutrons (a form called tritium). These various forms of hydrogen are called isotopes of hydrogen. In the same way, there may be different isotopes of other elements. (To indicate an isotope, scientists use a notation that shows the mass number--the total number of protons and neutrons--along with the symbol for the element. Ordinary hydrogen is H-1, deuterium H-2, and tritium H-3.) With its single proton, hydrogen is the lightest element. The heaviest natural element is uranium. Its isotopes include uranium 235 and uranium 238. Scientists have made heavier elements in the laboratory. But these elements are not found in nature because they are unstable--they begin to change almost as soon as they are formed. These changes are related to conflicting forces in the nucleus. The positively charged protons and uncharged neutrons are held together by a powerful attraction called the strong force. At the same time, however, the protons repel each other--just as particles with opposite charges attract each other, those with the same charge tend to push each other away.
In the very heavy elements that scientists have created, the nuclei are very large and have many protons. In such nuclei, the tendency of the protons to repel each other overpowers the strong force. Thus the nuclei begin to break down into smaller nuclei and individual particles. This process is called radioactivity. A few natural elements, including uranium, are also radioactive. Nuclear Reactions Radioactivity is an example of a nuclear reaction--a change in the nucleus. It is considered a spontaneous nuclear reaction because it takes place without the action of any outside force. The unstable nuclei continue to give off particles until they reach a stable state. In this way, uranium gradually changes into lead, a lighter and more stable element. Radioactivity was discovered by the French scientist Antoine Henri Becquerel in 1896. As scientists learned more about this process, they discovered that similar reactions can be induced, or made to happen. If a uranium 235 nucleus is struck by a free neutron, for example, it may split. The result will be two nuclei of lighter elements. Some stray neutrons are also released. In early studies, scientists observed that the total mass of the particles left after a nuclear reaction was often less than the mass of the original nucleus. What had happened to the missing mass? It had been converted to energy. (The idea that mass and energy are two forms of the same thing had been proposed by Albert Einstein in 1905. To read more about this idea, see the article Relativity.) In the same way that heavy nuclei can break apart, light nuclei can combine. In both cases, the nuclei are seeking to reach a more stable state--elements with medium mass tend to be the most stable. And in both cases, small amounts of mass can be converted to large amounts of energy. Thus nuclear energy can be released in two types of reactions: fusion (the joining of nuclei) and fission (the splitting of a nucleus.) Nuclear Fusion In nuclear fusion, two light nuclei combine, or fuse, to form a heavier nucleus. When two deuterium nuclei fuse, for example, helium is formed, and energy is given off.
Fusion reactions power the sun and other stars. But these reactions have proved difficult to duplicate in a controlled, continuous way on earth. To fuse, two nuclei must be pushed together. But because each nucleus carries a positive charge, the nuclei tend to repel each other instead. This tendency of the nuclei to repel each other can be overcome by enormous pressures and temperatures of millions of degrees. At these extremes, matter is in a state called plasma, in which atoms break up into free electrons and free nuclei. The particles move so fast in the intense heat that they overcome the tendency to repel each other. Fusion achieved at high temperatures is called thermonuclear fusion. Humans use it in nuclear weapons (see the article Nuclear Weapons). Attempts to use it for peaceful purposes, such as the production of power, have failed. The plasmas required for thermonuclear fusion are difficult to produce and contain. A device called a tokamak, invented in the Soviet Union in 1969, uses a magnetic field to confine the plasma in a doughnut shape. But magnetic confinement, as this method is called, requires extremely strong magnetic fields. These fields generally use more energy than the fusion produces. Another technique, called inertial confinement, uses laser beams or beams of charged particles. The beams are directed at tiny pellets that hold deuterium or another light element. They cause the pellet to heat rapidly, forcing the nuclei together. So far, this technique too has failed. In 2005, the United States and five other nations announced that France had been chosen as the site of the International Thermonuclear Experimental Reactor (ITER). ITER uses a tokamak design. If ITER maintains a fusion reaction, it will become a test power plant. Nuclear Fission In nuclear fission, a single nucleus is split into two nuclei of lighter elements. In the process, a small amount of mass is converted into a great amount of energy. The fission of a given amount of uranium 235 produces about 160,000 times as much energy as the burning of the same amount of coal. Thus the discovery that energy could be produced through nuclear fission was truly monumental.
The fission of a single nucleus would not be of much use. But in the right conditions, fission can develop into a chain reaction. For example, the fission of a uranium 235 nucleus releases two or three neutrons. These neutrons can hit other nuclei of uranium 235 and cause them to break up, releasing energy and neutrons. In this way the reaction builds. For the chain reaction to continue, the uranium 235 must be arranged so that most of the neutrons that are released encounter other nuclei and cause further fission. When material is arranged so that a chain reaction can take place, the arrangement is called a critical assembly. The amount of uranium 235 or other fuel required to keep the reaction going is called the critical mass. In nuclear weapons that make use of fission, the critical assembly is designed to release a sudden, enormous burst of energy. But a critical assembly can also be designed to produce continuous, controllable energy. This is the idea behind a nuclear reactor. The development of nuclear reactors has made fission an important source of energy, chiefly for the production of electrical power. Nuclear Power At an electric power plant, fanlike machines called turbines drive generators that produce electricity. In most plants, these turbines are powered by steam. Some form of energy is needed to boil water and produce the steam. In a traditional power plant, this energy is produced by burning fossil fuels, such as coal or oil. In a nuclear power plant, the source of the energy is a nuclear reactor. Nuclear Reactors Various types of nuclear reactors are used in power production. In all types, however, the central part of the reactor is the core, which contains the fuel, the moderator, and the coolant. Other important parts of the reactor include the control rods and the containment structure. Fuel. The fuel used in a reactor is generally a form of uranium. Uranium ores are mined, and the uranium is separated from the ores at mills near the mines. Natural uranium is a mixture of isotopes. It is less than 1 percent uranium 235, which is the isotope used for fission.
Before uranium is used in U.S. reactors, it is enriched, or processed to increase the level of uranium 235. (Canadian reactors, which have a slightly different design, use natural uranium as fuel.) The fuel is then formed into small pellets and put into long, thin metal rods. As many as 50,000 of these fuel rods, bundled into groups called fuel assemblies, stand upright in the reactor core. One type of reactor, the breeder reactor, produces as well as consumes fuel. In a breeder reactor, free neutrons produced by fission are absorbed by uranium 238, which is transformed into plutonium. Plutonium is highly fissionable and can be used in weapons as well as in reactors. For security and safety reasons, breeder reactors are not used for civilian power production in the United States. France is the only country that uses breeder reactors in this way. Moderator. The moderator is a substance that surrounds the fuel rods. Its purpose is to slow the speed of the free neutrons that are released in fission. By slowing the neutrons, the moderator increases the chance that they will be absorbed by the fuel and cause fission. Most U.S. reactors use ordinary water as a moderator. Canadian reactors use heavy water. (Water is a mixture of hydrogen and oxygen. In heavy water, deuterium takes the place of the ordinary hydrogen.) Besides slowing the free neutrons, ordinary water absorbs some of these particles. Heavy water absorbs fewer free neutrons, so that more neutrons are available to cause fission. This is why Canadian reactors are able to use natural uranium as fuel. A type of reactor common in the former Soviet Union and some other countries uses blocks of graphite (a form of carbon) as a moderator. Breeder reactors use fast neutrons and generally have no moderator. Coolant. The coolant carries away heat produced by fission in the core. In reactors that use ordinary or heavy water as a moderator, the same fluid acts as the coolant. In a boiling water reactor, the heat from the core turns the coolant into steam, which drives the turbines for electric power production. The steam then passes through a condenser, which cools it and turns it back into water. In a pressurized water reactor, the water that acts as the moderator and coolant is kept under pressure so that it cannot boil, even at very high temperatures. This water is used to heat a second supply of water and turn it into steam for driving the turbines.
Reactors that rely on graphite as a moderator must use another substance as a coolant. This may be water or a gas, such as carbon dioxide, that is blown through the core. A common type of breeder reactor uses liquid sodium as a coolant. Control Rods. To produce a steady supply of energy, the chain reaction in a nuclear reactor must be carefully controlled. If too little fission takes place, the reaction will die out. If there is too much fission, the reaction may begin to run out of control. To regulate the chain reaction, nuclear reactors are equipped with control rods that can be lowered into the core. The control rods are made of a material such as the metal cadmium that absorbs free neutrons. The more control rods are lowered, the slower the reaction becomes. In most designs, the control rods will drop into the core automatically in an emergency, to shut the reactor down. Containment. The reactor is housed in a reactor vessel with heavy steel walls. This vessel is generally placed in a containment building with walls, floor, and roof of thick concrete. These structures are designed to prevent the escape of radioactive materials. Safety and Waste Disposal The makeup of the fuel assembly in a reactor makes it virtually impossible that the core could explode like a nuclear bomb. However, should a reactor fail in some way and release radioactive material, exposure to radiation could harm people, animals, and crops over a wide area. Radiation could poison food and water supplies and make its effects felt for years. Thus the enormous energy and the radioactive materials contained in a reactor are sources of concern. The safe handling of nuclear fuel and the wastes produced by nuclear reactors are also important issues. Reactor Safety. Nuclear reactors are designed with many safety features. For example, most reactors have emergency cooling systems that automatically flood the core with cold water if it begins to overheat. Reactor operators are highly trained, and constant watchfulness is part of the operation of all plants. Reactor construction and operation are
closely regulated by governments. (In the United States, the agency that oversees reactors is the Nuclear Regulatory Commission.) Despite these measures, serious accidents have occurred at nuclear reactors. The most feared type of accident involves loss of a reactor's coolant. When this happens, the core can heat to enormous temperatures, and the fuel rods can melt. In a full core meltdown, the molten fuel could burn through the floor of the containment structure and enter the ground, where it could pollute water supplies. So far, this has not happened. In a serious accident in 1979 at the Three Mile Island nuclear plant, near Harrisburg, Pennsylvania, about half the fuel melted. But the accident was brought under control before the fuel broke through the reactor vessel, and little radioactivity was released. The reactor was permanently shut down. An even more serious accident took place in the Soviet Union in 1986, at the Chernobyl power plant near Kiev. The reactor's graphite moderator overheated, causing a series of explosions and fires. The roof was blown off the building, and radioactive material was released into the air. Over 30 people were killed immediately, and hundreds of nearby residents developed severe radiation-related illnesses. In 1999, Japan experienced its worst nuclear accident at a fuel processing plant in the town of Tokaimura. Workers mixed too much uranium at once, setting off an uncontrolled chain reaction. Although it was brought under control, dozens of people were exposed to radiation. Accidents such as these have produced worldwide concern. Designers have proposed several ways to improve safety. One proposal is to build smaller reactors and to submerge them in pools of borated water (water containing boron, which can absorb radiation). If the reactor vessel were to crack, this water would enter the reactor and stop the chain reaction. Human error has been the main cause of nuclear accidents. Much of the daily operation of nuclear plants is handled by computers. The accidents at Three Mile Island, Chernobyl, and Tokaimura showed that it is important to have well-educated operators and well-designed computer systems to help operators make the right decisions in an emergency. Fuel and Waste Handling.
Nuclear fuel travels through many steps from the mine to the reactor and to its eventual disposal. Together, these steps make up the nuclear fuel cycle. Because the fuel is radioactive, each step raises risks of radiation exposure. In recent years, regulations have improved safety. For example, large quantities of mill tailings (material left over from milling operations, in which uranium is separated from its ore) were once allowed to lie in heaps. Dust from the heaps blew away, exposing people for miles around to radiation. Regulations now require these heaps to be covered. The disposal of spent fuel from nuclear reactors poses a more difficult problem. After fuel rods have been in place about three years, most of the uranium 235 is used up. But the rods contain highly radioactive materials, including some unused uranium 235 and plutonium. The fuel rods can be processed to recover this material, and this is done in France. In the United States, commercial reactors were barred from reprocessing fuel in the 1970's, for security and safety reasons. Instead, spent fuel is stored in water pools at the reactors. By the end of 1987, almost 16,000 tons of spent fuel had been stored in this way. The materials in spent fuel are high-level nuclear wastes. These wastes will remain radioactive for thousands of years. Thus people need to find safe ways of storing them. In a plan developed by the U.S. Department of Energy, spent fuel and other high-level nuclear wastes would be stored in deep underground rock formations. Some of the wastes would first be melted together with other materials to form a glasslike substance (a process called vitrification). Besides high-level wastes, reactors produce low-level wastes--building materials, equipment, and clothing that may have absorbed radiation. This material is not as harmful as high-level waste, but there is a lot of it. It is stored at special disposal sites. History Scientists began searching for a way to set off a nuclear chain reaction in the 1930's. Several researchers succeeded in splitting atomic nuclei at this time. But it was two Austrian scientists, Lise Meitner and Otto Frisch, who in 1939 first explained the release of energy obtained in nuclear fission.
During World War II, the United States government supported work in nuclear fission by a number of scientists. The goal of this program, called the Manhattan Project, was to produce an atomic bomb before Nazi Germany could do so. As part of this work, the Italian-born scientist Enrico Fermi headed a team of researchers at the University of Chicago. They created the first artificially produced nuclear chain reaction in 1942. The first use of nuclear energy was in weapons. Scientists were quick to realize nuclear energy's potential for other uses, however. The first nuclear-powered submarine, the USS Nautilus, was launched in 1954. The world's first nuclear power plant began operation in England two years later. A commercial nuclear power plant was built in the United States near Pittsburgh, Pennsylvania, in 1957. That same year, the International Atomic Energy Agency was set up by the United Nations to promote the peaceful use of nuclear energy. This agency, with headquarters in Vienna, Austria, also attempts to prevent the use of nuclear materials for war. During the 1960's and 1970's, utility companies turned increasingly to nuclear power. By the end of the 1980's, more than 100 nuclear plants were providing just under 20 percent of the electricity used in the United States. The percentage of electricity provided by nuclear energy was much higher in some countries--more than 70 percent in France, 66 percent in Belgium, and almost 50 percent in Sweden, for example. In the United States, only a few nuclear plants were built after 1975, however. Interest in nuclear power declined for three reasons. First, demand for electricity did not grow as much as had been expected. Second, the cost of building and running a safe nuclear power plant proved to be very high. Third, public support for nuclear power fell off as concern over the safety and cost of nuclear power plants grew. Concern over these issues has grown in other countries, too. For example, Sweden halted construction of new nuclear power plants after the Three Mile Island accident in 1979. Because it does not pollute the air or use up the world's limited supplies of fossil fuels, nuclear energy continues to have great promise. But its future will depend on how the problems of safety, waste disposal, and cost are solved. Meanwhile, scientists continue to experiment with nuclear fusion. Fusion fuels are more plentiful than fission fuels--deuterium, for example, can be extracted from seawater. Thus fusion might one day provide an almost limitless source of energy. And scientists think that the waste problems associated with fusion would be less severe than those that result from fission. Bala R. Nair Westinghouse Electric Corporation
Indira Nair Carnegie Mellon University How to cite this article: MLA (Modern Language Association) style: Nair, Bala R., and Indira Nair. "Nuclear Energy." The New Book of Knowledge. 2009. Grolier Online. 22 Apr. 2009 <http://expertspace.grolier.com/article?id=a2021270-h&product_id=nbk>. APA (American Psychological Association) style: Nair, B. R., & Nair, I. (2009). Nuclear Energy. The New Book of Knowledge. Retrieved April 22, 2009, from Grolier Online http://expertspace.grolier.com/article?id=a2021270-h&product_id=nbk Chicago Manual of Style: Nair, Bala R., and Indira Nair. "Nuclear Energy." The New Book of Knowledge. Grolier Online http://expertspace.grolier.com/article?id=a2021270-h&product_id=nbk (accessed April 22, 2009). Source: The New Book of Knowledge