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Nuclear Energy ? Essay, Research Paper
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 fiss 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.
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 gaslike 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 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.
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