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Nuclear p ower is electrical power generated from a nuclear reactor. Electricity is produced by the heat released from the atomic core though a controlled nuclear chain reaction. Waste products include heat and spent nuclear materials.
Nuclear power plants use a nuclear reactor, which is also called an atomic reactor or an atomic pile. Nuclear reactors have seven primary parts. Some of these are systems and some are protective parts. These primary parts are the atomic core, the moderator, control rods, a coolant system, a pressurized vessel, and a biological shield. In addition there are safety systems and a containment unit to house the reactor.
The atomic core is the center of the reactor. It is where nuclear fission takes place. Nuclear fission, which is also called atomic fission, occurs naturally when atoms that are high in the scale of chemical elements decay or split into smaller elements and at the same time give off energy. Albert Einstein’s famous formula E = MC2 says that energy is equivalent to mass times the speed of light squared. The splitting of atoms involves the transformation of small portions of the matter into energy in the nuclear reaction.
When the nucleus of an atom splits, its component protons and neutrons are released. The atom that is split separates into nuclei; free neutrons, photons, and gamma rays are also usually generated. Of the photons, gamma rays are usually the most common form. There may be other particles generated, such as alpha particles and beta particles. The atomic reaction produces energy in the form of gamma rays and kinetic energy in the form of heat. The amount of heat released is millions of times the amount of energy released from chemical reactions that occur from the burning of fossil fuels such as coal, gas, or oil.
Radioactive atomic materials release neutrons naturally. There are radioactive materials that are not used as nuclear fuel in nuclear reactors. Some of these are the waste products of nuclear power plants, while others, such as radium, are used in medicine. Atomic energy was recognized shortly after the discovery of radioactivity. In the case of radium, it was noticed that its decay created heat that naturally surrounded quantities of it. In 1939 the discovery of uranium fission made possible the development of the atomic bomb and, after World War II, the development of nuclear power.
The radioactive material used in atomic reactions is generally referred to as special nuclear material (SNM). The term comes from a definition used in the United States Atomic Energy Act. The special nuclear materials used most commonly in nuclear reactors are plutonium, uranium 233 (U233) and uranium 235 (U235). Uranium is widely distributed in the earth. It was originally formed in a star that was a part of the material that formed the earth. About 99 percent of the uranium on earth is uranium 238 (U238). Less than one percent is U235.
A nuclear reaction, if uncontrolled in a unit of special nuclear materials, will cause a chain reaction. The neutrons that are causing radioactive decay naturally will continue to increase rapidly so that as a neutron strikes the nucleus of an individual atom the split atom also releases free neutrons. These increase rapidly in number releasing increasing quantities of energy.
Creating an uncontrolled chain reaction of nuclear fission is the principle used to create an atomic explosion. Oddly, a nuclear weapon is designed so that a controlled set of induced atomic reactions is used to create an uncontrolled chain reaction in unstable U235 resulting in a huge explosion with its attendant atomic blast, radiation, and enormous heat.
If an uncontrolled reaction occurs in a nuclear power plant an atomic meltdown can occur. The atomic core of the reactor becomes so hot that it melts. If the pressure vessel is not able to contain all
of the heat and steam produced, then a terrible accident can occur. This happened at Chernobyl in the Ukraine in 1986. The radioactive materials released were spread over northern Europe. The area around the Chernobyl nuclear power plant for some miles distant, including the city of Chernobyl, had to be abandoned. The health consequences since have been very serious: A much higher rate of cancers, birth defects, and other illnesses caused by exposure to high amounts of radiation.
The nuclear reactions generated in an atomic core are controlled. The core is part of a nuclear power plant that contains the nuclear fuel; nuclear fission reactions take place in the core. The core is composed of nuclear material-usually pellets of uranium oxide (UO2). These are put into tubes to form fuel rods, and the fuel rods are then arranged into fuel assemblies in the reactor’s core.
The core’s reactions are managed by control rods, which regulate the rate of chain reactions. Control rods are usually made of boron, cadmium, cobalt, europium, gadolinium, hafnium, indium, silver, or other materials that can absorb neutrons without becoming fissionable. These elements capture neutrons at different rates so their use as control rods is guided by the spectrum of the kind of nuclear fission reactions the atomic core is designed to generate. The reaction is started, slowed, and stopped by means of the control rods. The rods are slowly removed from their positions in the core. Nuclear reactors are designed so that the rods effectively cover the top and sides of the fuel rods. The fuel rods rest on a floor of neutron absorbing material.
As the control rods are pulled upward from covering the fuel rods, the nuclear reaction begins to take place. The neutrons are no longer absorbed by the moderator material in the control rods. This allows free neutrons to create fission reactions and more neutrons, which then drive the reaction. By manipulating the control rods the reactor is turned on or off. The effect is like that of allowing more oxygen to enter a fire so the fire burns hotter. The removing of the control rods allows the atomic reaction to occur. The further the rods are removed the faster the reaction increases.
Nuclear power plants are operated with a large number of monitors that measure the heat being generated. Special instruments measure radiation levels in the core as well. The moderator in mater reactors is a material used to stimulate nuclear chain reactions. Moderators may be composed of several different types of materials. Most moderators have used graphite or heavy water. Water is hydrogen oxide (H2O); however, heavy water is the common name of an uncommon isotope of hydrogen called deuterium.
The moderator material captures the free neutrons and allows them to be evenly available to the fissionable material. U238 is not atomically unstable like U235 so it will normally absorb the free neutrons. The principle is that when an atom of U235 is hit by a neutron it creates a high probability of fission. The U235 atom splits, and on average produces about two or more neutrons. The effect is to create a self-sustaining chain reaction. No additional neutrons are needed.
The fissionable reactions can be held constant if the surplus neutrons escape from the system. However, if they are allowed to increase, the reactions diverge. In nature the probability that a high energy neutron will directly cause fission in another atom is low. To create the fission reactions in a nuclear core, enriched uranium is used. An increased amount of U235, which is expensive to produce, is consolidated in the nuclear fuel. What the moderator materials do is slow down fast neutrons so that slower neutrons have a better opportunity to be captured in the core’s fuel. The moderator becomes a medium for reducing the velocity of fast neutrons so that they are changed into thermal neutrons that can sustain nuclear chain reactions.
Nuclear power plants are built and named after the kind of moderator material used. The most common forms of moderators are water and heavy water. Light water reactors use water; heavy water reactors use deuterium. To be useful as a moderator a material should have the lowest atomic number possible and resist absorbing neutrons to a very high degree. Besides deuterium and graphite, beryllium and hydrocarbons have been studied for possible use. Deuterium has the disadvantage of being expensive; huge quantities of water have to be processed to capture the deuterium atoms.
Types of Reactors
Another way of naming nuclear power plants is after the type of reactor used. Nuclear fission reactors use a critical mass of fissionable material. There are currently several types of these reactors: Generations I, II, and III, and other subtypes. All of these types use a reactor cooled by pressurized water. The water is under immense pressure, which allows it to absorb greater quantities of heat than if it were at open atmospheric pressure. Most nuclear power plants have pressurized water reactors. In the opinion of nuclear experts, these are the reactors with the most reliable technology. Some of these are fastspectrum and others are thermal-spectrum reactors. Usually the fast-spectrum reactors produce waste with a shorter half-life (the time it takes for half of the radioactive material to completely decay).
Fast reactors can also be designed to act as breeders of even more radioactive material. Thermal reactors cannot produce reusable radioactive material. Most reactors in use in power plants are thermal-spectrum reactors and pressurized water reactors. These types of reactors have proved to be the safest and the most reliable, even though the Three Mile Island plant in Pennsylvania was the site of an accident.
Boiling water reactors are water pressure reactors, but the pressure is less and the water is allowed to boil in the reactor. A problem with this type of reactor is that the boiling water in the reactor stresses the components. This increases the risk of an accident that will permit radioactive water to escape. However, the use of this type of thermal reactor is widespread.
Another type of reactor is the Pressurized Heavy Water Reactor (PHWR). This type of reactor uses heavy water as a coolant and also as a moderator. This type of reactor uses hundreds of pressure tubes to hold the fuel, rather than housing it in a single large containment vessel. An advantage of the Pressurized Heavy Water Reactor is that it does not have to be taken off line to be refueled. Canada has developed this type of reactor and has sold units to a number of countries, including Argentina, China, India, Pakistan, Romania, and South Korea.
The former Soviet Union built a number of plutonium reactors. These have proven to be dangerous and unstable. They are water cooled, but use graphite as a moderator. An advantage is that they can be refueled while in operation. However, they are too large to have containment buildings. The Chernobyl plant was of this design. Gas Cool Reactors were been developed in Great Britain. They use a graphite moderator with carbon dioxide as a coolant and have high thermal energy efficiency.
Reactor designs have been advancing since the first reactor went on line in the 1950s. A Generation IV reactor design is being developed. This type of reactor will be like a light water reactor, but it will allow the water to be heated to a critical level so that the thermal efficiency is very high. The critical level is the point at which liquid water under pressure acts like a gas.
Some reactors are liquid metal fast breeder reactors. The liquid metal acts as both moderator and as coolant. There are two types-lead-cooled and sodium-cooled. Several reactors using liquid metal have been built in France. Another type of reactor is the radioisotope thermoelectric generator. It produces heat through passive radioactive decay. Space probes, unmanned lighthouses, some pacemakers, and other devices have been designed to use micro nuclear power plants.
The coolant system in a nuclear power plant distributes the heat generated in the nuclear reactions to where it can be used. The other systems of the nuclear power plant use the heat generated to cool the nuclear reactor and to produce electricity. Cooling the reactor is vital to avoid major accidents. Materials used as coolants in nuclear reactors include gases, liquids, and liquid metals. In light water reactors the moderator also functions as coolant. Heavy water reactors use a different system.
The pressure vessel or a set of pressure tubes hold the core in a nuclear power plant. The core is covered by a very strong vessel. Usually some kind of robust steel is used to contain the reactor core, the moderator, and in systems where the moderator is also the coolant, the coolant as well. Sometimes the pressure vessel is a series of pressure tubes. The tubes hold the fuel and move the coolant through the moderator.
The electrical production in a nuclear power plant comes from steam generation. The portion of the cooling system that diverts heat delivers it to sources of water that can be converted to steam. The steam is used to drive turbines. It can also be used to do mechanical work as well. The turbines in nuclear ships drive the ship’s propeller blade as well as supplying electrical power. Nuclear power reactors have been used by several of the world’s navies; submarines and aircraft carriers have been atomic powered since the 1950s.
The containment structure surrounds the reactor core. It is designed to protect the core from outside intrusion. Storms or other invasive disruptions of the core’s reactions could cause a major nuclear accident. Another important function of the containment structure is to act as a biological shield to protect the outside world from the deadly effects of radiation. The material used to make the containment is usually very thick concrete and steel. The structure is often more than a yard (meter) thick. This thickness is needed if a malfunction or accident should occur.
The safety system is really a series of systems to handle emergencies so that serious accidents can be prevented. In general the safety systems act to stop the atomic reaction or cool the core. Safety rods in the core are a very important feature. They will stop a reaction very rapidly and will be automatically inserted into the core if a rapid increase or abnormal amount of neutrons is detected.
Another safety action to stop the atomic reaction is the employment of samarium oxide balls. The balls are made of oxygen and samarium. If dropped into a core they will immediately stop the reaction by absorbing the neutrons. Emergency core cooling systems seek to prevent an accidental meltdown. If the normal coolant is lost or if the coolant system fails then the emergency cooling system can be employed. It floods the core with water that absorbs heat to prevent a nuclear core meltdown.
Benefits and Risks
Today a growing portion of the world’s electrical supply is coming from nuclear power plants. Nuclear power plants are supplying 15 percent of the world’s electricity. In the United States 20 percent of the electrical supply comes from nuclear power plants. In France the percentage is 80 percent.
The great benefit of nuclear power is that enormous quantities of electricity can be produced at a lower cost. They also have the benefit that they do not produce smoke or carbon dioxide, which contributes to global warming. However, the great drawbacks are the waste products and the threat of destruction posed by accidents and nuclear weapons.
The enormous quantities of heat generated by nuclear power plants can be handled with appropriate engineering. Uses of waste heat include cogeneration or supplying heat to winter agriculture in northern climates or other practical uses. The key problem is preventing any contamination by radioactive materials, because these can pose serious health problems.
The problem of storing nuclear waste is a major issue. Temporary sites where materials are buried must be supervised to prevent leakage of radioactive materials. Eventually a permanent site where radioactive materials can be kept for thousands of years has to be developed.
The fact that nuclear reactors can be designed to produce nuclear materials that can be used in nuclear weapons is a grave concern to the world. As the knowledge of nuclear fission spreads it is becoming easier for countries or even private corporations to develop these. Many supporters of nuclear power argue that the knowledge is now so widespread that to not use atomic power because it will allow nuclear fissionable materials to be created is folly.
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