Nuclear Power
Nuclear Power
Nuclear power is any method of doing work that makes use of nuclear fission or nuclear fusion reactions. In its broadest sense, the term refers to both the uncontrolled release of nuclear energy, as in fission or fusion weapons, and to the controlled release of energy, as in nuclear power plants. Most commonly, however, the expression “nuclear power” is reserved for the latter. Approximately 430 nuclear reactors devoted to the manufacture of electricity are operating worldwide.
The world’s first exposure to nuclear power came with the detonation of two fission (atomic) bombs over the Japanese cities of Hiroshima and Nagasaki in 1945, events that coincided with the end of World War II. (It has long been assumed by many historians and members of the general public that these atomic bombings were necessary to produce a Japanese surrender and to forestall a United States invasion of Japan; this belief has been disputed by some historians in recent years.) A number of scientists and laypersons perceived an optimistic aspect of these terrible events; they hoped that the power of nuclear energy could be harnessed for human good. Those hopes have been realized, but to only a modest degree. Starting in the 1970s, intense political opposition to nuclear power arose in many nations, including the U.S. Some technical problems associated with the use of nuclear power have never been satisfactorily solved, although proponents of this technology argue that none are insurmountable. After three decades of progress in the development of controlled nuclear power, interest in this energy source has leveled off and, in many nations, declined. Indeed, strong popular opposition to nuclear power exists in many countries today, and nuclear power has become a locus of political struggle, with citizens’ groups (supported by some scientists) ranged, typically, against industry and government experts. Nuclear advocates see nuclear opponents as irrationally fearing nuclear technology; nuclear opponents see nuclear advocates as irresponsibly advocating use of unreliable nuclear technology. The nuclear–power issue remains a highly contentious one in many nations.
The nuclear power plant
A nuclear power plant is a system in which some of the energy released by nuclear fission is used to generate electricity. Every such plant contains four fundamental elements: reactor, coolant system, electrical–power generating unit, and safety system.
The source of energy in a nuclear reactor is a fission reaction in which neutrons collide with nuclei of uranium–235 or plutonium–239 (the fuel), causing them to split apart. The products of a fission reaction include not only energy but also new elements (known as fission products) and free neutrons. A constant and reliable flow of neutrons is insured in the reactor by a moderator, which slows down the speed of neutrons, and by control rods, which limit the number of neutrons available in the reactor and, hence, the rate at which fission can occur. In a nuclear weapon, the fission chain reaction, once triggered, proceeds at an exponentially increasing rate, resulting in an explosion; in a nuclear reactor, it proceeds at a steady, controlled rate. Most commercial nuclear power plants are incapable of undergoing an explosive nuclear chain reaction, even should their safety systems fail; this is not true of all research reactors (e.g., some breeder reactors).
Energy produced in the reactor is carried away by means of a coolant such as pressurized water, liquid sodium, or carbon dioxide gas. The circulating coolant absorbs heat in the reactor; once outside the reactor, it is allowed to boil or the heat it contains is used to boil water in a secondary loop. Steam produced in either of these ways is then piped into the electrical generating unit, where it turns the blades of a turbine. The turbine, in turn, turns a generator that produces electrical energy.
The high cost of constructing a modern nuclear power plant—three to four billion dollars, in the U.S.—reflects in part the wide range of safety features needed to protect against various possible mishaps, especially those which could release to the environment any of the plant’s inventory of radioactive substances. (Small special–purpose reactors, such as those used to power nuclear submarines or aircraft carriers, have different costs and technical features from the large, land–based reactors used to supply electrical grids.) Some of those features are incorporated into the reactor core itself. For example, all of the fuel in a reactor is sealed in a protective coating made of a zirconium alloy. The protective coating, called a cladding, helps retain heat and radioactivity within the fuel, preventing it from escaping into the plant itself.
Every nuclear plant is also required to have an elaborate safety system to protect against the most serious potential problem of all, loss of coolant. If such an accident were to occur, the reactor core might melt itself down, possibly breaching the structures which contain it and releasing radioactive materials to the rest of the plant and, perhaps, to the outside environment. To prevent such an accident, the pipes carrying the coolant to and from the reactor are required to be very thick and strong. In addition, back–up supplies of the coolant must be available to replace losses in case of a leak.
On another level, the whole plant itself (in much of the world, including Europe and the U.S.) is required to be encased within a dome–shaped containment structure made of steel–reinforced concrete several feet thick. The containment structure is designed to prevent the release of radioactive materials in case of an accident within the reactor. The containment also serves against as a barrier against efforts to deliberately damage the reactor from outside, as by firing shoulder–launched missiles at the reactor or crashing a hijacked airplane into it.
Another safety feature is a system of high–efficiency filters through which all air leaving the building must pass. These filters are designed to trap microscopic particles of radioactive materials that might otherwise be vented to the atmosphere. Other specialized devices and systems have also been developed for dealing with other kinds of accidents in various parts of the power plant.
Types of nuclear power plant
Nuclear power plants differ from each other primarily in the methods they use for transferring heat produced in the reactor to the electricity generating unit. Perhaps the simplest design of all is the boiling water reactor plant (BWR), in which coolant water surrounding the reactor is allowed to boil and form steam. This steam is then piped directly to turbines, which produce rotary mechanical power that turns electrical generators. A very different type of plant is one that was popular in Great Britain for many years; this design that carbon dioxide as a coolant. In this type of plant, carbon dioxide gas passes through the reactor core, absorbs heat produced by fission reactions, and is piped into a secondary system where it gives up some of its energy to water, which begins to boil. This steam is then used to power a turbine and generator.
Safety concerns
In spite of all the systems developed by nuclear engineers, the general public has long had serious concerns about the use of such plants as sources of electrical power. In France, for example, where more than half of the electrical power supply comes from nuclear power plants, public support for nuclear power has declined in recent years; France has now placed a moratorium on the construction of new plants. Germany, which relies on nuclear power for approximately 30% of its electricity, has decided to phase out all of its nuclear power plants as quickly as it can obtain replacement power from other sources. In the U.S., enthusiasm for nuclear power in the 1950s and 1960s faded in the 1970s, giving way to widespread—but often unscientific or ill–founded—public unease about nuclear dangers, a vigorous antitechnology and antinuclear activist movement, and investor concern over the high monetary cost of nuclear power; no new nuclear power plants have been ordered in the U.S. for over 20 years. The nuclear–power industry remains strongest in Asia, where approximately 25 new plants are now under construction.
One concern about nuclear power plants, of course, is an echo of the world’s first exposure to nuclear power, the atomic bomb blasts. Many people fear that a nuclear power plant may go out of control and explode like a nuclear weapon. In spite of experts’ insistence that such an event is impossible, a few major disasters have perpetuated the fear of nuclear power plants exploding or failing catastrophically in some other mode. Although commercial nuclear power plants cannot explode, they have a demonstrated potential to pass out of the control of their operators, with unpredictable consequences. By far the most serious of those events was the explosion that occurred at the Chernobyl nuclear power plant near Kiev in the Ukraine in 1986.
On April 16 of that year, one of the four power–generating units in the Chernobyl complex exploded, blowing the top off the containment building. (The explosion was caused by hydrogen gas released by the overheating core; it was not a nuclear explosion.) Hundreds of thousands of nearby residents were exposed to dangerous levels of radiation and were evacuated from the area. Radioactive clouds released by the explosion were detected downwind in Scandinavia and western Europe. More than a decade later, the remains of the Chernobyl reactor remain far too radioactive for anyone to spend more than a few minutes near the former reactor core. The Soviet government had, of course, always insisted that such a disaster was impossible. In the U.S., the most famous nuclear incident to date is the accident at the Three Mile Island nuclear plant in 1979. The reactor suffered a partial meltdown that was contained by the giant, pill–shaped steel vessel containing the core; some radioactive gas was released to the environment, and a hydrogen explosion raised pressures inside the containment dome to within a few pounds per square inch of the dome’s design pressure. Again, experts had reassured the public that the chances against such an event were literally astronomical—comparable to those of a giant meteor striking a major city.
Critics of nuclear power also worry about the amount of radioactivity released by nuclear power plants on a day–to–day basis. This concern is probably of less importance than is the possibility of a major disaster; studies have shown that nuclear power plants are so well shielded that the amount of radiation to which nearby residents are exposed is no more than that of a person living many miles away. Nevertheless, some epidemiological evidence hints that the small amounts of radioactive material released during routine operation may have detectable medical effects on nearby populations. These claims are, of course, intensely disputed.
In any case, safety concerns in the United States have been serious enough to essentially bring the construction of new plants to a halt in the last decade. Licensing procedures are now so complex and so expensive that few industries are interested in working their way through the bureaucratic maze to construct new plants. However, it should also be noted that orders for new nuclear power plants in the U.S. had practically ceased in the late 1970s, years before the Chernobyl or Three Mile Island accidents or the regulatory response to them. Some analysts argue that the stagnation of the nuclear–power industry should be attributed primarily to economics, rather than to political opposition.
Nuclear waste management
Perhaps the single most troubling issue for the nuclear power industry is waste management. Nuclear wastes can be classified into two general categories, low–level wastes and high-level wastes. The former consist of materials that release a relatively modest level of radiation and/or that will soon decay to a level where they no longer present a threat to humans and the environment. Storing these materials in underground or underwater reservoirs for a few years or in some other system is usually a satisfactory way of handling these materials.
High-level wastes are a different matter. After a period of time, the fuel rods in a reactor are no longer able to sustain a chain reaction and must be removed. These rods are still highly radioactive, however, and present a serious threat to human life and the environment that can be expected to last for tens of thousands of years. These rods and any materials derived from them (as, for example, during chemical dismantling of the rods to extract their plutonium for the production of nuclear weapons or for use as a nuclear fuel), are considered high-level wastes.
For more than two decades, the United States government has been attempting to develop a plan for the storage of high-level nuclear wastes. At one time, the plan was to bury the wastes in a salt mine near Lyons, Kansas. Objections from residents of the area and other concerned citizens made that plan infeasible. More recently, the government decided to construct a huge crypt in the middle of Yucca Mountain in Nevada for the burial of high-level wastes. The government insists that Yucca Mountain is safe and will eventually become the long–term storage site for the nation’s high-level radioactive wastes. Until the site is put into full operation, however (if it is), the wastes slated for burial in it are held in “temporary” storage on the grounds nuclear power plants throughout the United States.
History
The first nuclear reactor was built during World War II as part of the Manhattan Project to build an atomic bomb. This reactor was constructed under the direction of Italian physicist Enrico Fermi (1901–1954) in a large room beneath the squash courts at the University of Chicago. Until the day on December 2, 1942, when the Chicago reactor was first put into operation, scientists had relied entirely on mathematical calculations to determine the effectiveness of nuclear fission as an energy source; thus, the scientists who constructed the first reactor were taking an extraordinary chance.
That reactor consisted of alternating layers of uranium and uranium oxide with graphite (carbon) as a moderator. Cadmium control rods were used to control the concentration of neutrons in the reactor. Since the various parts of the reactor were constructed by piling materials on top of each other, the unit was at first known as an “atomic pile.”
Nuclear fusion
Many scientists believe that the ultimate solution to the world’s energy problems may be in the harnessing of nuclear fusion. A fusion reaction is one in which two small atomic nuclei combine with each other to form one larger nucleus. For example, two hydrogen nuclei may combine with each other to form the nucleus of an atom known as deuterium, or heavy hydrogen. Fusion reactions are responsible for the production of energy in stars. Most commonly, four hydrogen atoms fuse in a series of reactions to form a single helium atom. An important byproduct of these reactions is the release of an enormous amount of energy. By weight, a fusion reaction releases many times more energy than does a fission reaction.
The world was introduced to the concept of fusion reactions in the 1950s when first the United States and then the Soviet Union exploded fusion (hydrogen) bombs. The energy released in the explosion of each such bomb was more than 1,000 times greater than the energy released in the explosion of a single fission bomb such as that used by the U.S. to destroy the city of Hiroshima.
KEY TERMS
Cladding —A material that covers the fuel elements in a nuclear reactor in order to prevent the loss of heat and radioactive materials from the fuel.
Containment —Any system developed for preventing the release of radioactive materials from a nuclear power plant to the outside world.
Coolant —Any material used in a nuclear power plant to transfer the heat produced in the reactor core to another unit in which electricity is generated.
Generator —A device for converting kinetic energy (the energy of movement) into electrical energy.
Neutron —A subatomic particle that carries no electrical charge and that has a mass of zero.
Nuclear fission —A reaction in which a larger atomic nucleus breaks apart into two roughly equal, smaller nuclei.
Nuclear fusion —A reaction in which two small nuclei combine with each other to form one larger nucleus.
Nuclear pile —The name given to the earliest form of a nuclear reactor.
Turbine —A device consisting of a series of baffles mounted on a wheel around a central shaft used to convert the energy of a moving fluid into the energy of mechanical rotation.
As with fission, scientists and nonscientists alike expressed hope that fusion reactions could someday be harnessed as a source of energy for everyday needs. This line of research has been much less successful, however, than research on fission power plants. In essence, the problem has been to find a way of producing, in a controlled, sustainable fashion, the very high temperatures (millions of degrees Celsius) needed to sustain fusion. Optimistic reports of progress on a fusion power plant appear in the press from time to time, but some authorities now doubt that fusion power will ever be an economic reality.
The future of nuclear power
Proponents of nuclear power argue that a fresh round of power–plant construction should be undertaken to meet growing electricity demand. Coal–fired plants presently produce most of the world’s electricity, but release air pollution and greenhouse gases; nuclear power, its advocates argue, does not release these substances. Given the threat of global climate change, it follows that it is of utmost importance to build more nuclear power plants. Opponents of nuclear power respond that greater reductions in air pollution and greenhouse–gas emissions could be achieved by spending the same money on non-nuclear technologies (i.e., efficiency improvements and renewable energy sources such as solar power). They also argue that a massive increase in the number of nuclear power plants bring increased risks for nuclear accidents and for proliferation of nuclear weapons to states that do not as yet have them. Globally, nuclear power is approximately stagnant as of 2006; wind power is being deployed more rapidly than nuclear power, and at a fraction the cost per kilowatt–hour delivered. Whether it is about to undergo the renaissance urged by its advocates or the accelerated phaseout urged by its opponents may not be apparent for some years.
See also Accelerators; Atomic models; Bioterrorism.
Resources
BOOKS
Caldicott, Helen. Nuclear Power is Not the Answer. New York: New Press, 2006.
Ramsey, Charles B. Commercial Nuclear Power: Assuring Safety for the Future. Charleston, SC: BookSurge Publishing, 2006.
Richard, L., and Georges Charpak. Megawatts and Megatons: The Future of Nuclear Power. University of Chicago Press, 2002.
PERIODICALS
Ahearne, J.F. “Intergenerational issues regarding nuclear power and nuclear waste.” Risk Analysis 20 (2001): 763–770.
Hoffert, Martin, et al. “Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet.” Science vol. 298, no. 5595 (Nov. 1, 2002): 981–987.
David E. Newton
Larry Gilman