The term nuclear power, which describes any method of performing work that makes use of nuclear fission or nuclear fusion reactions, 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. More than 440 nuclear reactors, devoted to the manufacture of electricity, are in operation globally.
The world first experienced nuclear power with the detonation of two fission (atomic) bombs over the Japanese cities of Hiroshima and Nagasaki in 1945, as part of a U.S. effort to end World War II. Although these events caused immeasurable destruction, scientists and laypersons hoped the power of nuclear energy could be harnessed for human benefit. Beginning in the 1970s, intense political opposition to nuclear power arose in various nations, including the United States, as some technical problems associated with nuclear power became known and were not satisfactorily resolved despite assurances from proponents. After three decades of progress in the development of controlled nuclear power, interest in this energy source leveled off and, in many nations, declined, with strong popular opposition continuing today. Nuclear advocates regard opponents as irrationally fearing nuclear technology; nuclear opponents see advocates as irresponsibly supporting use of unreliable nuclear technology.
Background and scientific foundations
The first nuclear reactor, constructed under the direction of Italian physicist Enrico Fermi (1901–1954), was built during World War II as part of the Manhattan Project to build an atomic bomb. Until December 2, 1942, when the 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.
Today's nuclear power plant is a system in which some of the energy released by nuclear fission is used to generate electricity. Such plants contain 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, with the products of the reaction including 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, whereas 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 (for example, 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, but 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, which then turns a generator that produces electrical energy.
The high expense of constructing a modern nuclear power plant—three to four billion dollars, in the United States—partially reflects the wide range of safety features needed to protect against various possible mishaps, especially those that could release to the environment radioactive substances from the plant. (Small special-purpose reactors, like 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.) An example of some of those features, which are incorporated into the reactor core itself, include the reactor's fuel being sealed in a protective coating of zirconium alloy, called a cladding; it helps retain heat and radioactivity within the fuel, preventing it from escaping into the plant itself.
Nuclear plants are required to include an elaborate safety system to protect against the most serious potential problem of all, loss of coolant. If such an accident occurs, the reactor core might melt down, possibly breaching the structures that contain it and releasing radioactive materials to the rest of the plant and even to the outside environment. To prevent such accidents, the pipes carrying the coolant to and from the reactor are required to be thick and strong. In addition, back-up supplies of the coolant must be available to replace losses in case of a leak.
The entire plant (in much of the world, including Europe and the United States) is required to be encased within a dome-shaped containment structure made of steel-reinforced concrete several feet thick. Such structures are designed to prevent the release of radioactive materials if an accident occurs within the reactor. The containment also serves as a barrier against efforts to deliberately damage the reactor from the outside, such as crashing a hijacked airplane into it. Another safety feature is a system of high-efficiency filters that are designed to trap microscopic particles of radioactive materials that might otherwise be vented to the atmosphere.
Nuclear power plants differ in the methods they use for transferring heat produced in the reactor to the electricity generating unit. The simplest design is the boiling water reactor plant (BWR), in which coolant water surrounding the reactor is allowed to boil and form steam, which is then piped directly to turbines that produce rotary mechanical power that turns electrical generators. A different type of plant, one popular in Great Britain for many years, uses carbon dioxide as a coolant, passing through the reactor core and absorbing heat produced by fission reactions, before being 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.
The general public has long had serious concerns about the safe use of nuclear power plants to produce electricity. A few major disasters have perpetuated the fear of nuclear power plants failing catastrophically, with their operators losing control of the reactor with unpredictable consequences. The most serious disasters were the explosion at the Chernobyl nuclear power plant in Ukraine in 1986, which exposed hundreds of thousands of nearby residents to dangerous levels of radiation, and the nuclear accident in Fukushima, Japan, that occurred after the Tohoku earthquake and tsunami in 2011. Such disasters contributed to Germany's plan to phase out its nuclear power plants by 2022.
In the United States, enthusiasm for nuclear power in the 1950s and 1960s faded by 1980 due to widespread public unease about nuclear dangers, a vigorous antitechnology and antinuclear activist movement, and investor concerns over the high financial cost of nuclear power. Such concerns were compounded by a nuclear incident at the Three Mile Island plant in 1979 when the reactor suffered a partial meltdown and radioactive gas escaped into the environment. Until 2012, no new nuclear power plants had been ordered or approved by regulators in the United States in over thirty years.
Although critics of nuclear power worry about the amount of radioactivity released by nuclear power plants each day, studies show that nearby residents are exposed to radiation at around the same level as a person living many miles away. Nevertheless, some epidemiological evidence suggests that the small amounts of radioactive material released during routine operation may have detectable medical effects on nearby populations, although these claims are disputed intensely.
Nuclear waste management
Perhaps the single most troubling issue for the nuclear power industry is waste management. Nuclear wastes are classified into two general categories, low-level wastes and high-level wastes. The former consists 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 different because after a while, 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 to produce nuclear weapons or for use as a nuclear fuel), are considered high-level wastes. For more than two decades, the U.S. government has attempted to develop a plan for the storage of high-level nuclear waste to no avail. As a result, the wastes slated for burial at a permanent national storage facility are held in storage on the grounds of nuclear facilities throughout the United States.
Many scientists believe that the ultimate solution to the world's energy problems may lie 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 United States to destroy the city of Hiroshima. As with fission, scientists and nonscientists alike express hope that fusion reactions will 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 no significant breakthroughs have occurred.
Issues and developments
Proponents of nuclear power argue that the construction of new power-plants would help meet growing electricity demand. They contend that nuclear power is cleaner than coal-fired plants, which release air pollution and greenhouse gases, contributing to global climate change. Opponents of nuclear power contend that nonnuclear technologies, particularly renewable energy such as solar power, are the best sources to pursue.