The term nuclear power refers to any method of energy transfer that makes use of nuclear fission or fusion reactions. In its broadest sense, the term refers to both the uncontrolled release of energy (as in fission or fusion weapons) and to the controlled release of energy (as in a nuclear power plant). Most commonly, however, the expression “nuclear power” is reserved for the latter of these two instances. Nuclear fission power plants are a technology that has proven economically feasible but is plagued by many safety and environmental concerns. The economic feasibility of nuclear fusion power plants, on the other hand, has not been established, but the safety and environmental concerns are much less than with nuclear fission.
Nuclear fission is a process in which the nucleus of an atom splits, usually into two pieces. When a fission reaction occurs, fragments fly apart with a large release of energy. This energy includes the kinetic energy of the fragments as well as released photons, usually in the form of gamma rays. The fission reaction was the basis of the atomic bomb, which was developed by the United States during World War II. After the war, controlled energy release from fission was applied to the development of nuclear reactors. Reactors are utilized for production of electricity at nuclear power plants, for propulsion of ships and submarines, and for the creation of radioactive isotopes used in medicine and industry.
A fission nuclear power plant contains four fundamental elements: the reactor, coolant system, electrical power generating unit, and the 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 any fission reaction include not only huge amounts of energy, but also waste products (known as fission products) and additional neutrons. A constant and reliable flow of neutrons is insured in the reactor by means of a moderator–which slows down the speed of neutrons–and control rods–which control the number of neutrons available in the reactor and, hence, the rate at which fission can occur.
The first nuclear reactor was built during World War II as part of the Manhattan Project to build an atomic bomb. The reactor was constructed under the direction of Enrico Fermi in a large room beneath the squash courts at the University of Chicago. The reactor consisted of alternating layers of uranium and uranium oxide with graphite 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.” The moment at which Fermi directed the control rods to be withdrawn occurred at 3:45 p.m. on December 2, 1945, and that date can legitimately be regarded as the beginning of the age of controlled nuclear power in human history.
Energy produced in reactors is carried away by means of a coolant–a fluid such as water, liquid sodium, or carbon dioxide gas. The fluid absorbs heat from the reactor and then begins to boil itself or to cause water in a secondary system to boil. 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, powers a generator that produces electrical energy.
The high cost of constructing a modern nuclear power plant reflects in part the enormous range of safety features needed to protect against various possible mishaps. 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 could melt down, releasing radioactive materials to the rest of the plant and, perhaps, to the outside environment. To prevent such an accident from happening, the pipes carrying the coolant are required to be very thick and strong. In addition, backup supplies of the coolant must be available to replace losses in case of a leak.