Nuclear Power

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.