National Geographic : 1979 Apr
less than the original. The loss in mass turns into energy. The fissioning atom also gives off neu trons, heavy subatomic particles. Under the right conditions these strike other fission able atoms and cause them to split, thus cre ating a chain reaction. A reactor is a remarkable device to stimu late this splitting of nuclei on a grand, but controlled, scale. Resulting energy, as heat, can be harnessed to make steam that drives turbine generators to produce electricity. The scientific principle is simple; putting it into use safely calls for complex, highly sophisticated engineering. If you could look into the heart of a typical pressurized water reactor-the commonest kind in use-you would see the nuclear core, scores of thick bundles about 12 feet long, made up of slender, shiny tubes. Each tube, or fuel rod, is filled with some 200 pellets of enriched uranium (pages 468-9). The pellets are small-about twice as thick as a pencil and slightly more than half an inch long. But they are mighty: The ener gy content of each pellet is about the same as a ton of coal or four barrels of crude oil. The cost? Five to ten dollars. A large reactor operating today holds tens of thousands of fuel rods in which are sealed some eight million uranium pellets weighing about a hundred tons. The capacity of such a reactor is roughly 1,000 megawatts (one mil lion kilowatts) of electricity, enough for a city of 600,000 people. But the life of the core is limited. A third of the fuel must be re placed annually during the expected 30-to 40-year life of the reactor. Once reactor operations begin, the core is surrounded and infiltrated with water thousands of tons-which circulates under high pressure to carry away the intense heat and keep the reactor temperature within limits. The water also moderates, or slows, the flow of neutrons and thus helps control the chain reaction. Core and water are con tained in a heavy steel pressure vessel. It, in turn, is shielded by a steel-and-concrete con tainment structure to prevent radioactivity from escaping into the outside world. Suppose radioactivity does leak. Why is that a matter (Continued on page 466) Troubled Seabrook: Blue-collar workers march in support of the Seabrook reactor in New Hampshire. Protesting against it and other plants, concerned groups have opened persistent debate regarding safety, radiation, and long-term storage of wastes. California and Maine have banned future plants until the storage question is resolved.