How It Works

As plumbing goes, nuclear power plants exceed Rube Goldberg's wildest fantasies. The basic idea sounds simple--unstable heavy atoms, like those of uranium 235, break up (fission). Scattered in all directions are electrically neutral particles called neutrons as well as fission products such as shortlived radioactive xenon, krypton and iodine. The neutrons hit still other atoms like errant billiard balls in a chain reaction that produces heat. But obtaining useful energy from this process can be extremely complex. Pennsylvania's Three Mile Island nuclear plant has two pressurized water reactors. Such reactors are based on a design pioneered for nuclear submarines by the redoubtable Admiral Hyman Rickover.

The reactor's power center is its fuel core. Housed in a pressure-cooker-like reactor vessel, the core is filled with pellets of fissionable uranium packed in bundles of thin cylindrical zircaloy rods. Inserted into the core are still other rods, usually made of cadmium or boron, which absorb and retain neutrons given off by the uranium atoms--in effect, stopping the billiards and regulating the intensity of the reaction. To start the reactor, the control rods are raised to precisely calculated levels. The chain reaction begins, converting mass into energy and producing great quantities of heat in the process. That raises the temperature of the water surrounding the core to nearly 600DEG F. Under high pressure this water is carried off by the cooling system's primary loop, a complex system of pipes, to a heat exchanger called a steam generator. The heat is transferred from the radioactive water of the primary loop to the uncontaminated water of the separate secondary loop, where the water is quickly heated to steam that drives a turboelectric generator, which in turn produces electricity.

At the slightest hint of trouble, the reactor's computer is programmed to drop the control rods back into the core. That curtails the core's chain reaction, but heat is still given off. If the core's temperature rises precipitously as a result of the problem, possibly a loss of cooling fluid (a "blowdown"), the computer will activate an emergency core cooling system. That system should quickly dump thousands of gallons of water on the hot core, preventing what has become known as a "meltdown," in which the fuel melts through the floor of the containment building into the ground and possibly erupts in a geyser of steam and debris upon hitting the ground water, releasing a radioactive cloud into the air. As the final precaution, the reactor and primary loop are shielded by a thick concrete containment dome, which should prevent the venting of any radioactivity into the atmosphere--as long as a meltdown does not occur and if there are no other mishaps or blunders. Obviously, at Three Mile Island, these fail-safe systems somehow failed.

This file is automatically generated by a robot program, so viewer discretion is required.