Nuclear power plants rely on the chain reaction of fission to create nuclear power. The real trick, however, is to control the chain reaction, releasing its energy slowly so that ends other than destruction might be achieved.
The secret to controlling a chain reaction is to control the neutrons. If the neutrons can be controlled, then so can the energy that is released. That's what scientists have done with nuclear power plants.
Most people believe that the concepts behind nuclear power plants are tremendously complex. That's really not the case. Nuclear power plants are very similar to conventional fossil fuel plants, in which a fossil fuel (coal, oil, natural gas) is burned, and the heat is used to boil water, which, in turn makes steam. The steam then turns a turbine that is attached to a generator that produces electricity.
The big difference between a conventional power plant and a nuclear power plant is that the nuclear power plant produces heat through nuclear fission chain reactions.
In a nuclear reactor, a fissionable isotope is contained in fuel rods in the reactor core. Control rods, commonly made of boron or cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay. Operators can stop a chain reaction completely by pushing the control rods all the way into the reactor core, where they absorb all the neutrons. The operators can then pull out the control rods a little at a time to produce the desired amount of heat.
A liquid (water, or sometimes liquid sodium) is circulated through the reactor core, and the heat generated by the fission reaction is absorbed. The liquid then flows into a steam generator, where steam is produced as the heat is absorbed by water. This steam is then piped through a steam turbine that's connected to an electric generator. The steam is condensed and recycled through the steam generator. This forms a closed system; that is, no water or steam escapes — it's all recycled. This closed system helps ensure that no contamination of the air or water takes place.