Nuclear fission involves the splitting of a heavy element into lighter elements. The reaction is set off by the random capture of a stray neutron.

For instance, in the above reaction, a uranium atom captures a neutron and for a brief while exists as uranium-236, an unstable isotope. This unstable isotope then "fissions" into the smaller elements barium and krypton and releases 3 more neutrons. This reaction results in the creation of 10 BILLION KJ per mole of energy.

Notice that more neutrons are released by the fission process. These neutrons go on to react with other uranium atoms and continue the process.

One might imagine that with each successive reaction more and more neutrons would be produced, and consequently more and more fissioning of uranium would occur. In a nuclear weapon, that is exactly what would happen. However, in nuclear power plants the reactions are controlled.

As shown in the image above, reactive uranium rods are alternated with absorbent carbon or lead rods. Thus, although some neutrons continue to react and continue the fission reaction, others are absorbed, and the reaction does not get out of control.

The energy produced by fission is used primarily to heat a liquid (usually water) to boiling. The steam generated by the boiling liquid is used to power a turbine that generates electricity.