Chain reactions

Nuclear fission (see here) is caused when an atom absorbs an extra neutron, causing the nucleus to split apart. The resultant nuclear reaction releases more neutrons, which can go on to cause further fission events. This sequence, where released neutrons go on to cause further fissions, is known as a chain reaction.

If the number of neutrons released matches the number of neutrons that caused the fission then the chain reaction will proceed at a constant rate. But if more neutrons are released than caused the initial fission then future fissions will occur at an ever increasing rate. This would result in a huge release of energy in a very short space of time, whereas if we want to generate electricity using the heat released by nuclear fission then we need the energy release to occur at a constant rate over a long period of time.

In fact, the nuclear fission process that is used for electricity generation DOES produce an excess of neutrons but not in a way that causes a run-away chain reaction. To generate a reliable and constant electrical output, we need to both reduce the number of free neutrons and make the remaining neutrons more effective. This is done using control rods and a moderator, as explained in the next post.