What is a nuclear chain reaction?
In a nuclear fission power station (or a nuclear fission bomb) an unstable nucleus of uranium is hit by a neutron, causing it to break up, releasing a small quantity of energy plus several more neutrons. These new neutrons go on to hit more nuclei, causing them to break up, releasing even more energy and neutrons. This quickly builds up to a nuclear chain reaction. In a power station, control rods are used to regulate the rate of energy release.