What is a chain reaction in nuclear fission?

A chain reaction in nuclear fission is a self-sustaining series of reactions where neutrons split atomic nuclei.

In more detail, nuclear fission occurs when the nucleus of an atom, such as uranium-235 or plutonium-239, absorbs a neutron and becomes unstable. This instability causes the nucleus to split into two smaller nuclei, releasing a significant amount of energy, along with additional neutrons. These newly released neutrons can then go on to collide with other nearby fissile nuclei, causing them to split as well. This process repeats, creating a chain reaction.

For a chain reaction to be sustained, there must be a sufficient quantity of fissile material, known as the critical mass. If the material is below this critical mass, the neutrons escape without causing further fission events, and the reaction dies out. However, if the material is at or above the critical mass, the neutrons are more likely to collide with other fissile nuclei, maintaining the chain reaction.

In a nuclear reactor, this chain reaction is carefully controlled using materials called control rods, which absorb excess neutrons. By adjusting the position of these rods, operators can manage the rate of the reaction, ensuring it remains steady and safe. In contrast, in a nuclear bomb, the chain reaction is uncontrolled, leading to a rapid, explosive release of energy.

Understanding chain reactions is crucial for both harnessing nuclear energy for electricity and recognising the potential dangers of nuclear weapons. It highlights the importance of careful control and regulation in the use of nuclear technology.