Chain Reactions

Definition of Chain Reactions

A chain reaction in nuclear physics is a self-sustaining series of nuclear fissions. When a nucleus undergoes fission, it releases neutrons that can then cause further fission events in other nuclei. This leads to an exponential increase in the number of fissions, as each fission event produces more neutrons to continue the process.

Because the reaction feeds itself, it can either be controlled or uncontrolled depending on how the neutrons are managed.

Example: If one fission event releases 3 neutrons, and each neutron causes another fission, the number of fissions grows rapidly: 1 fission leads to 3, then 9, then 27, and so on.

Chain Reactions in Nuclear Fission

Chain reactions occur mainly in certain isotopes like Uranium-235 and Plutonium-239. When these nuclei absorb a neutron, they become unstable and split into two smaller nuclei, releasing:

• Energy (mainly kinetic energy of the fragments)
• Additional neutrons (usually 2 or 3)

These released neutrons can then collide with other Uranium-235 or Plutonium-239 nuclei, causing them to split as well. This is the basis of the chain reaction.

The energy released in each fission is very large compared to chemical reactions, which is why nuclear fission is used as a powerful energy source.

For instance, when a Uranium-235 nucleus absorbs a neutron, it splits into two smaller nuclei (fission products) and releases about 2 or 3 neutrons plus energy:

These neutrons then cause further fissions, continuing the chain reaction.

If each fission causes more than one further fission, the reaction grows rapidly (exponentially). If exactly one neutron from each fission causes another fission, the reaction is steady and controlled.

Control of Chain Reactions

To safely use nuclear fission in reactors, the chain reaction must be controlled to avoid an explosion or meltdown. This is done by:

• Control rods: Made from materials like boron or cadmium, these rods absorb excess neutrons, reducing the number available to cause further fission. By adjusting how far the rods are inserted, the reaction rate can be controlled.
• Moderators: Substances such as water or graphite are used to slow down fast neutrons produced by fission. Slower neutrons are more likely to cause further fission in Uranium-235 nuclei, helping maintain a steady reaction. This is because slow neutrons are more easily absorbed by the nuclei.

Together, control rods and moderators keep the chain reaction at a steady rate, ensuring a constant release of energy without the reaction running away.

If the control rods are fully withdrawn, the reaction can become uncontrolled, leading to a rapid increase in energy release.

Applications and Risks

Controlled chain reactions are the basis of nuclear reactors, which generate electricity by using the heat from fission to produce steam that drives turbines.

In contrast, uncontrolled chain reactions occur in atomic bombs, where the reaction happens extremely rapidly, releasing a huge amount of energy in a very short time.

Risks associated with chain reactions include:

• Radiation hazards: Fission produces radioactive waste and radiation that can harm living organisms.
• Accidents: If chain reactions are not properly controlled, they can lead to dangerous releases of radiation or explosions.
• Safety measures: Nuclear reactors have multiple safety systems, including shielding, containment buildings, and emergency shutdown procedures to protect workers and the public.