When an uranium nucleus splits, two neutrons are emitted. These neutrons can carry on splitting other uranium nuclei, resulting in a chain reaction.

Conditions for chain reaction

1. There is a critical size ( about the size of a cricket ball) which must be attained before a chain reaction can start.

Reasons:

• If the amount of uranium-235 is too small, the neutrons will escape from the reacting region and the chain reaction will stop.
• Some neutrons may be absorbed by other nuclei (e.g. uranium-238) without causing fission. Therefore, there must be enough uranium-235 both in concentration and in terms of total amount.

2. We need to control the velocities of the neutrons emitted during the fission.

Reason:

• Only slow neutrons have a high probability of being captured by uranium-235 and continue with the fission. It has been done by letting neutrons collide and transfer kinetic energy to another object with mass similar to that of the neutron, such as hydrogen nuclei.

Then, we can have a balance between the rate at which neutrons are produced by fission and the rate at which they are lost by causing fission, by absorption, or by leaving the reacting region.

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