### Theory:

When a neutron strikes a uranium nucleus ($$U^{235}$$), the process of fission occurs, creating three neutrons. These three neutrons produce a fission reaction in three more uranium nuclei in the sample, thus resulting in nine neutrons. These nine neutrons undergo fission and make another twenty-seven neutrons, and so on. This type of reaction is known as a Chain reaction.

Chain reaction
A chain reaction is a self-propagating process in which the number of neutrons multiplies rapidly in a geometrical progression.
Generally, two types of chain reactions are possible. They are
(i) Controlled chain reaction
(ii) Uncontrolled chain reaction
Controlled chain reaction:
The number of neutrons released in a controlled chain reaction is always one. This is accomplished by using a neutron absorber to absorb the excess neutrons released during the process, leaving only one neutron to produce further fission. As a result, the reaction is kept under control.

Controlled chain reaction

The energy released by a regulated chain reaction can be used for many constructive purposes. In a nuclear reactor, the controlled chain reaction is employed to produce energy in a predictable manner over a long period.

Energy generation from a nuclear reactor
Uncontrolled chain reaction:
The quantity of neutrons multiplies endlessly in an uncontrolled chain reaction, causing fission in a huge mass of fissile material. This causes a massive amount of energy to be released in a fraction of a second. This chain reaction is employed in an atom bomb to create an explosion.

Uncontrolled chain reaction

It is calculated that $$10^{23}$$ uranium atoms undergo fission in $$one\ minute$$ in an uncontrolled chain reaction.

The main difference between the controlled and uncontrolled chain reactions is that an enormous amount of energy is released in a short span of time and causes an explosion in an uncontrolled reaction. Whereas in the controlled reaction, the energy is released in a slow and controlled manner, which can be effectively used without causing any destructive explosions.
Reference:
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