Nuclear fission is the process of the breakdown of a large nucleus into a smaller stable nucleus.
A large nucleus breaks into smaller nuclei - Fission
In an uncontrolled reaction, the number of atoms increases exponentially.
The number of atoms grows exponentially - Uncontrolled reaction
Some minimum amount of mass of an atom is required for a reaction, which is termed as critical mass.
This is the amount of material necessary for a reaction to occur - Critical mass
To make a reaction controlled extra neutrons are provided. The absorption of extra neutrons makes it a controlled reaction.
Extra neutrons are absorbed - Controlled reaction