Step-by-step explanation:
An uncontrolled chain reaction is a chain of nuclear reactions that take place subsequently, but not under controlled conditions. When the number of atoms grows exponentially the reaction is not under control.
The definition of nuclear fusion is: "a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy". So in a fusion reaction small nuclei join to form a bigger nuclei.
The definition of nuclear fission is: "a reaction in which the nucleus of an atom splits into two or more smaller nuclei". So in a fission reaction a large nucleus breaks into smaller nuclei.
A fission chain reaction happens when a neutron and a fissile isotope interact with each other. This causes the release of some neutrons from the fissile nucleus. These released neutrons can interact with other fissile isotopes and cause the initiation of subsequent fission reactions. This is a controlled fission reaction. So, when extra neutrons are absorbed we have a controlled fission reaction.
The definition of critical mass is: "is the smallest amount of fissile material needed for a sustained nuclear chain reaction". So the critical mass is the amount of material necessary for a reaction to occur.
Answer:
The number of atoms grows exponentially the reaction ----> uncontrolled reaction
Small nuclei join to form a bigger nuclei ----> fusion
A large nucleus breaks into smaller nuclei ----> fission
Extra neutrons are absorbed ----> controlled reaction
This is the amount of material necessary for a reaction to occur ---> critical mass