Question

Why is nuclear fission a chain reaction?

Answer 1

A nuclear fission is a chain reaction because it produces its own reagents, thus allowing more nuclear fissions.

Explanation:

Be a radioactive atom `A` which, when hit by a neuttron `n`, disintegrates into two lighter atoms `B` and `C` and `x` neutrons. The nuclear fission's equation is

`n+A rarr B+C+x*n`

You can see that if one neutron is thrown at a group of atoms `A`, one disintegration will be triggered, releasing `x` neutrons.
Each neutron released by the first reaction can, and probably will, encounter another atom `A` of the group and trigger another disintegration, releasing `x` more neutrons, etc.

Any reaction following the first is a consequence of the previous reaction, due to the previous reaction releasing a reagent of the current reaction. This is a chain reaction.