Question

What do you mean by the chain reaction in nuclear fission? How is it controlled?

Answer 1

When slow neutrons strike the radioactive nuclei, three new neutrons and a tremendous amount of energy is released. These new neutrons can further strike the other radioactive nuclei under the suitable conditions.
A chain reaction in nuclear fission can be controlled using moderators by absorbing neutrons emitted in the fission process and preventing them from striking other radioactive nuclei. Moderators like graphite, heavy water have a unique property of absorbing neutrons.