Explain the Nuclear Chain Reactions

Fission is the breaking up of a massive nucleus into two fragments, with the release of energy and some particles. Uranium 235 fissions upon capturing a neutron, thus releasing a large amount of energy and two or more neutrons. If the uranium is carefully arranged so that the neutrons released will cause other Uranium nuclei to fission, a chain reaction can occur. Boron rods which can be inserted into the nuclear reactor, or pile can be used to absorb the escaping neutrons and slow down the reaction. Cadmium can also be used in the control rods. Fission of Uranium 235 proceeds best with slow-moving neutrons, so moderators such as graphite and deuterium (used in heavy water) are used to slow down the neutrons.

The A-bomb or fission bomb splits large amounts of fissionable materials quickly, releasing large amounts of energy and radioactive material. Not all radioactive materials are serious radiation hazards. Some are not man-made but instead are naturally occurring.  The ones with very long half-lives and ones that emit very high energy radiation are most dangerous.