Fission, by definition, means splitting, and nuclear fission means the splitting of a nucleaus. In most cases, energy is required to tear apart a nucleus. However, some heavy isotopes have nuclei that can be split part into products that have less mass than the original nucleus. From such nuclei, energy can be obtained from the mass difference.

The device that allows us to extract energy from controlled nuclear fission is called nuclear reactor. Atomic bomb is opposite. It releases energy in an uncontrolled manner, so it is destructive.