Atom consists ofnucleus and electronsin orbits.

The nucleus consists of protons (positively charged particles) and nuetrons (nuetrally charged particles)

Energy is obtained from the nucleus in the following methods :

Nuclear Fusion:Breaking a heavy nuclei into 2 or more smaller lighter nuclei

Nuclear Fission:Combining two lighter nuclei to form a heavy nucleus

Uncontrolled chain reaction: In an uncontrolled nuclear fission, aheavy nucleus in bombarded with a neutron. It splits into lighter nuclei releasingtwo or more neutrons. Eachneutron again bombards nuclei of atomand splitsfurther and further. Theneutrons and thereby the fission reaction exponentially increases. Eg : Atom bomb

Controlled chain reaction: In an controlled nuclear fission, aheavy nucleus in bombarded with a neutron. It splits into lighter nuclei releasingtwo or more neutrons. But in this case,except one neutron all the other neutrons are absorbed. So, only one neutron is left. This single neutron again bombards nuclei of atom and splits further and further. Again only one neutron is left behind and the others are absorbed.So, the reaction is controlled. Eg: Nuclear generator.

In a nuclear generator, nuclear energy is harnessed in a controlled manner to generate electricity.