What is nuclear fission and nuclear fusion ?

Nuclear fission is a nuclear reaction in which a relatively large nucleus such as uranium splits into two smaller nuclei and releases energy in the process. Nuclear fission can be spontaneous or induced. Very large nuclei are unstable normally so some of them have a chance of splitting up spontaneously. Nuclear fission can also be initiated when the nucleus of an atom is hit by other particles. For example, uranium may split up when hit by a neutron.

Nuclear fusion is a nuclear reaction in which two nuclei join to form a larger nucleus and release energy. The energy released in nuclear fusion is usually larger than that in nuclear fission. It is hydrogen fusion (producing helium) in the Sun that results in light and heat coming out of it.