When a heavy nucleus breaks up into two or more fragments with the liberation of energy, the process is called fission.
When two light nuclei fuse together to form a comparatively heavy nucleus, the process is called fusion. Both processes are accompanied by release of energy.
The most common example of fission is the fission of Uranium nucleus, which absorbs a slow neutron and breaks into Barium, Krypton, a few neutrons and energy.
U(235)+neutron→Ba(139)+Kr(94)+3 neutrons+200 MeV
Each fission releases around 200 MeV of energy.
The fusion reaction is responsible for energy generated by stars.
Protons in the stars combine to form a Helium nucleus with the liberation of energy.
4 protons→Helium+2 positrons+ nuetirnos+ 26.7 MeV
Fusion reactions can happen only at very high temperatures, which is required ,so that the protons have enough kinetic energy to overcome their mutual electrostatic repulsion. The temperatures that can result in fusion are of the order 10^8 K is present in stars , hence stars generate energy by fusion.
However, such temperatures are difficult to maintain and control on Earth, and hence controlled fusion reaction is still not available commercially. Experiments are on , using large magnetic fields to contain the ions that undergo fusion, since no container can hold the nuclei while they undergo fusion.
Fusion is also called clean energy, since the byproducts of fusion are not radioactive, but many fission byproducts are generally radioactive.
