Examples,
Although energy per fusion is less compared to that of fission, the energy released per unit mass is greater in fusion than in fission.
Nuclear fusion occurs at very high temperatures i.e., 107 K because only under such conditions that protons come close together against their mutual repulsion. Further, high density is also desirable for such collisions to occur. These conditions are found in the Sun. Thus, the source of Sun's energy is nuclear fusion.Research has been going on for nearly fifty years to produce fusion in a controlled manner without much success. The only practical device that has been developed is the hydrogen bomb.
Enormous amount of energy is released in the explosion that accompanies an uncontrolled nuclear chain reaction.
