Nuclear EnergyIn a nucleus, protons and neutrons are held together at distances of the order of 10-15 m by some energy, which binds them together to form the nucleus. This energy is called nuclear energy. Consider a helium nucleus. It consists of two protons and two neutrons held together to form a helium nucleus.
Experiments have shown that the mass of a helium nucleus is less than the sum of the masses of two protons and two neutrons. This mass difference is equivalent of an energy. This energy is released in a nuclear reaction involving the fusion of two protons and two neutrons to form a helium nucleus. This reaction is called nuclear fusion.
In nuclear fission, a heavy unstable nucleus (Uranium-235) is broken up into two lighter stable nuclei with the release of nuclear energy.