Overview#The relation E = mc2 states the equivalence of mass and energy. In a fusion reaction, some reactant mass energy is converted to kinetic energy of the products. Binding energy is the energy equivalent of the mass difference between a whole nucleus and its individual constituent protons and neutrons. For energy release in fusion or fission, the products need to have a higher binding energy per nucleon (proton or neutron) than the reactants.
Fusion only releases energy for light elements and fission only releases energy for heavy elements.
The actual fusion reaction occurs when two nuclei approach within about 1.0E-15 m, so that the attraction, via the residual strong interaction between the nuclei, overcomes the electrical repulsion between the protons. Such close encounters only occur when nuclei collide with sufficient kinetic energy. Only at high temperatures do enough energetic particles exist for there to be many fusion reactions.