 The amount of energy that is needed to overcome this repulsion is known as the Coulomb barrier.The amount of energy released by the fusion reaction when it occurs may be greater or less than the Coulomb barrier.Generally, lighter nuclei with a smaller number of protons and greater number of neutrons will have the greatest ratio of energy released to energy required, and the majority of fusion power research focusses on the use of deuterium and tritium, two isotopes of hydrogen.