Crucial to the formation of a fusion is the cross-section ofaction, the measure of the probability that colliding nuclei will react with each other.The effective cross-section is usually only sufficiently large if the two cores collide with high energy. This is necessary to overcome the coulombbarrier, the electrical repulsion between the positively charged cores, or to tunnelits narrow maximum.Beyond the barrier, at a distance of only about 10 x 15 m, the attraction prevails due to the strong interaction and the nuclei merge with each other.
Fusion reactions can be exothermic (energy-disenmiting) or endothermic (absorbing energy).Exothermic fusion reactions can maintain the high temperatures needed for thermal energy to lead to further fusion reactions. Such thermonuclear processes take place in stars and fusion bombs under extreme pressure. In contrast to nuclear fission, a chain reaction with fusion reactions is not possible.
The fusion reaction as a thermonuclear process shown above is intended to serve in the future for power generation in nuclear fusion reactors: nuclei of deuterium (2 H) and tritium (3 H) merge into a helium nucleus (4 He) with the release of a Neutrons energy (3.5 MeV + 14.1 MeV).