What is the fusion chain

  Nuclear fusion describes a nuclear reaction in which two atomic nuclei "fuse" to form a new nucleus.

In principle, this reaction can be exothermic (energy-producing) or endothermic (energy-consuming); Significantly large cross-sections (probability that the colliding nuclei react with one another) only exist in exothermic fusion reactions. The latter in the form of an energetic chain reaction are an essential subject of research and development because this energy generation is in the foreground of interest.

In nuclear fusion, the Coulomb barrier (electrical repulsive force) between the positively charged nuclei must first be overcome. The tunnel effect makes this process more likely. If the distance is then only 10-15 m, the strong interaction binds the nuclei together.

Energy balance

If the mass of the nuclei / particles created during the fusion is less than the sum of the mass of the starting nuclei, the mass difference (the so-called mass defect) is calculated according to Einstein's mass-energy equivalence formula E = m * c2 free in the form of energy (as kinetic energy of the reaction products and as radiation energy). Such exothermic fusion reactions are only possible in the area of ​​light nuclei, since the binding energy per nucleon increases with increasing mass number up to about 60. It has a strong local maximum with the nuclide helium-4. The most favorable reactions for the production of fusion energy (see nuclear fusion reactor) therefore produce He-4. In the picture above, the easiest of these reactions to initiate is, D + T →4He + n, shown.

The formation of 1 kg of helium by means of this reaction provides around 115 million kilowatt hours of energy. With an ideal, complete conversion, this would cover the entire German electricity demand for 2 hours.

Nuclear fusion in stars

Nuclear fusions are the energy source of the stars, including our sun. In most of the stars, hydrogen fuses to helium via several intermediate steps ("hydrogen burning"); At the pressure prevailing in these stars, the temperature required for this is around 10 million Kelvin.

Reactions (selection):

In the sun, among other things, the proton-proton reaction takes place, a series of reactions that also produce helium-4 with a corresponding gain in energy. In addition, a carbon-catalyzed fusion cycle takes place in the sun, the Bethe-Weizsäcker cycle, which accounts for around 1.6% of the sun's energy production.

The temperature required for the fusion depends, among other things, on the pressure. The temperature on earth required for hydrogen fusion is around 100 million ° C, since no such pressure as the gravitational pressure prevailing in the sun can be generated here.

When a star's hydrogen is used up and turned into helium, the energy comes from the fusion of helium or even heavier atomic nuclei. These fusions provide less energy and require higher fusion temperatures. Larger stars can also generate a stronger gravitational pressure with their mass, which ultimately also creates heavier elements through fusion (up to mass number 60). Elements with even larger mass numbers, however, can no longer be created in this way, since such fusions are endothermic, i. H. deliver less energy than they need. They are formed by neutron (s and r process) and proton accumulation (p process) (see supernova, core collapse).


Physical research, neutron sources

Fusion reactions without a chain reaction effect, i.e. without the reaction products causing further nuclei to fuse through collisions, can be carried out like other nuclear reactions using particle accelerators in the laboratory for physical research purposes. The above-mentioned deuterium-tritium reaction is used to generate fast free neutrons. The Farnsworth-Hirsch Fusor is also a source of free neutrons for research and technical purposes.


Uncontrolled fusion chain reactions take place in nuclear weapons (hydrogen bombs). While conventional corecleavageweapons (such as the Hiroshima bomb Little Boy) released an explosive force of around 15-60 kilotons of TNT depending on the typefusionweapons or H-bombs, explosive forces of up to approx. 57 megatons of TNT (see Tsar bomb). To ignite the fusion chain reaction, there is a nuclear fission bomb inside a hydrogen bomb in order to achieve a sufficiently high temperature. Hydrogen-containing alloys are placed around the uranium bomb, in which the fusion chain reaction takes place.

Civil energy generation

see also nuclear fusion reactor; on the danger of the military use of nuclear fusion reactors, see Risks with regard to nuclear proliferation

Great lines of development

To use the deuterium-tritium reaction as an energy source, nuclear fusion reactors with magnetic confinement of the plasma are being developed in international cooperation (see also fusion by means of magnetic confinement and ITER). There are also smaller development programs for fusion with inertial confinement - to put it simply, the ignition of micro hydrogen bombs in a reactor vessel in rapid succession using laser or ion beams (see inertial fusion).

Cold fusion

Cold fusion describes various concepts and experiments that describe fusion chain reactions that take place at significantly lower temperatures. This approach became known primarily through the work of Martin Fleischmann from 1990. Some of the approaches have not been proven reproducible, sometimes the physical and technical possibility is beyond doubt, but in the case of a negative or controversial total energy balance (e.g. that caused by cavitation Vesicle fusion).


  • RealVideo: Do we need nuclear fusion? (from the TV show Alpha Centauri)


  • Alexander M. Bradshaw, Thomas Hamacher: Nuclear Fusion - A Sustainable Energy Source of the Future. Naturwissenschaftliche Rundschau 58 (12), pp. 629 - 637 (2005), ISSN 0028-1050

See also

Categories: Nuclear Fusion | Nuclear chemistry | Nuclear physics