The nuclear reactions that produce energy are of two types: fission, in which a heavy atomic nucleus separates into two lighter nuclei, and fusion, which releases energy following the combination of two light nuclei in a heavier nucleus. Nuclear fusion is a phenomenon that takes on great importance in nature because it is the basis of the functioning of the stars. On Earth, fusion has so far been exploited to make the so-called thermonuclear bombs or hydrogen bombs. While since the post-war period, the possibility of carrying out "controlled" nuclear fusion reactions to produce electricity on an industrial scale, as is already the case, is being studied with fission reactors.
For the fusion to take place, it is necessary to approach the two nuclei at an extremely close distance (of the order of a millionth of a billionth of a meter). To overcome their electrostatic repulsion, the nuclei must be confined to a density and very high temperatures, for example, by means of magnetic fields: a very complicated operation from a technical point of view and costly in terms of energy. For this reason, even if the research has been going on for decades, it has not yet come to the realization of a true fusion reactor. However, being able to do so would bring numerous advantages, especially because fusion energy is substantially "clean," i.e., it is not accompanied by the production of waste (as happens instead in fission reactors).
Nuclear fusion is the phenomenon that involves the transformation of hydrogen isotopes into helium atoms. In nature, as is well known, everything tends to a state of minimum potential energy. An object held at one meter from the ground, if left free, will fall on the floor, or it will tend to a minimum potential (gravitational, in this case) state of energy.
Although the potential energy at the atomic level has a different nature, everything works in a similar way. The nuclei of the two isotopes merge to bring both of them in a lower energy state, releasing the excess. However, the fusion reaction requires environments with very high pressures and temperatures, so that the nuclei are sufficiently close to each other, in the plasma state (the so-called "fourth state of matter").
Isotopes are atoms that have more neutrons in the nucleus. The hydrogen atom has a nucleus formed by a single proton, the hydrogen isotopes possess, depending on the type of isotope, a nucleus with a proton and a neutron ( deuterium ), two neutrons ( tritium ), three neutrons ( hydrogen four ) and so on.
Nuclear fusion can be considered the inverse process with respect to fission, in which two nuclei of an element with a low atomic number unite to form a nucleus of higher atomic number.
The typical process of fusion is that which takes place naturally in the stars, and therefore also in the Sun, whereas an overall result four hydrogen nuclei (hence four protons, 11H) "merge" giving rise to a helium nucleus (formed by two protons and two neutrons, 42He). The process takes place through a series of intermediate reactions (including the transformation of two protons into two neutrons) and is accompanied by the release of large amounts of energy. The hydrogen nuclei, which on the stars are abundantly present in the ionized state, have such high kinetic energies, due to the high temperatures inside the stars. That's why it overcomes the electrostatic repulsions and join together to form heavier nuclei (a gas of ionized particles is called plasma).
For two nuclei to be able to get close enough to each other to make the fusion take place, the temperatures must be around millions or tens of millions of degrees. For this reason, it is very difficult to artificially trigger fusion processes that supply more energy than it is spent to produce them.
In the fusion produced in the laboratory the nuclei of the common hydrogen are not used, but those of its isotopes: the deuterium (21H), formed by a proton and a neutron, and the tritium (31H), formed by a proton and two neutrons, which produce helium according to the reaction.
In this reaction, energy equal to 17.6 MeV is released, also due to the difference between the initial and final masses. To use the energy produced by the fusion, it would be necessary to construct a fusion reactor capable of "retaining," i.e., confining the plasma and heating it to temperatures of tens of millions of degrees. Almost all types of experimental fusion reactors in the study use high magnetic fields to confine the plasma, exploiting the fact that the ionized state particles, therefore electrically charged, are affected by the magnetic force. Plasma heating can take place by means of very intense electric currents.
The possibility of exploiting nuclear fusion, which is considered the energy source of the future, it requires the overcoming of formidable technological problems that involve very high research costs in the initial phase. The studies in progress in various countries have allowed us to achieve some important experimental results, which make us consider reasonable the expectation that within a few decades, we can reach the realization of a prototype of a fusion nuclear power plant.
In the field of nuclear fusion, the ITER project is aimed at the realization of a nuclear fusion reactor, which would allow producing a large quantity of energy, as has never been possible in history. The ITER project, an acronym of the International Thermonuclear Experimental Reactor, brings together the collaboration of 35 countries including European and some Asian countries. ITER is an experimental project, whose purpose is not exactly the production of energy for civil use, but intends to demonstrate the feasibility and controllability of physical processes, in the face of technical challenges that are anything but trivial.