Generally, the concept of Température is related to that of thermodynamic balance, however that the systems which one considers in all the fields are more or less in imbalance.

If, for an unspecified reason (natural or not), a system has a local imbalance compared to the remainder, the unit will evolve to a return towards another balance, which can prove very close to primitive balance if the quantity of energy introduced at the time of the event is very weak compared to the thermal energy of the system in its entirety.

In physics-chemistry in particular, one very often meets minor amounts of matter, or volumes (which one will call in what follows the “particles” which are put brutally under conditions very different from the remainder of their environment. One witnesses an evolution then bringing back these particles to a thermodynamic balance with the remainder. It is about a thermalization .

The phenomenon of thermalization of the neutrons in is a typical example. In a reaction of Fission, of Uranium 235, for example, some fast neutrons are produced, whose kinetic energies (several MeV) are much larger than the kinetic energies associated with the temperature T of the environment. The thermalization then will be carried out by successive shocks between the neutrons and the cores which they meet. When one studies the effects of the successive shocks (which they are elastic or not) one realizes that the neutrons lose their kinetic energy rather quickly to be stabilized with kinetic energies compatible with the statistics associated with the temperature T .

With regard to the existing nuclear plants, the research of obtaining slow neutrons (suitable for induce new reactions of fission) is paramount. This indicates the interest of the phenomenon of thermalization.

It is in particular the role of the Modérateur light S, elements, nonsuitable for absorb the neutrons. In a power station, the cores of the regulators are at the temperature of the engine, and the shocks which they undergo at the time of a meeting with the fast neutrons cause an exchange of impulse which considerably will slow down the neutrons, even in the event of elastic shock.