Thermalization of the neutrons
The thermalization of the neutrons is the deceleration of the neutrons by a succession of shocks with the atoms of a moderating . A Neutron thermalized is known as thermal neutron or slow neutron , it has a kinetic energy lower than 0.625 EV. One thus distinguishes it from the neutrons known as fast neutrons whose energy is higher than 0.907 MeV. The spectrum of intermediate energies is known as epithermal.
A reactor thermal neutrons or engine with slow neutrons uses a regulator to slow down the neutrons resulting from the reactions of fission. In the absence of regulator, the engine is known as Fast reactor.
The main reason for which one uses this process of deceleration of the neutrons is to make it possible the neutrons to interact with the atoms Fissile S (Uranium 235 or Plutonium 239) present in fuel of a Nuclear reactor. Indeed, when a fissile atom breaks after absorption of a thermal neutron, it emits two or three neutrons having a speed comparable with 20.000 km/s (energy about 2 MeV). At this speed, it is not very probable that another fissile atom absorbs this neutron, and this for two raisons : if there is collision, the probability of starting a fission is weaker for a fast neutron, and in addition, the fast neutron goes essentially, in straight line and leaves the c&oelig quickly; ur of the engine, while the thermal neutron carries out a Brownian Movement random giving him a trajectory much longer. This is why in a nuclear plant, it is preferable to carry out the thermalization of the neutrons using a moderating to allow a better output of the engine.
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