|Avantage aux neutrons rapides|
The surplus of energy carried by fast neutons favors fission of fragile heavy nuclei. Fast neutrons are able to fission all heavy nuclei present in the reactors including those considered non fissile by slow neutrons. The figure compares the percentages of neutron captures followed by fission for the main isotopes of uranium, plutonium and minor actinidesin the cases of the slow neutrons of a conventional pressurized water reactor (PWR) and those of an hybrid reactor. In the first case the neutrons are slowed down by a moderator, in the second case they are or very fast.