Breeding with plutonium 239
For breeding to take place, the balance of fuel regeneration must be positive. This balance varies rapidly with the energy of the neutrons that trigger fissions. For plutonium 239, the feedback is positive for very slow neutrons and mainly for those with energies over 10 keV. This is the reason why plutonium breeder reactors operate with fast neutrons.
IN2P3 (Source A.Nuttin/LPSC Grenoble).