Comparison of secondary neutrons and neutrons available for a new fission
This diagram compares the neutrons produced by the fission of uranium 235, uranium 233 and plutonium 239, caused by the absorption of a slow neutron. Of the three nuclei, plutonium 239 produces the largest number of secondary neutrons. As not all neutron absorptions lead to nuclear fission, the important figure is how many neutrons are available to induce fission. When this number is equal to 1 a chain reaction can be maintained, and at 2 the fuel can maintain itself (breeding).