Fission probability of uranium-235
The fission probability - called fission cross-section by physicists - varies greatly with the neutron energy. It is measured in barns For a fissile nucleus such as uranium-235, this fission probability is the highest for kinetic energies of a few electronvolts. Since the proportion of uranium-235 is only of a few percents in most reactor fuels, it is necesseray to compensate for this low proportion by a large fission probalility of Uranium-235, therefore to operate with flux of a slow neutrons.