Fission probability of uranium-235 with neutron energy
The fission probability of uranium-235 nuclei by fast neutrons whose energy is large compared to that of slow neutrons called "thermal" is only of a few barns compared to 584 barns for thermal neutrons of 0.025 eV. To compensate, reactors using these neutrons require nuclear fuel rich in fissile material and high neutron flux.
IN2P3