 This is much more efficient than series of steps 238U + n → 239Np → 239Pu in replacing fissile isotope consumption.Uranium-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral - the average of neutrons having a range of intermediate energies.In a nuclear reactor non-fissile isotopes 234U and 238U both capture a neutron breeding fissile isotopes 235U and 239Pu respectively.