Fissile / Fertile Material Cross-sections. Uranium 238.
Source: JANIS (Java-based nuclear information software)
238U is a fissionable isotope, but is not fissile isotope. 238U is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand 238U can be fissioned by fast neutron with energy higher than >1MeV. During the fuel burning, the content of the U-235 continuously decreases and the content of the plutonium increases (up to ~1% of Pu ). 238U belongs also to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile 239Pu and other transuranic elements. This is the way how 238U contributes to the operation of nuclear reactors and production of electricity through this plutonium. For example, at a burnup of 40GWd/tU, about 40% of the total energy released comes from bred plutonium. This corresponds to a breeding ratio for this fuel burnup of about 0.4 to 0.5. That means, about half of the fissile fuel in these reactors is bred there. This effect extends the cycle length for such fuels to sometimes nearly twice what it would be otherwise. MOX fuel has a smaller breeding effect than 235U fuel and is thus more challenging and slightly less economic to use due to a quicker drop off in reactivity through cycle life.