Why is uranium-238 not fissionable like uranium-235?

Uranium-238 is not considered "fissile" like uranium-235 because it cannot undergo nuclear fission when struck by slow, low-energy neutrons. While all heavy nuclides have the potential to fission in an excited state, only specific isotopes—termed fissile materials—can sustain a chain reaction consistently when bombarded by these slow neutrons.123

Instead, uranium-238 is classified as a fertile material. This means that while it does not fission readily, it can be transformed into a fissile isotope through neutron capture. When a uranium-238 nucleus absorbs a neutron, it undergoes a series of radioactive decays to eventually become plutonium-239, which is a fissile material capable of sustaining a chain reaction.234

Because uranium-238 makes up the vast majority of naturally occurring uranium, this conversion process is essential for the nuclear industry, allowing fertile materials to be used to generate new fuel in nuclear reactors.143