Why is plutonium-239 more useful for fission than uranium-238?

Plutonium-239 is more useful for fission than uranium-238 because it is fissile, whereas uranium-238 is only fertile.12

In nuclear physics, the distinction is critical:
* Fissile materials, such as plutonium-239, can undergo nuclear fission when struck by both fast and slow (low-energy) neutrons, allowing them to sustain a chain reaction.13
* Fertile materials, such as uranium-238, cannot sustain a chain reaction on their own because they do not fission readily with slow neutrons. Instead, they must first capture a neutron and undergo radioactive decay to be transformed into a fissile isotope.32

Because uranium-238 cannot support the rapid, self-sustaining fission required for nuclear weapons or certain reactor operations, it is primarily used as a source material to produce plutonium-239 through neutron capture in nuclear reactors.13