U238 will undergo fission by
U238 will undergo fission by high-energy (fast) neutrons alone.
U 238 is a fissionable isotope, meaning that it can undergo nuclear fission, but the neutrons fired at it would need much more energy in order for fission to take place.
U238 has an even mass, and odd nuclei are more fissile because the extra neutron adds energy - more than what is required to fission the resulting nucleus.
Because of the large amount of energy needed, U238 will not normally undergo fission in a nuclear reactor. however, U- 238 can also form Pu 239, which can undergo fission in a nuclear reactor.