the aim of nuclear reactors is to generate energy through nuclear fission reactions.
U-#235# is a fissile isotope, meaning that it can split into smaller molecules when a lower-energy neutron is fired at it.
this process releases neutrons and energy in the form of both gamma rays and kinetic energy (stored in neutrons and fragment nuclei).
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. U-#238# 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, U-#238# 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.