Why is U-235 used more in nuclear reactors than U-238?

1 Answer
Dec 31, 2017

U-#238# does not undergo fission with lower-energy neutrons, while U-#235# does.


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.