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

Dec 31, 2017

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

#### Explanation:

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.