Specifically, it's the uranium-235 isotope that is used in nuclear reactors.

Here's why:

* Fissionable: Uranium-235 is fissionable, meaning its nucleus can be split by neutrons, releasing a huge amount of energy.

* Abundance: While not the most abundant isotope, it is present in sufficient quantities in naturally occurring uranium ore to be economically viable.

However, it's important to note that other elements can also be used for nuclear power, although they are less common:

* Plutonium: This is a man-made element produced in nuclear reactors from uranium-238. It can be used in breeder reactors and for weapons.

* Thorium: This is a naturally occurring element that can be used in thorium-based reactors, which are considered safer and less proliferative than uranium-based reactors.

While uranium-235 is the most common, research is constantly exploring other potential fuels for nuclear power.