Here's why:

* Fissionable: U-235 is a fissile material, meaning it can be split by neutrons.

* Abundance: While not the most abundant isotope of uranium, U-235 is present in a significant enough amount to make it practical for nuclear power.

* Chain Reaction: When a neutron strikes a U-235 atom, it splits the atom, releasing more neutrons, which can then cause further fission reactions, leading to a chain reaction.

Other Fissionable Materials:

While U-235 is the most common fuel, other fissionable materials are also used in nuclear reactors, including:

* Plutonium-239 (Pu-239): A byproduct of uranium fission and can be used as fuel itself.

* Uranium-233 (U-233): Produced by bombarding thorium-232 with neutrons.

However, it's important to note that U-235 remains the primary fuel source for fission reactions.