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Thorium 232 is rather abundant on Earth and is now coming into use as a breeder fuel.

The varieties of chemical elements known as isotopes have the same number of. Isotopes are variations of chemical elements that have a varied number of neutrons but the same number of protons and electrons. In other words, isotopes are different forms of the same element that have different amounts of nucleons (the sum of protons and neutrons) because of variations in the total number of neutrons in each of their individual nuclei.

Examples of radioactive isotopes include carbon-14, tritium (hydrogen-3), chlorine-36, uranium-235, and uranium-238.

Thorium- $232$is rather stable, with a half-life of more than 14 billion years.

Thorium- $231$ can be fissile material, but nature contains only trace amounts.

In a thorium-fueled reactor, the thorium $232$absorbs neutrons eventually to produce uranium- $234$.

This is like in the uranium reactors, where fertile uranium- $238$absorbs a neutron to form fissile Uranium- $239$ , then forms Plutonium $239$ by beta decay and serves as fuel for the reactor.

So, the fissile material in a thorium reactor is uranium- $238$.