Which substance is a fuel used in nuclear power plants?

Uranium, or more specifically Uranium-235.

uranium-235 and plutonium-239

Explanation:

This fuel is mostly used in nuclear reactors to sustain a nuclear chain reaction. Being a radioactive substance used in heating water in turbines to produce fission where atoms are split into smaller particle sizes, and the center of the reactor is the core, which contains uranium fuel. This fuel can be used with some forms of alloy, metals, and oxides. Unlike fossil fuels, nuclear fuel doesn't tend to produce carbon dioxide and sulfur oxides due to the radioactivity. Most of the radioactive material has certain issues and hence significant issues of incidence can take place.