Overview of Material Types:

The reactor core is the heart of any nuclear reactor and consists of fuel elements made of a suitable fissile material. Presently there are four radioactive materials that are suitable for fission through thermal neutrons. They are plutonium-239 (²³⁹Pu), uranium-233 (²³³U), uranium-235 (²³⁵U), and plutonium-241 (²⁴¹Pu).  The isotopes uranium-238 (²³⁸U) and thorium-232 (²³²Th) are fissionable through fast neutrons.   The subsequent text discusses uranium, plutonium, and thorium as used for nuclear fuel