Thorium, not being fissile, can’t split to make a nuclear chain reaction like uranium. However, if it is bombarded by neutrons from an energy fuel that is fissile — like uranium-235 or plutonium-239 — it can be converted into uranium-233. The process creates energy and is self-sustaining after it begins; fission of uranium-233 turns more thorium nearby into the same nuclear fuel. There are many more complex processes involved, but this relationship between thorium and fissile materials serves as the foundation for the technology in thorium reactors.