Non-fissionable material from which fissionable material is generated by neutronUncharged elementary particle with a mass of 1.67492716 ·10... absorption and subsequent nuclei conversions. Fertile materials are thorium-232 which can be converted into fissionable uranium-233, and uranium-238 which can be converted into fissionable plutonium-239.
Th-232 + n -> Th-233 -> Pa-233 -> U-233
U-238 + n -> U-239 -> Np-239 -> Pu-239.