Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption. Naturally occurring fertile materials Naturally occurring fertile materials that can be converted into a fissile material by irradiation in a reactor include: thorium-232 which converts into uranium-233 uranium-234 which converts into uranium-235 uranium-238 which converts into plutonium-239Artificial isotopes formed in the reactor which can be converted into fissile material by one neutron capture include: plutonium-238 which converts into plutonium-239 plutonium-240 which converts into plutonium-241Some other actinides need more than one neutron capture before arriving at an isotope which is both fissile and long-lived enough to probably be able to capture another neutron and fission instead of decaying. plutonium-242 to americium-243 to curium-244 to curium-245 uranium-236 to neptunium-237 to plutonium-238 to plutonium-239 americium-241 to curium-242 to curium...