Fertile materials

Uranium-238 (' or U-238') is the most common isotope of uranium found in nature, with a relative abundance above 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing abso

Plutonium-238 (' or Pu-238') is a radioactive isotope of plutonium that has a half-life of 87.7 years.

nuclides from which fissile material can be generated

Uranium-234 (' or U-234') is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million, or 1/18,000) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of 238U. Thus the ratio of to in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of 234U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, 234Th nuclei emit a

Plutonium-240 (' or Pu-240') is an isotope of plutonium formed when plutonium-239 captures a neutron without undergoing fission. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.

Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half-life of 14.0 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay chain terminates at stable lead-208.