Fissile versus fertile

A fissile nucleus, such as uranium-235 or plutonium-239, can be split by a neutron of any energy, including a slow one, and so can sustain a chain reaction. A fertile nucleus cannot sustain a chain reaction itself, but after absorbing a neutron it turns, through two radioactive decays, into a fissile one. Uranium-238 and thorium-232 are the two fertile materials that matter in practice, becoming plutonium-239 and uranium-233 respectively. Uranium-238 can be split by fast neutrons, and supplies a small share of the power in an ordinary reactor this way, but not often enough to keep a chain reaction going on its own. That is why a thorium reactor still needs a starting charge of fissile fuel.

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