To obtain plutonium-239 and uranium-233 in amounts useful for nuclear energy production, they must be manufactured from materials that occur in relative abundance. Plutonium-239 is produced from reactions following the absorption of a neutron by uranium-238; uranium-233 is produced by neutron absorption in thorium-232. Uranium-238 and thorium-232 are called fertile materials, and the production of fissile materials from them is called breeding.
The reactions for plutonium-239 are
92-U-238 + n ===> 92-U-239
92-U-239 ====> 93-Np-239 + beta particle (electron)
93-Np-239 ====> 94-Pu-239 + beta particle (electron)
For uranium-233 the reactions are:
90-Th-232 + n ===> 90-Th-233
90-Th-233 ===> 91-Pa-233 + beta particle (electron)
91-Pa-233 ====> 92-U-233 + beta particle.
The symbol Pa stands for the element protactinium.
