Plutonium, a heavy metal, is produced from a common uranium isotope called U-238. Plutonium accumulates in the fuel rods that power nuclear reactors. As U-235 nuclei undergo fission, they emit free neutrons. Non-fissile U-238 nuclei absorb these neutrons and convert to plutonium isotope Pu-239. As quantities of Pu-239 build up in a reactor, it, like U-235, starts to undergo fission. This adds a significant amount of energy near the end of the nuclear fuel's life, as just one gram of Pu-239 can yield as much energy as one ton of oil. Plutonium, then, is both a by-product and a fuel.
Once thought to be among the deadliest substances known to man, plutonium is indeed hazardous. But it is relatively easy to contain its radiation because most plutonium isotopes emit a type of radiation called alpha particle radiation. Alpha particles are actually the nuclei of helium atoms, that is, two protons and two neutrons bound together. During fission, the energy stored in atomic nuclei is released, producing heat and several forms of radiation, most commonly alpha and beta particles and gamma rays. Alpha particles can travel only a few inches in the air and can be stopped by a sheet of paper or a person's skin. This type of radiation is only harmful to humans if swallowed or inhaled. But because it takes a long time for plutonium to decay to a completely harmless substance -- its half-life is 24,000 years -- plutonium is still considered hazardous to humans and the environment and must be safely contained.
The United States doesn't reprocess the plutonium in spent fuel rods because of several disadvantages associated with the practice. These include the additional costs of building and operating reprocessing facilities and the potential for increased radiation exposure to both facility workers and those who transport the spent fuel rods. Moreover, reprocessing would make larger quantities of plutonium potentially accessible to terrorists or others intent on obtaining it for use in weapons. Instead, plutonium removed from U.S. reactors is being stored temporarily in thick steel containers that prevent radiation from escaping, until a permanent home is found for it and other kinds of nuclear waste.