This policy is the result of the high costs of disposal of uranium and thorium compounds, (particularly nitrates) originally purchased as staining agents for electron microscopy when they were no longer useful.
Natural uranium consists of three isotopes of mass numbers 234 (0.005 percent), 235 (0.711 percent), and 238 (99.283 percent). All three isotopes are radioactive. Its most abundant isotope, uranium-238, decays by emitting an alpha particle with a half-life of 4.47 × 10 9 years. Uranium can take many chemical forms. In nature, uranium is generally found as an oxide, such as in the olive-green-colored mineral pitchblende.
Uranium nitrate is considered both radioactive and an oxidizer. Uranium nitrate wastes are considered "mixed wastes." Other uranium compounds are considered to be radioactive waste when they are no longer useful.
More than two dozen isotopes of thorium are known. All are radioactive. The isotope with the longest half life is thorium-232. Its half life is about 14 billion years. Thorium and its compounds have relatively few uses. The most important thorium compound commercially is thorium dioxide.
Thorium nitrate is considered both radioactive and an oxidizer. Thorium nitrate wastes are also considered "mixed wastes." Other thorium compounds are also considered to be radioactive waste when they are no longer useful.