Primarily, the sandy process waste material from a conventional uranium recovery facility. This naturally radioactive ore residue contains the radioactive decay products from the uranium chains (mainly the U-238 chain) and heavy metals. Although the milling process recovers about 93 percent of the uranium, the residues (known as "tailings" contain several naturally occurring radioactive elements, including uranium, thorium, radium, polonium, and radon. For further information, see Uranium Mill Tailings and the Backgrounder on Uranium Mill Tailings.