There are a number of different proposals for permanent disposal of spent nuclear fuel. Here are summaries of some possible techniques.
Deep geological repository: This proposal consists of digging a deep hole in a geologically stable formation and burying sealed containers that would not leak into the water table. A depository in Yucca Mountain in Nevada was approved in 2002 and was under development until 2010 when it was defunded by an act of Congress.
Dry cask storage: In this technique, fuel rods are sealed into steel cylinders filled with an inert gas. The cylinders are welded or bolted shut and some are encased in a concrete, steel or a shell of another material to block radiation. These casks can be used for both transportation and storage. They can be stood on end or stacked horizontally in concrete and steel enclosures. This is considered to be only a temporary storage option pending development of a long term option.
Ducrete: Ducrete is a special form of high density concrete. The name is derived from “Depleted Uranium Concrete”. Crushed ceramic pellets of depleted uranium dioxide is used in place of gravel in a Portland cement binder. The depleted uranium absorbs gamma rays and the water used in the cement absorbs neutrons. The Ducrete was developed as a means of shielding other nuclear waste and disposing of depleted fuel pellets.
Ocean floor disposal: This alternative consists of burying spent nuclear fuel in the ocean floor where it won’t be disturbed by either geological or human activity. Some options discussed included dropping concrete encased waste into holes drilled in the ocean floor or using torpedoes to send the encased waste into the ocean floor. It would be difficult to recover the waste at a future time if that was desirable. There is an international treaty in force to prevent dumping radioactive materials in the oceans that will be in force until 2018 at which time such a disposal method could be considered.
Deep borehole disposal: In this type of disposal, extremely deep holes would be bored into the earth as much as five miles. Some proposals call for lining the bore hole. Other proposals would rely on hot waste to melt the borehole and its contents which would then solidify. The borehole above the waste would be filled with clay, cement, crushed rock, asphalt and/or other materials to further isolate the waste.
Nuclear transmutation: In this form of waste treatment, spent nuclear fuel is irradiated with fast neutrons in a reactor. Dangerous and long-lived transuranic radioisotopes such as plutonium, neptunium americium and curium fission and are transmuted into other elements which have shorter half-lives or are not radioactive.
Other techniques of spent nuclear fuel disposal are being discussed and developed. However, none of the techniques covered above or other proposals has been selected as being the definitive safe economical method of permanent spent nuclear fuel disposal.
Underground storage diagram from geo.arizona.edu
