There are hundreds of thousands of tons of depleted uranium hexafluoride in temporary storage in the United States. A very small amount has been used in the manufacture of such things as tank armor, armor piercing artillery, shields for industrial radioactive materials and other applications.
Research supported by the U.S. Department of Energy was begun in 1993 into the use of depleted uranium in heavy concrete. There was a dual goal in the research. They were looking for a way to dispose of large quantities of depleted uranium and also a new type of material for the creation of dry casks (covered in last post) used for storage and transportation of spent nuclear fuel. Scientists at Idaho National Engineering and Environmental Laboratory conceived of the new type of concrete and were awarded patents for it in 1998 and 200
Ducrete is a special type of concrete that was invented to help solve the radioactive waste problem. The name is derived from depleted uranium concrete. Concrete is made from a gravel aggregate bound together with Portland cement, water and sand. In ducrete, the gravel aggregate is replaced by granular depleted uranium left over from the process of creating enriched uranium for nuclear reactor fuel or nuclear weapons. The depleted uranium is mostly U-238 with a much lower proportion of U-235 than natural uranium ore. Ducrete is a very efficient shielding material. The depleted uranium shields against gamma rays and the water and sand absorbs neutrons.
There were a series of technical problems that had to be overcome in order to develop ducrete. Depleted uranium in the form of uranium hexafluoride is too reactive to be used in ducrete. It has to be oxidized to form depleted uranium trioxide (DUO3) or depleted triuranium octaoxide (DU3O8). Hydrogen gas is then used to produce uranium dioxide (DUO2) in a powdered form. The second issue is the fact that concrete requires aggregate or gravel. The powder that comes out of the chemical processing has to be turned into a granular form. The powder is heated but does not reach the melting point of the material. Individual atoms diffuse across the boundaries between the powder grains and form larger chunks called depleted uranium aggregate that are suitable for use in concrete.
The ducrete can be used to build dry casks and containment storage building to store radioactive waste. Ducrete casks are smaller and lighter dry casks than containment buildings made of conventional concrete. Because of the additional shielding properties of ducrete, it is actually cheaper to produce ducrete dry casks than using steel, lead and depleted uranium metal for shielding in addition to concrete. While ducrete may be a good cheap alternative for temporary storage, the mild radioactivity of the ducrete casks makes them more expensive to dispose of than conventional dry casks.
However superior ducrete may be for constructing dry casks and containment buildings, it has not actually been utilized for storage of spent nuclear fuel in the United States. No facilities utilizing ducrete have been licensed in the U.S.
