A description of the uranium enrichment process, including gaseous diffusion.
Enrichment Process and Products
Using uranium as a fuel in the types of nuclear reactors common in the United States requires that the uranium be enriched so that the percentage of the uranium-235 isotope is increased, typically to 3 to 5%. Uranium enrichment is an isotopic separation process that increases the proportion of the uranium-235 isotope in relation to uranium-238 in natural uranium.
The enrichment process used in the United States involves combining uranium with fluorine to make uranium hexafluoride (UF6) followed by gaseous diffusion (see below). The UF6 output from gaseous diffusion is in two streams - one is increased, or enriched, in its percentage of U-235, and the other is reduced, or depleted, in its percentage of U-235. The depleted uranium hexafluoride product is referred to as "depleted UF6." After gaseous diffusion, the enriched uranium hexafluoride is subjected to further processing, while the depleted UF6 is generally stored.
The uranium enrichment process was first developed in the 1940s as part of the Manhattan Project.
Uranium - From Ore to Reactor Fuel
In gaseous diffusion, uranium hexafluoride is divided into two separate streams, and U-235 selectively diffuses from one stream to the other.
Before processing by gaseous diffusion, uranium is first converted from uranium oxide (U308) to uranium hexafluoride (UF6). The uranium hexafluoride (UF6) is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF6 molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF6 streams, with one stream having a higher concentration of uranium-235 than the other. The stream having the greater uranium-235 concentration is referred to as enriched UF6, while the stream that is reduced in its concentration of uranium-235 is referred to as depleted UF6. The depleted UF6 can be converted to other chemical forms, such as depleted uranium oxide or depleted uranium metal.
Most of the depleted UF6 produced in the United States is stored at the locations where it was produced as a product of the gaseous diffusion enrichment process. These locations are the gaseous diffusion plants near Paducah, Kentucky, and Portsmouth, Ohio, and at the East Tennessee Technology Park (formerly the K-25 Site) at the Oak Ridge Reservation in Oak Ridge, Tennessee.
In addition to gaseous diffusion, other methods can be used to enrich uranium, producing depleted uranium as a by-product. The most common enrichment process used outside of the United States is gas centrifuge enrichment. Laser-based enrichment processes are also under development.
For more information on uranium processing, see the Depleted UF6 Production and Handling slide show.