URANIUM ENRICHMENT BASICS
Uranium is a radioactive element that occurs naturally in varying but small amounts in soil, rocks, water, plants, animals and all human beings. Small amounts of uranium are found almost everywhere. However, concentrated deposits of uranium ores are found in just a few places, usually in hard rock or sandstone. These deposits are normally covered over with earth and vegetation. Uranium has been mined in Canada, the southwest United States, Australia, parts of Europe, the former Soviet Union, Namibia, South Africa, Niger and elsewhere.
Uranium is the heaviest naturally occurring element, with an atomic number of 92. In its pure form, uranium is a silver-colored heavy metal that is nearly twice as dense as lead. In nature, uranium atoms exist as several isotopes, which are identified by the total number of protons and neutrons in the nucleus: uranium-238, uranium-235, and uranium-234. (Isotopes of an element have the same number of protons in the nucleus, but a different number of neutrons. The three naturally occurring isotopes of uranium are each radioactive, which means the nuclei spontaneously disintegrate or "decay.") In a typical sample of natural uranium, most of the weight (99.27%) consists of atoms of uranium-238. About 0.72% of the weight consists of atoms of uranium-235, and a very small amount (0.0055% by weight) is uranium-234.
In addition to being naturally radioactive, the uranium-235 isotope of uranium is capable of fission, the splitting of the nucleus into two parts, triggered by absorption of a neutron. When this splitting occurs, considerable energy is released.
For many years, uranium was used primarily as a colorant in ceramic glazes, producing colors that ranged from orange-red to lemon yellow. It was also used for tinting in early photography. Its radioactive properties were not recognized until 1896, and its potential for use as an energy source was not realized until the middle of the 20th century. Its primary use is as fuel in nuclear power reactors to generate electricity. It is also used in weapons applications, and in small nuclear reactors to produce isotopes for medical and industrial purposes around the world.
To use uranium for defense or commercial fuel applications, it must be processed or "enriched." The enrichment process used in the United States involves combining the uranium with fluorine to make uranium hexafluoride (UF6) followed by gaseous diffusion. The UF6 output from gaseous diffusion is in two streams - one is increased, or enriched, in its percentage of uranium 235, or 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. To produce fuel, uranium must be enriched so that the percentage of uranium-235 is increased 3-5 percent. For defense applications, the percentage of uranium-235 must be increased to 90-plus percent.
The gaseous diffusion process used to enrich uranium requires uranium in the form of UF6. In the first step of UF6 production, uranium ore is mined and sent to a mill where uranium oxide (often called "yellowcake") is produced. The uranium oxide is then sent to a UF6 production facility. At the production facility, the uranium oxide is combined with anhydrous (without water) hydrogen fluoride and fluorine gas in a series of chemical reactions to form the chemical compound UF6. The product UF6 is placed into steel cylinders and shipped as a solid to a gaseous diffusion plant for enrichment.
Uranium hexafluoride is used in uranium processing because its unique properties make it very convenient. It can conveniently be used as a gas for processing, as a liquid for filling or emptying containers or equipment, and as a solid for storage, all at temperatures and pressures commonly used in industrial processes.
In the gaseous diffusion enrichment plant, uranium hexafluoride (UF6) gas is slowly fed into the plant's pipelines where it is pumped through special filters called barriers or porous membranes. The holes in the barriers are so small that there is barely enough room for the UF6 gas molecules to pass through. The isotope enrichment occurs when the lighter UF6 gas molecules (with the uranium-234 and uranium-235 atoms) tend to diffuse faster through the barriers than the heavier UF6 gas molecules containing uranium-238. One barrier isn't enough, though. It takes many hundreds of barriers, one after the other, before the UF6 gas contains enough uranium-235 to be used for fuel or defense purposes.
Uranium hexafluoride is a chemical compound consisting of one atom of uranium combined with six atoms of fluorine. It is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt.
Uranium hexafluoride does not react with oxygen, nitrogen, carbon dioxide, or dry air, but it does react with water or water vapor. For this reason, UF6 is always handled in leak tight containers and processing equipment. When UF6 comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive hydrogen fluoride (HF) and a uranium-fluoride compound called uranyl fluoride (UO2F2).
Uranium Enrichment Basics was compiled from the following web sites: http://web.ead.anl.gov/uranium/guide/index.cfm http://www.nrc.gov/materials/fuel-cycle-fac/ur-enrichment.html