Uranium Enrichment and Conversion

Uranium from mines arrives at enrichment plants in the form of yellowcake, or U3O8. This is is the raw material for nuclear fuel.

Uranium is generally used in reactors in the form of either uranium dioxide (UO2) or uranium metal; nuclear weapons use the metallic form. Production of uranium dioxide or metal requires chemical processing of yellowcake. Further, most civilian and many military reactors require uranium that has a higher proportion of uranium-235 than present in natural uranium. The process used to increase the amount of uranium-235 relative to uranium-238 is known as uranium enrichment.

U.S. civilian power plants typically use 3 to 5 percent uranium-235. Weapons use "highly enriched uranium" (HEU) with over 90 percent uranium-235. Some research reactors and all U.S. naval reactors also use HEU.

To enrich uranium, it must first be put in the chemical form uranium hexafluoride (UF6). After enrichment, UF6 is chemically converted to uranium dioxide or metal. A major hazard in both the uranium conversion and uranium enrichment processes comes from the handling of uranium hexafluoride, which is chemically toxic as well as radioactive. Moreover, it reacts readily with moisture, releasing highly toxic hydrofluoric acid. Conversion and enrichment facilities have had a number of accidents involving uranium hexafluoride.

The bulk of waste from the enrichment process is depleted uranium, so-called because most of the uranium-235 has been extracted from it. Depleted uranium has been used by the U.S. military to fabricate
armour-piercing conventional weapons and tank armour plating. It was incorporated into these conventional weapons without informing armed forces personnel that depleted uranium is a radioactive material and without procedures for measuring doses to operating personnel.
The enrichment process can also be reversed.

Highly enriched uranium can be diluted, or "blended down" with depleted, natural, or very low-enriched uranium to produce 3 to 5 percent low-enriched reactor fuel. Uranium metal at various enrichments must be chemically processed so that it can be blended into a homogeneous material at one enrichment level. As a result, the health and environmental risks of blending are similar to those for uranium conversion and enrichment.