Yellowcake

Overview

Originally, raw uranium ore was extracted by traditional mining, and this is still the case in many mines. It is first crushed to a fine powder by passing it through crushers and grinders to produce "pulped" ore. This is further processed with concentrated acid, alkaline, or peroxide solutions to leach out the uranium. However, nearly half of yellowcake production is now produced by in situ leaching in which the solution is pumped through the uranium deposit without disturbing the ground. Yellowcake is what remains after drying and filtering. The yellowcake produced by most modern mills is actually brown or black, not yellow; the name comes from the color and texture of the concentrates produced by early mining operations.

Initially, the compounds formed in yellowcakes were not identified; in 1970, the U.S. Bureau of Mines still referred to yellowcakes as the final precipitate formed in the milling process and considered it to be ammonium diuranate or sodium diuranate. The compositions were variable and depended upon the leachant and subsequent precipitating conditions. The compounds identified in yellowcakes include uranyl hydroxide, uranyl sulfate, sodium para-uranate, and uranyl peroxide, along with various uranium oxides. Modern yellowcake typically contains 70% to 90% triuranium octoxide (U₃O₈) by weight. Other oxides such as uranium dioxide (UO₂) and uranium trioxide (UO₃) exist.

Yellowcake is produced by all countries in which uranium ore is mined.

Further processing

Yellowcake is used in the preparation of uranium fuel for nuclear reactors, for which it is smelted into purified UO₂ for use in fuel rods for pressurized heavy-water reactors and other systems that use natural unenriched uranium.

Purified uranium can also be enriched into the isotope U-235. In this process, the uranium oxides are combined with fluorine to form uranium hexafluoride gas (UF₆). Next, the gas undergoes isotope separation through the process of gaseous diffusion, or in a gas centrifuge. This can produce low-enriched uranium containing up to 20% U-235 that is suitable for use in most large civilian electric-power reactors. With further processing, one obtains highly enriched uranium, containing 20% or more U-235, that is suitable for use in compact nuclear reactors—usually used to power naval warships and submarines. Further processing can yield weapons-grade uranium with U-235 levels usually above 90%, suitable for nuclear weapons.