Watch our interactive graphic on how nuclear fuel is produced, used and stored.
Before its use in a reactor, uranium must undergo four processing steps to convert it from an ore to solid ceramic fuel pellets. These processes are: mining and milling, conversion, enrichment and fabrication.
Mining and Milling
Uranium miners use several techniques to obtain uranium: surface (open pit), underground mining and in-situ recovery. Uranium also is a byproduct of other mineral processing operations. Solvents remove the uranium from mined ore or in-situ leaching, and the resulting uranium oxide—called yellowcake—undergoes filtering and drying.
The yellowcake then goes to a conversion plant, where chemical processes convert it to uranium hexafluoride. The uranium hexafluoride is heated to become a gas and loaded into cylinders. When it cools, it condenses into a solid.
Uranium hexafluoride contains two types of uranium, U-238 and U-235. The percentage of U-235, which is the type of uranium that fissions easily, is less than 1 percent. To make the uranium usable as a fuel, its U-235 content is increased to between 3 percent and 5 percent. This process is called enrichment. The concentration of U-235 is so low in enriched uranium that an explosion is impossible.
After the uranium hexafluoride is enriched, a fuel fabricator converts it into uranium dioxide powder and presses the powder into fuel pellets. The fabricator loads the ceramic pellets into long tubes made of a noncorrosive material, usually a zirconium alloy. Once grouped together into a bundle, these tubes form a fuel assembly.
For more information, see the U.S. Nuclear Regulatory Commission's website pages on "Stages of the Nuclear Fuel Cycle," "Uranium Conversion," and "Fuel Fabrication."
Size of Nuclear Fuel Assemblies
A single fuel assembly for a boiling water reactor (BWR) is approximately 14.5 feet high and weighs approximately 704 pounds. A single fuel assembly for a pressurized water reactor (PWR) is approximately 13 feet high and weighs approximately 1,450 pounds. The PWR fuel assembly weighs more because it contains 264 fuel tubes, while the BWR fuel assembly contains 63.