Key Facts
- Before its use in a nuclear reactor, a series of processing steps converts mined uranium ore into ceramic pellets, which are loaded into fuel rods.
- Ore mined from open-pit and underground mines travels to a conventional mill. Solvents or ion exchange processing removes the uranium, resulting in uranium oxide, or yellowcake, which is then filtered, dried and packaged. Uranium can also be recovered through a process known as in situ recovery (ISR) mining where oxygenated groundwater is injected through a porous orebody to dissolve the uranium oxide and bring it to the surface. The uranium oxide is then recovered from the solution as in a conventional mill.
- A conversion plant removes impurities from yellowcake and chemically converts the material to uranium hexafluoride. This is heated to become a gas and then loaded into cylinders, where it cools and condenses into a solid.
- Enrichment increases the fissionable isotope, uranium-235 (U-235), within natural uranium from under 1 percent by weight to 3 to 5 percent. This allows the controlled chain reaction required for electricity generation.
- A fuel fabricator converts the enriched uranium hexafluoride into uranium dioxide powder and presses it into fuel pellets. The fabricator loads the ceramic pellets into noncorrosive long tubes, or fuel rods. Once bundled together, the fuel rods form a fuel assembly.
From Ore to Pellets: A Multistep Process
Most power plants create electricity by boiling water into steam that drives a turbine generator. Splitting uranium atoms—the fission process—creates heat to boil the water.
Before its use in a reactor, uranium must undergo four major processing steps to convert it from its raw state to a usable fuel source: mining and milling, conversion, enrichment, and fuel fabrication. The resulting pellets are loaded into fuel rods. When grouped, the rods form bundles, or fuel assemblies, for insertion into the reactor.
Mining and Milling: From Ore to Yellowcake
Uranium miners employ several techniques: surface or open-pit mining, underground mining, and in-situ recovery mining, which involves using liquids to recover minerals from underground ore. Uranium also can be a byproduct of other mineral processing operations.
Open-pit mining involves the removal of a great deal of overlying rock—known as burden—by drilling and blasting. A significant amount of waste rock also is removed. Steps are cut into the exposed ore body to facilitate ore removal by large loaders and dump trucks.
Ore mined from open-pit and underground mines travels to a conventional mill, where it is crushed, ground and leached to dissolve the uranium. Most of the ore is barren rock or other minerals that are not dissolved in the process. These solids, also called tailings, are separated from the uranium solution, usually by allowing them to settle out. Solvents or ion exchange processing removes the uranium from the ore. The resulting uranium oxide, or yellowcake, is filtered, dried and packaged.
In-situ recovery mining in the United States involves the injection of carbonated water through specially drilled wells into an ore body several hundred feet underground. The injected solutions penetrate the ore deposits and dissolve the uranium. This process brings the uranium-bearing solution to the surface, where the uranium is extracted.
Conversion: From Yellowcake to Uranium Hexafluoride
Yellowcake requires further chemical processing before it’s used as a fuel. A conversion plant removes impurities and converts the material to uranium hexafluoride, the form required for enrichment. This compound is heated to become a gas that is then loaded into cylinders, where it cools and condenses into a solid.
Enrichment: Boosting the Fuel’s Potency
Natural uranium contains U-238 and U-235. The lighter U-235 is fissionable and thus usable in various nuclear applications, but makes up less than 1 percent of natural uranium. To optimize uranium for use in nuclear energy facilities, a process called enrichment increases the U-235 content to between 3 and 5 percent by weight. Uranium producers sell enrichment services in “separative work units” (SWU). An SWU measures the amount of energy needed to raise the concentration of U-235 to a specified level.
Fabrication
After enrichment, a fuel fabricator converts enriched uranium hexafluoride into uranium dioxide powder and presses it into fuel pellets. The fabricator loads the ceramic pellets into long tubes, or fuel rods, made of a noncorrosive material, usually a zirconium alloy. Once grouped together into a bundle, these fuel rods form a fuel assembly.
Fuel assemblies, though similar, are designed to meet the specific requirements of each nuclear reactor. Fuel fabricators employ stringent quality-control measures throughout the production process to tailor the fuel assemblies to each reactor.
A typical pressurized water reactor (PWR) contains 193 fuel assemblies composed of about 51,000 fuel rods containing more than 18 million uranium dioxide fuel pellets. A typical boiling water reactor (BWR) contains 764 fuel assemblies composed of about 75,000 fuel rods. In the United States, there are 64 PWRs and 32 BWRs.
A fuel assembly’s life in a reactor typically ranges from 36 to 72 months, after which the majority of the U-235 has fissioned, and there is an inadequate amount to support the chain reaction. Operators schedule an outage to replace about one-third of the fuel assemblies every 18 to 24 months.