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Fuel Production

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.