Uranium dioxide

Uranium dioxide (UO₂) is a black, crystalline compound of uranium and oxygen. Its high melting point and ability to sustain nuclear fission make it the principal ceramic fuel in many reactors.

Fluorite structure: A crystal arrangement in which cations form a face-centered cubic lattice and anions occupy tetrahedral sites. UO₂ adopts this structure, which accommodates oxygen defects in the fuel.
Nuclear fuel rod: A sealed metal tube containing nuclear fuel pellets for use in a reactor core. UO₂ is pressed into pellets and stacked inside these tubes.
Uranium: A naturally occurring radioactive element with atomic number 92. Uranium supplies the metal atoms in UO₂ and its fissionable isotopes.
Spent nuclear fuel: Nuclear fuel removed from a reactor after irradiation, containing residual fissile material and radioactive products. Irradiated UO₂ pellets become spent fuel and remain intensely radioactive.
Uranium-238: The most abundant naturally occurring uranium isotope, with 146 neutrons in its nucleus. It makes up most uranium in ordinary UO₂ fuel and can capture neutrons to form plutonium.
Pressurized water reactor: A nuclear reactor that keeps water under high pressure so it can carry heat without boiling in the core. Most pressurized water reactors use UO₂ pellets as their fuel.
Uranium oxide: A family of compounds formed from uranium and oxygen. UO₂ is one member of this family, alongside oxides with different oxygen content.
Nuclear fuel cladding: A metal layer surrounding nuclear fuel pellets to retain fission products and separate fuel from coolant. Cladding confines UO₂ and its radioactive products during reactor operation.
Uranium-235: A fissile uranium isotope that can sustain a nuclear chain reaction with slow neutrons. Its fission supplies much of the heat released by enriched UO₂ fuel.
Boiling water reactor: A nuclear reactor in which water boils in the core and steam directly drives a turbine. Its fuel assemblies commonly contain UO₂ pellets.

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