Uranium is the heaviest primordial element in nature and, to date, the only nuclear-fuel metal deployed at large commercial scale. Its central use is as fuel for nuclear power stations: about 0.7% of natural uranium is fissile uranium-235, which is enriched by gas centrifugation to roughly 3%–5%, fabricated into uranium dioxide pellets, and loaded into fuel assemblies that generate low-carbon baseload electricity in hundreds of reactors around the world, supplying a substantial share of the grid. In the military sphere, uranium-235 or reactor-produced plutonium-239 is the core material of nuclear weapons, a use that drove the build-out of large-scale uranium enrichment and reprocessing industries during the Cold War. The "depleted uranium" left over after enrichment (with a uranium-235 abundance below that of natural uranium) is still radioactive, but its very high density and good mechanical properties have made it useful as ballast for aircraft and helicopters, as ship ballast, as shielding material in radiotherapy equipment, and as penetrator cores in tank armor-piercing ammunition; the latter use is controversial and restricted in several countries. Uranium also has a historical, non-nuclear use: small amounts of uranium oxide added to glass melt produce "uranium glass," which fluoresces yellow-green and was widely used from the nineteenth century through the early twentieth century in decorative glassware, vessels, and tableware. Production ceased long ago, and such pieces now form a specialty category on the collectors' market.
Uranium dioxide (UO2) is the main form of nuclear fuel pellets and, after sintering, serves for long periods in reactor cores. Triuranium octoxide (U3O8) is the most common oxide form in natural uranium ores (pitchblende) and is the principal component of "yellowcake," the product into which uranium ore concentrates are converted before transport and trade. Uranium hexafluoride (UF6) sublimes at only about 56 °C and is the only conveniently gaseous uranium compound used in gas-centrifuge enrichment; the enriched UF6 is then converted back into uranium dioxide fuel. Uranyl nitrate (UO2(NO3)2) is the key product of the extraction flowsheets in uranium ore milling and front-end fuel processing: uranium is extracted and purified from leach liquor as uranyl nitrate and then converted further into oxides. Other uranium compounds were historically used as colorants in glass and ceramics, but they have now largely disappeared from consumer use.
