Uranium

Uranium is a heavy, silvery metal whose chemical behavior reflects both its metallic character and radioactivity. It is an actinide with several oxidation states, most commonly +4 and +6, allowing it to form compounds such as uranium dioxide and soluble uranyl salts.¹ In air, freshly exposed metal tarnishes and can eventually form an oxide coating.

Uranium occurs naturally in rocks, soils, oceans, and living organisms at low concentrations. Commercial ores are processed into a concentrated material called yellowcake, usually consisting largely of uranium oxides. Important ore minerals include uraninite and carnotite. Uranium deposits are distributed across many regions, and their economic value depends on ore grade, extraction costs, regulation, and market conditions.³

Uranium’s isotopes differ in neutron number but share the same chemical identity. Natural uranium is predominantly uranium-238, with a much smaller fraction of uranium-235 and a trace amount of uranium-234; all are radioactive, but they decay at different rates.¹

Uranium-235 is fissile: after absorbing a neutron, its nucleus can split and release energy and additional neutrons, sustaining a chain reaction under suitable conditions. Uranium-238 is not readily fissile with slow neutrons, but it can absorb neutrons and eventually produce plutonium-239 in a reactor. The proportion of uranium-235 can be increased through uranium enrichment; material with a higher concentration is used in many reactor fuels, while highly enriched material has applications restricted by national and international controls.²

Uranium was identified as a distinct element in 1789 by Martin Heinrich Klaproth, who named it after the recently discovered planet Uranus. For much of the nineteenth century it was mainly a source of colored glass and ceramic glazes. The discovery of radioactivity transformed its scientific significance, and research during the Manhattan Project demonstrated the military potential of a sustained nuclear chain reaction.

Today, uranium is principally associated with nuclear fuel. Uranium dioxide pellets are assembled into fuel rods for power reactors, where fission heat produces steam and electricity. Depleted uranium, left after enrichment, has specialized industrial and military uses because of its density, though those uses remain subject to health, environmental, and regulatory scrutiny. Uranium isotopes also support radiometric dating, including the uranium-lead dating of very old rocks.¹

Uranium’s hazards depend on both radioactivity and chemical toxicity. Inhaled or ingested uranium can affect the kidneys, while its radioactive decay contributes to radiation exposure; the extent of risk depends on chemical form, dose, route, and duration.

A notable natural phenomenon occurred at Oklo natural nuclear fission reactors in present-day Gabon, where uranium deposits sustained self-regulating fission roughly two billion years ago when natural uranium contained more uranium-235 than it does today. The site provides evidence about how geological formations can retain or redistribute radioactive elements over immense periods.

Uranium also has a long-lived environmental dimension. Uranium-238 has a half-life of about 4.5 billion years, and its decay chain includes radium and radon. Mining, milling, tailings, groundwater movement, and waste management therefore require monitoring long after extraction. International safeguards seek to verify that civil nuclear materials are not diverted to weapons programs.³

Fissile

Able to sustain a nuclear fission chain reaction with neutrons of suitable energy.

Enrichment

Increasing the proportion of a particular isotope, especially uranium-235, in uranium.

Yellowcake

A concentrated uranium-oxide product made during ore processing.

Half-life

The time required for half the radioactive atoms in a sample to decay.