Overview
Uranium (chemical symbol U, atomic number 92) is a silvery-grey metallic element belonging to the actinide series of the periodic table. It has the chemical formula U and a standard atomic weight of approximately 238.03 g/mol. All isotopes of uranium are unstable, making it weakly radioactive. Despite its radioactivity, uranium occurs widely in the Earth's crust at concentrations of about 2–4 parts per million.
| Chemical Symbol | U |
|---|---|
| Atomic Number | 92 |
| Atomic Weight | 238.02891 g/mol |
| CAS Number | 7440-61-1 |
| Electron Configuration | [Rn] 5f³ 6d¹ 7s² |
| Density | 19.1 g/cm³ |
| Melting Point | 1,132 °C (2,070 °F) |
| Boiling Point | 4,131 °C (7,468 °F) |
| Oxidation States | +2, +3, +4, +5, +6 |
| Block | f-block (Actinide) |
| Period | 7 |
History and Discovery
Uranium was first identified as an element in 1789 by the German chemist Martin Heinrich Klaproth, who isolated it from the mineral pitchblende (uraninite). He named it after the recently discovered planet Uranus. For almost a century after its discovery, uranium was used primarily as a yellow colouring agent in ceramic glazes and glass.
In 1896, Henri Becquerel discovered that uranium emitted radiation spontaneously — a phenomenon later named radioactivity by Marie Curie. This discovery, along with Marie and Pierre Curie's subsequent work isolating radium and polonium from uranium ores, marked the beginning of nuclear science.
The fission of uranium-235 was demonstrated by Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch in 1938–1939. This discovery led directly to the development of both nuclear reactors and nuclear weapons during the 1940s.
Physical Properties
Uranium is a dense, silvery-white metal with a slight yellow tinge when freshly cut. It is harder than most metals but still malleable and ductile. Key physical properties include:
- Density: 19.1 g/cm³ — approximately 1.7 times denser than lead
- Melting point: 1,132 °C (2,070 °F)
- Boiling point: 4,131 °C (7,468 °F)
- Crystal structure: Orthorhombic at room temperature; undergoes phase transitions at higher temperatures
- Electrical conductivity: Moderate; resistivity ~0.28 µΩ·m
- Magnetic behaviour: Paramagnetic
- Hardness: Approximately 6 on the Mohs scale
When exposed to air, uranium oxidises rapidly. A thin layer of uranium oxide (UO₂ or U₃O₈) forms on the surface, giving it a dull, black or grey appearance. Uranium metal powder is pyrophoric and can ignite spontaneously in air.
Chemical Properties
Uranium is chemically reactive and can exist in several oxidation states: +2, +3, +4, +5, and +6. The most stable and common oxidation state in aqueous solution is +6, found in the uranyl ion (UO₂²⁺). The +4 state (U⁴⁺) is also stable under reducing conditions.
Notable reactions and compounds include:
- Uranium dioxide (UO₂): The primary form used as nuclear fuel in reactors. Dark brown–black solid with a fluorite crystal structure.
- Uranium hexafluoride (UF₆): A volatile solid used in the uranium enrichment process. Reacts violently with water.
- Uranium tetrachloride (UCl₄): Dark green solid produced by chlorination of uranium.
- Uranium trioxide (UO₃): Orange-yellow powder; an intermediate in uranium processing.
- Uranyl nitrate (UO₂(NO₃)₂): Yellow crystalline compound used in reprocessing spent nuclear fuel.
- Uranyl acetate: Historically used as a stain in electron microscopy.
Uranium reacts with most nonmetals, including halogens, sulfur, and nitrogen. It dissolves readily in nitric acid and hydrochloric acid but is resistant to alkalis.
Natural Occurrence
Uranium is found in trace amounts throughout the Earth's crust, ocean water, and even in the human body. The average crustal abundance is about 2–4 parts per million (ppm) by weight, making uranium more common than silver, mercury, or gold.
It occurs in over 200 minerals, but commercial extraction focuses on a few:
- Uraninite (pitchblende): Uranium dioxide (UO₂); the primary ore mineral
- Carnotite: K₂(UO₂)₂(VO₄)₂·3H₂O; important ore in the Colorado Plateau
- Autunite: Ca(UO₂)₂(PO₄)₂·10–12H₂O; fluorescent yellow mineral
- Coffinite: USiO₄; common in sedimentary uranium deposits
- Brannerite: (U,Ca,Ce)(Ti,Fe)₂O₆; found in Canadian ore bodies
The world's largest uranium producers (as of the time of this article) include Kazakhstan, Canada (notably the Athabasca Basin in Saskatchewan), and Australia. Other significant producers include Namibia, Niger, Russia, and Uzbekistan. The Cigar Lake and McArthur River mines in Canada are among the world's highest-grade deposits, containing ore assaying 15–25% uranium by weight — hundreds of times the world average.
Isotopes of Uranium
Uranium has 92 electrons and 92 protons. Its naturally occurring form is a mixture of three radioactive isotopes:
| Isotope | Abundance | Half-life | Decay Mode |
|---|---|---|---|
| Uranium-238 (²³⁸U) | 99.2742% | 4.468 × 10⁹ years | Alpha decay → Thorium-234 |
| Uranium-235 (²³⁵U) | 0.7204% | 7.038 × 10⁸ years | Alpha decay → Thorium-231 |
| Uranium-234 (²³⁴U) | 0.0054% | 2.455 × 10⁵ years | Alpha decay → Thorium-230 |
Uranium-235 is the only naturally occurring fissile isotope — it undergoes fission when struck by a slow (thermal) neutron, releasing enormous energy and additional neutrons that can sustain a chain reaction. Its relatively low natural abundance (0.72%) means that most reactor designs require enriched uranium, in which the concentration of ²³⁵U is raised to 3–5% for civilian power reactors, or to over 90% for weapons-grade material.
Uranium-238, while not itself fissile, is fertile — it can absorb a neutron and transmute (via beta decay) into plutonium-239, which is fissile. This property is exploited in breeder reactors to generate more fuel than they consume.
Nuclear Fission
When a ²³⁵U nucleus absorbs a neutron, it becomes highly unstable ²³⁶U, which almost immediately splits into two smaller nuclei (fission fragments), releasing an average of 2–3 neutrons and approximately 200 MeV of energy per fission event. A controlled chain reaction is the basis of nuclear power; an uncontrolled supercritical chain reaction is the basis of a nuclear explosion.
A typical 1 GW(e) nuclear power plant consumes around 27 tonnes of enriched uranium fuel per year. The same amount of energy would require burning approximately 2.5 million tonnes of coal, illustrating uranium's extraordinary energy density.
Uses of Uranium
Nuclear Power
The dominant use of uranium today is as fuel for nuclear power reactors. Uranium fuel is typically fabricated as ceramic pellets of uranium dioxide (UO₂), assembled into fuel rods and bundled into fuel assemblies. These assemblies are placed in the reactor core, where fission generates heat that is used to produce steam and drive turbines.
As of 2008, nuclear power provides approximately 15% of the world's electricity, with over 400 commercial reactors operating in more than 30 countries. France generates roughly 80% of its electricity from nuclear power; the United States has the largest absolute nuclear capacity with over 100 reactors.
Nuclear Weapons
Highly enriched uranium (HEU, >90% ²³⁵U) can be used in nuclear weapons. The first nuclear weapon used in warfare — the "Little Boy" bomb dropped on Hiroshima on 6 August 1945 — used a gun-type fission design with approximately 64 kg of HEU. Modern weapon designs often favour plutonium, but HEU remains a concern in non-proliferation efforts.
Depleted Uranium (DU)
Depleted uranium is the by-product of the enrichment process and consists mostly of ²³⁸U (with ²³⁵U content reduced to ~0.3%). It is extremely dense and hard, making it valuable for:
- Kinetic energy penetrators: Armour-piercing ammunition for military use
- Radiation shielding: In medical and industrial radiation equipment
- Aircraft counterweights: Used in some older commercial aircraft
The military and environmental implications of depleted uranium use — particularly its potential as both a chemical and radiological hazard in conflict zones — remain controversial.
Other Industrial Uses
- Ceramics and glass: Uranium compounds produce vivid yellow, orange, and green colours in glazes. This use largely ceased following the introduction of regulations on radioactive materials.
- Photography: Uranium nitrate was once used in toning photographic prints.
- Electron microscopy: Uranyl acetate and uranyl formate are used as staining agents to provide contrast in biological specimens.
Uranium Mining and Processing
Uranium is mined by three main methods:
- Open-pit (surface) mining: Used for shallow, low-grade deposits
- Underground mining: Used for deep, high-grade ore bodies such as those in the Athabasca Basin
- In-situ leaching (ISL): A chemical solution is pumped underground to dissolve uranium from the ore in place; the uranium-bearing solution is then pumped to the surface for processing. ISL accounts for the majority of world production and is the dominant method in Kazakhstan.
Mined ore is first crushed and ground, then leached with either sulfuric acid or an alkaline carbonate solution to dissolve uranium. The resulting solution is processed to produce yellowcake (U₃O₈), a concentrated uranium oxide powder that is the standard traded commodity.
Yellowcake is then converted to uranium hexafluoride (UF₆) for isotopic enrichment via gaseous diffusion or centrifuge methods. The enriched UF₆ is converted back to UO₂ for fabrication into fuel pellets.
Health and Safety
Uranium poses both radiological and chemical hazards. Although its radioactivity is relatively low (due to its long half-lives), prolonged or heavy exposure is harmful.
Chemical Toxicity
As a heavy metal, uranium is nephrotoxic — it can damage the kidneys. The kidney is the primary target organ for uranium's chemical toxicity. Uranium can also accumulate in bone tissue, where it may remain for years. Soluble uranium compounds (such as uranyl fluoride or uranyl nitrate) are significantly more toxic than insoluble compounds (such as uranium dioxide).
Radiological Hazard
Natural and depleted uranium emit predominantly alpha particles, which have low penetrating power and cannot penetrate the skin. However, if uranium dust or particles are inhaled or ingested, alpha emitters can irradiate internal tissues. This is the primary radiological concern for workers in uranium mines, conversion plants, and fuel fabrication facilities.
Regulatory exposure limits for uranium in the workplace are typically set at 0.2 mg/m³ (air) and are enforced by agencies such as the US Nuclear Regulatory Commission (NRC), the UK Health and Safety Executive (HSE), and the International Atomic Energy Agency (IAEA).
Environmental Considerations
Uranium mining and processing generate large volumes of radioactive waste, including mill tailings — the sandy residue remaining after uranium is extracted from ore. Mill tailings contain radioactive decay products such as radium-226 and radon-222 and must be carefully managed and contained. Long-term tailings management is a significant environmental challenge associated with uranium production.
Uranium in the Nuclear Fuel Cycle
The nuclear fuel cycle encompasses all steps from mining to waste disposal:
- Mining: Extraction of uranium ore from the ground
- Milling: Conversion of ore to yellowcake (U₃O₈)
- Conversion: Transformation to uranium hexafluoride (UF₆)
- Enrichment: Increasing ²³⁵U concentration (for most reactor types)
- Fuel fabrication: UO₂ pellets → fuel rods → fuel assemblies
- Reactor operation: Fission produces heat → electricity
- Spent fuel management: Interim storage, reprocessing (in some countries), or direct disposal
Some countries (notably France, the UK, Russia, and Japan) operate a closed fuel cycle, reprocessing spent fuel to recover plutonium and unused uranium for re-use. Others, including the United States, operate an open (once-through) fuel cycle, where spent fuel is stored and ultimately intended for direct geological disposal.
Related Chemicals and Elements
Uranium's chemistry and nuclear behaviour connect it closely to several other elements:
- Thorium (Th, Z=90): Another naturally occurring actinide; a fertile material that can breed ²³³U for use as reactor fuel.
- Plutonium (Pu, Z=94): Produced from ²³⁸U in reactors; fissile and used as both reactor fuel and in nuclear weapons.
- Radium (Ra, Z=88): A decay product of uranium; historically important in medicine and early radioactivity research.
- Radon (Rn, Z=86): A radioactive noble gas produced in the uranium decay chain; a health hazard in uranium mines and in homes built on uranium-bearing rock.
Key Facts Summary
- Uranium is element 92, the heaviest naturally occurring element.
- It was discovered in pitchblende by Martin Heinrich Klaproth in 1789.
- All uranium isotopes are radioactive; the most abundant is ²³⁸U (99.27%).
- Only ²³⁵U is naturally fissile, making it the primary fuel for nuclear reactors.
- One kilogram of ²³⁵U can theoretically release energy equivalent to about 3 million kg of coal.
- Uranium poses both chemical (nephrotoxic) and radiological hazards.
- Global uranium reserves are sufficient for many decades of nuclear power generation at current consumption rates.