Uranium (U, Z = 92): The Element with Contained Energy

Role of Uranium in Cosmology and Geology

Synthesis in Stars and Supernovae

Uranium is an element heavier than iron and cannot be synthesized by ordinary nuclear fusion in the cores of stars. It is mainly produced during cataclysmic events such as neutron star mergers or core-collapse supernovae, via the rapid neutron capture process (r-process). The presence of uranium on Earth thus testifies to violent stellar events that occurred before the formation of the solar system.

Geochronology and Earth's "Natural Clock"

The radioactive decay of uranium into lead is one of the most important dating systems in geology.

• Uranium-lead (U-Pb) dating: Uses the two decay chains \(^{238}\mathrm{U}\) → \(^{206}\mathrm{Pb}\) (half-life 4.47 billion years) and \(^{235}\mathrm{U}\) → \(^{207}\mathrm{Pb}\) (half-life 0.70 billion years). The \(^{207}\mathrm{Pb}/^{206}\mathrm{Pb}\) ratio provides very precise ages, allowing the dating of the formation of the oldest terrestrial (zircons) and lunar minerals, and establishing the age of the Earth at about 4.54 billion years.
• Uranium-thorium (U-Th) dating: Uses the disequilibrium in the \(^{238}\mathrm{U}\) chain to date more recent events (up to 500,000 years), such as corals, limestone concretions (stalagmites), and marine sediments, providing crucial data for paleoclimatology.

Source of Earth's Internal Heat

The radioactive decay of uranium, thorium, and potassium-40 is a major source of heat inside the Earth. This internal heat drives mantle convection, responsible for plate tectonics, volcanism, and the Earth's magnetic field (via the dynamo of the outer core). About half of the Earth's heat flow comes from this radioactivity.

History of the Discovery and Use of Uranium

Etymology and Origin of the Name

The element is named after the planet Uranus, discovered eight years earlier by William Herschel (1738-1822) in 1781. The German chemist Martin Heinrich Klaproth (1743-1817), who isolated uranium oxide in 1789, followed a tradition of naming new elements after celestial bodies. This practice links chemistry to astronomy, as evidenced by other elements:

• Cerium (Ce): Named after Ceres, the first dwarf planet and the largest object in the asteroid belt, discovered in 1801 by Giuseppe Piazzi.
• Selenium (Se): From the Greek Selene (Σελήνη), goddess of the Moon, due to its resemblance to tellurium (named after Tellus, the Earth).
• Tellurium (Te): From the Latin tellus, meaning Earth.
• Palladium (Pd): Named after the asteroid Pallas, discovered in 1802.
• Neptunium (Np) and Plutonium (Pu): Following uranium, these transuranic elements were named after the planets Neptune and Pluto.

From Discovery to Radioactivity

Klaproth believed he had isolated the pure metal, but it was actually an oxide (\( \mathrm{UO_2} \)). The metal was first isolated in 1841 by Eugène-Melchior Péligot (1811-1890). For over a century, uranium was considered a mundane chemical element, used mainly as a yellow or green pigment (uranium glass, "Vaseline glass" tableware) or as an additive in steels.

The revolution came in 1896 when Henri Becquerel (1852-1908) discovered "radioactivity" while studying uranium salts. This revolutionary property was then studied in depth by Marie Curie (1867-1934) and Pierre Curie (1859-1906), who discovered polonium and radium in pitchblende, a uranium ore.

The Nuclear Era: Fission and Weapons

The discovery of nuclear fission by Otto Hahn, Lise Meitner, and Fritz Strassmann in 1938 changed everything. Physicists understood that the nucleus of uranium-235, when struck by a neutron, could split into lighter nuclei, releasing colossal energy and additional neutrons, allowing a chain reaction.

• Manhattan Project: During World War II, a colossal scientific and industrial effort (United States, United Kingdom, Canada) was launched to produce a weapon based on fission. It led to the creation of the first atomic bomb ("Little Boy") with enriched uranium-235, dropped on Hiroshima on August 6, 1945.
• Nuclear Arsenal and Arms Race: Enriched uranium and plutonium (produced from uranium-238) became the raw materials for nuclear deterrence during the Cold War.

Civil Nuclear Energy

After the war, the focus shifted to the peaceful use of nuclear energy. The first nuclear power plant was connected to the grid in Obninsk (USSR) in 1954. Today, nuclear energy, mainly based on the fission of uranium-235 in light water reactors, provides about 10% of the world's electricity, with very low CO₂ emissions.

Deposits and Production

Uranium is a relatively abundant element in the Earth's crust (about 40 times more than silver). The main ores are:

• Pitchblende (\( \mathrm{UO_2} \))
• Carnotite (\( \mathrm{K_2(UO_2)_2(VO_4)_2·3H_2O} \))
• Autunite (\( \mathrm{Ca(UO_2)_2(PO_4)_2·10-12H_2O} \))

The main producing countries are Kazakhstan, Canada, Namibia, and Australia. Extraction is done through open-pit mines, underground mines, or in situ leaching (injection of solutions directly into the deposit).

Structure and Fundamental Properties of Uranium

Classification and Atomic Structure

Uranium (symbol U, atomic number 92) is an element of the actinide series. It is a heavy, dense, and radioactive metal. Its atom has 92 protons and, for its most abundant isotope \(^{238}\mathrm{U}\), 146 neutrons. Its electronic configuration is [Rn] 5f³ 6d¹ 7s², although the 5f and 6d electrons are energetically close, leading to variable valence chemistry.

Physical and Radioactive Properties

• High density: 19.1 g/cm³ (about 70% denser than lead).
• Alpha radioactivity: Natural uranium is weakly radioactive. The isotope \(^{238}\mathrm{U}\) has a half-life of 4.47 billion years, emitting an alpha particle of 4.27 MeV. Its specific activity is low (12.4 kBq/g for natural uranium).
• Metallic state: Silvery-gray metal, malleable and ductile. It has three allotropes (crystalline phases) depending on temperature: orthorhombic (α) up to 668°C, tetragonal (β) up to 776°C, then body-centered cubic (γ).
• Melting point: 1135 °C.
• Boiling point: 4131 °C.
• Pyrophoricity: Fine uranium powder or shavings can spontaneously ignite in air.

Chemical Reactivity

Uranium is a chemically reactive metal.

• Reaction with air: Forms a dark oxide layer (\( \mathrm{UO_2} \)) that partially protects it. In powder form, it ignites.
• Reaction with water: Reacts slowly with cold water and vigorously with hot water to form uranium dioxide and hydrogen.
• Reaction with acids: Dissolves in most acids.
• Oxidation states: The +4 and +6 states are the most common and stable.
  • U(IV): Stable compounds, such as dioxide \( \mathrm{UO_2} \) (black, nuclear fuel).
  • U(VI): Forms the linear uranyl ion \( \mathrm{UO_2^{2+}} \) (bright yellow in solution), present in compounds such as trioxide \( \mathrm{UO_3} \) or uranyl nitrate \( \mathrm{UO_2(NO_3)_2} \).

Main Characteristics

Atomic number: 92.
Group: - (Actinide).
Electronic configuration: [Rn] 5f³ 6d¹ 7s².
Main oxidation states: +3, +4, +5, +6.
Most abundant isotope: \(^{238}\mathrm{U}\) (T½ = 4.47×10⁹ years).
Appearance: Silvery-gray, dense metal.

Table of Uranium Isotopes (natural)

Electronic Configuration and Electron Shells of Uranium

N.B.:
Electron shells: How electrons are organized around the nucleus (new window).

Uranium has 92 electrons distributed across seven electron shells. Its full electronic configuration is: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f³ 6s² 6p⁶ 6d¹ 7s². It is often noted as [Rn] 5f³ 6d¹ 7s², showing that the valence electrons are located in the 5f, 6d, and 7s orbitals.

Detailed Structure of the Shells

K shell (n=1): 2 electrons (1s²).
L shell (n=2): 8 electrons (2s² 2p⁶).
M shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰).
N shell (n=4): 32 electrons (4s² 4p⁶ 4d¹⁰ 4f¹⁴).
O shell (n=5): 21 electrons (5s² 5p⁶ 5d¹⁰ 5f³).
P shell (n=6): 9 electrons (6s² 6p⁶ 6d¹).
Q shell (n=7): 2 electrons (7s²).

Valence Electrons and Chemical Properties

The valence electrons of uranium (5f³ 6d¹ 7s²) give it a complex and rich chemistry. It can lose these electrons (and sometimes more internal 5f electrons) to form several oxidation states.

• +3 state (U³⁺): Strong reducer, slowly oxidizes in water. Configuration [Rn] 5f³.
• +4 state (U⁴⁺): Stable, forms compounds such as \( \mathrm{UO_2} \) or \( \mathrm{UF_4} \) (green). Configuration [Rn] 5f².
• +6 state (UO₂²⁺): The uranyl ion is extremely stable in aqueous solution and in the solid state. Its linear O=U=O structure is characteristic. It is the most mobile form in the environment.

This ability to change oxidation states is crucial for its nuclear fuel cycle (extraction, conversion, reprocessing) and its environmental behavior.

Applications of Uranium

• Nuclear energy: Fuel in nuclear power plants. Natural uranium (0.7% U-235) is enriched (to 3-5% U-235) to power most reactors. Depleted uranium (mostly U-238) is also used in some reactors (fast neutron reactors) or as fertile material to produce plutonium-239.
• Nuclear weapons: Highly enriched uranium (HEU, >90% U-235) is a material of choice for fission nuclear weapons. Depleted uranium is used in penetrators (kinetic projectiles) due to its very high density and self-sharpening ability on impact.
• Naval propulsion: Enriched uranium reactors power nuclear submarines and aircraft carriers, giving them considerable autonomy without the need to refuel for decades.
• Scientific applications:
  • Geological dating (U-Pb, U-Th).
  • Targets for particle accelerators to produce transuranic elements.
  • Source of radiation in certain industrial or research applications.
• Historical applications: Pigments for glass and ceramics (uranium yellow, uranium green) before the 1940s. Counterweights in aircraft control surfaces (depleted uranium).

The Nuclear Fuel Cycle

From Mine to Reactor

1. Exploration and mining.
2. Concentration and purification: Production of yellowcake (\( \mathrm{U_3O_8} \)) pure at ~80%.
3. Conversion: Transformation into gaseous uranium hexafluoride (\( \mathrm{UF_6} \)) for enrichment.
4. Enrichment: Increase in U-235 content by gaseous diffusion or gas centrifugation.
5. Fuel fabrication: Conversion of enriched UF₆ into uranium dioxide powder (\( \mathrm{UO_2} \)), then pressed and sintered into pellets, which are loaded into zirconium alloy tubes (fuel rods).
6. Use in reactor: Irradiation for 3 to 5 years, with energy production and fission products.

Management of Spent Fuel

• Pool storage: Initial cooling for several years.
• Dry storage: In specific containers.
• Reprocessing (optional): Recovery of reusable uranium and plutonium, separation of ultimate waste (fission products, minor actinides). France is a country that reprocesses its fuel.
• Deep geological storage: Long-term solution for high-level and long-lived waste (Cigéo project in France).

Health, Environment, and Radiation Protection

Chemical and Radiological Risks

Uranium presents a dual toxicity:

1. Chemical toxicity (renal): Like other heavy metals, uranium is toxic to the kidneys. The occupational exposure limit is mainly based on this chemical effect, which becomes critical before radiological effects for natural or depleted uranium.
2. Radiological toxicity (carcinogenic): Due to alpha emissions (and minor gamma/beta emissions from descendants). The main risk is related to inhalation or ingestion of insoluble dust that remains in the body long-term (lungs, bones).

Environmental Management

• Former mining sites: May present risks of contamination of water and soil by uranium and its descendants (radium, radon). Rehabilitation is mandatory.
• Controlled releases: Nuclear facilities release very small amounts of uranium into the environment, strictly regulated and monitored.

Radiation Protection

Handling uranium, especially enriched uranium, requires precautions:

• Confinement (enclosures, gloves) to avoid inhalation/ingestion.
• Criticality protection: For enriched uranium, specific measures prevent any geometric configuration that could initiate an accidental chain reaction (criticality accident).
• Monitoring: Dosimetry, contamination control.

Geopolitical and Economic Issues

A Strategic Resource

• Security of supply: Crucial for countries dependent on nuclear energy.
• Nuclear non-proliferation: The Non-Proliferation Treaty (NPT) and the International Atomic Energy Agency (IAEA) monitor uranium-related activities to prevent its diversion for military purposes. Enrichment is a particularly sensitive technology.
• Volatile market: The price of uranium fluctuates depending on energy demand, political decisions (nuclear phase-out), and the discovery of new deposits.

Future Challenges

• Development of Generation IV reactors: Could use uranium (including U-238) more efficiently and burn their own waste.
• Management of long-lived waste.
• Societal acceptance of nuclear energy in the face of the climate challenge.

Perspectives

Uranium, once an unremarkable element, became in the 20th century the symbol of atomic power, both destructive and civilizing. Its future is intimately linked to that of nuclear energy. Faced with the climate emergency, this low-carbon energy source is experiencing renewed interest, but it must meet the challenges of the circular economy (reuse of materials, waste minimization), absolute safety, and democratic transparency. Whether it remains an energy pillar or is gradually replaced, uranium will remain in history as the element that unleashed the energy of the nucleus, forever changing the destiny of humanity.