What Is Uranium?

Uranium is a naturally occurring radioactive metallic element with atomic number 92 and the chemical symbol U, positioned at the top of the actinide series on the periodic table. It is the heaviest naturally occurring element, with a density of approximately 19,050 kg/m^3, and it exists in nature as a mixture of three isotopes: uranium-238 (accounting for about 99.27 percent of natural uranium), uranium-235 (approximately 0.72 percent), and uranium-234 (a trace amount in secular equilibrium with uranium-238). The International Atomic Energy Agency characterizes uranium as a weakly radioactive element found in low concentrations in soil, rock, and water worldwide, with mining concentrated in deposits where uranium oxide minerals such as uraninite and coffinite reach economically recoverable grades. The element was first identified in 1789 by Martin Heinrich Klaproth in the mineral pitchblende and isolated in metallic form by Eugène-Melchior Péligot in 1841.

In electrical engineering and energy technology, uranium is relevant primarily as a fuel material for nuclear fission reactors, which account for approximately ten percent of global electricity generation. Its significance lies in uranium-235, the fissile isotope capable of sustaining a chain reaction when struck by thermal neutrons, releasing energy several orders of magnitude greater per unit mass than any chemical fuel.

Physical and Chemical Properties

Uranium metal is silvery-white in its freshly processed state and oxidizes rapidly in air to form a black oxide layer. It is harder and denser than steel and exhibits three allotropic crystal structures below its melting point of 1135 degrees Celsius. Uranium is chemically reactive and forms compounds in oxidation states ranging from +2 to +6, with the +4 and +6 states most common in engineering contexts. Uranium hexafluoride (UF6) is the compound used in gaseous diffusion and centrifuge enrichment processes due to its high volatility. Uranium dioxide (UO2) is the ceramic form used as nuclear reactor fuel pellets, selected for its chemical stability, high melting point of 2847 degrees Celsius, and compatibility with zirconium alloy cladding. IAEA thermophysical properties of materials for nuclear engineering tabulates the thermal conductivity, heat capacity, and density of UO2 as functions of temperature across the operational range of light water reactors.

Isotopic Composition and Enrichment

Natural uranium contains too low a concentration of uranium-235 to sustain a self-sustaining chain reaction in light water reactors, which require fuel enriched to between three and five percent uranium-235 by mass. Enrichment is accomplished by converting uranium ore concentrate (commonly called yellowcake, chemical formula U3O8) to uranium hexafluoride, then separating isotopes by exploiting the small mass difference between UF6 molecules containing uranium-235 and those containing uranium-238. Gas centrifuge technology, which became the dominant enrichment method from the 1990s onward, spins UF6 gas at high rotational speeds to concentrate the heavier isotope near the centrifuge wall. The U.S. Energy Information Administration overview of the nuclear fuel cycle describes the full chain from uranium mining through conversion, enrichment, fuel fabrication, reactor use, and spent fuel management.

Uranium in Nuclear Fuel Cycles

In a light water reactor, enriched UO2 fuel pellets are stacked in zirconium alloy tubes to form fuel rods, which are bundled into fuel assemblies. During operation, uranium-235 fissions when absorbing thermal neutrons, releasing approximately 200 million electron volts per fission event and breeding plutonium-239 from uranium-238 through neutron capture. After three to five years of irradiation, the fuel is removed and stored as spent nuclear fuel, which remains intensely radioactive for thousands of years due to fission products and transuranics. Depleted uranium, the uranium-238-rich byproduct of enrichment, has engineering applications as radiation shielding and in high-density penetrator ammunition, as described by IAEA documentation on depleted uranium.

Applications

Uranium has applications in a range of scientific and industrial fields, including:

• Nuclear power generation in light water, heavy water, and fast neutron reactors
• Fuel for naval propulsion reactors in submarines and aircraft carriers
• Radiation shielding in medical, industrial, and transportation applications
• Scientific research using enriched and depleted isotopic standards
• Space reactor and radioisotope power systems for deep-space missions