Enriched uranium
Enriched uranium is uranium in which the proportion of the isotope uranium-235 (²³⁵U) has been increased above its natural level through a process known as isotope separation. Natural uranium consists predominantly of uranium-238 (²³⁸U), which accounts for over 99% of its composition, with small amounts of uranium-235 and uranium-234. Only ²³⁵U is fissile with thermal neutrons, making it essential for most types of nuclear reactors and for the manufacture of nuclear weapons. Enrichment therefore plays a fundamental role in civil nuclear power production, military technology, and selected scientific and medical applications.
The residue left after enrichment, known as depleted uranium (DU), contains less ²³⁵U than natural uranium. Although less radioactive, DU retains very high density and is consequently used in shielding materials and armour-piercing ordnance.
Natural Uranium and the Need for Enrichment
Uranium extracted from the Earth contains only about 0.72% ²³⁵U and is unsuitable for most nuclear reactors. A few reactor types, such as CANDU and certain graphite-moderated reactors, can operate using natural uranium because they use efficient neutron moderators. Most commercial reactors, however, require uranium in which the ²³⁵U concentration has been increased to between 3% and 5%.
Before enrichment can occur, mined uranium ore undergoes milling, where chemical treatment concentrates the uranium into yellowcake, a material typically containing roughly 80% uranium. After milling, the material must be converted—either to uranium dioxide for reactors using natural uranium, or to uranium hexafluoride (UF₆), the chemical form used in enrichment facilities.
Grades of Enriched and Related Uranium
Uranium is categorised according to its ²³⁵U content, reflecting the differing requirements of reactors, research programmes, and military applications.
Reprocessed Uranium (RepU)Reprocessed uranium is recovered from spent nuclear fuel through chemical and physical separation techniques. It contains slightly more ²³⁵U than natural uranium but also includes uranium-236, an isotope that absorbs neutrons and complicates reactor performance by reducing fuel efficiency. RepU may also contain trace transuranic elements and fission products, demanding strict control during fuel fabrication and reactor operation.
Low-Enriched Uranium (LEU)LEU contains less than 20% ²³⁵U, with commercial light-water reactors typically using fuel enriched to between 3% and 5%. Slightly enriched uranium (SEU) contains under 2% ²³⁵U. Research reactors sometimes use LEU enriched from 12% up to about 19.75%, the upper limit chosen to replace highly enriched uranium while maintaining acceptable reactor performance.
High-Assay Low-Enriched Uranium (HALEU)HALEU falls between 5% and 20% ²³⁵U and is increasingly important in advanced reactor systems, including small modular reactors (SMRs). Its higher fissile content allows more compact and efficient designs.
Highly Enriched Uranium (HEU)HEU contains 20% or more ²³⁵U and is essential for specialised reactors, naval propulsion systems, certain research facilities, and nuclear weapons. Weapons-grade uranium contains about 85% or more ²³⁵U, but enrichment above 90% is common in military stockpiles. HEU enables compact, highly efficient fission assemblies. The first nuclear weapon used in warfare—the “Little Boy” bomb dropped on Hiroshima—used uranium enriched to about 80%.
Although theoretically possible, weapons made with low-enrichment uranium would require impractically large masses of material. As enrichment decreases, the critical mass required for an explosive chain reaction increases sharply. Modern weapons often use plutonium in primary stages, while HEU may appear in secondary stages due to its favourable nuclear properties.
HEU is also used in the production of important medical isotopes such as molybdenum-99, which decays to technetium-99m, a widely used diagnostic imaging agent.
Enrichment Methods
Isotope separation is challenging because chemical methods cannot distinguish effectively between ²³⁵U and ²³⁸U. The difference in mass between the two isotopes is just over 1%, and even smaller when processed as hexafluoride gas. Industrial enrichment therefore relies on physical processes that exploit mass differences over many repeated stages.
Gaseous DiffusionHistorically the first large-scale method, gaseous diffusion forces UF₆ through porous membranes. Because lighter molecules move slightly faster, each stage produces a minutely enriched stream. Thousands of stages are needed to achieve reactor-grade enrichment. This process consumes large amounts of energy and has largely been replaced.
Gas CentrifugationCentrifuge technology spins UF₆ at high speeds to separate isotopes by mass. It is far more energy efficient than gaseous diffusion, using only about 2–2.5% as much power. Centrifuges are the dominant commercial technology and a key factor in modern enrichment capacity.
Emerging ResearchOther enrichment concepts, such as those based on nuclear magnetic resonance or laser isotope separation, continue to be explored. However, evidence for widespread commercial viability remains limited.
Depleted Uranium and By-products of Enrichment
Depleted uranium (DU), composed mainly of ²³⁸U, has reduced radioactivity but retains uranium’s natural density. These characteristics make DU useful for radiation shielding, counterweights, and armour-piercing projectiles. In powder or aerosolised form, DU must be handled with caution due to its chemical and radiological properties.
Enrichment also produces intermediate and side products such as uranium oxides and other uranium compounds that require controlled handling, storage, and disposal.
Civil, Military, and Scientific Uses of Enriched Uranium
Enriched uranium supports a wide range of important applications:
- Civil nuclear power generation — LEU fuels commercial light-water reactors around the world.
- Research and isotope production — Research reactors use tailored enrichment levels for neutron flux management. HEU historically supported high-flux reactors for isotope production, though many programmes are now shifting to LEU.
- Naval propulsion — Certain submarine and naval reactor designs rely on HEU for compact, long-lived cores.
- Nuclear weapons — HEU enables efficient, compact weapon designs, especially in early and some modern stages of thermonuclear weapons.
- Medical technologies — Production of radioisotopes essential for diagnostics and therapy.
