Highly enriched uranium

Highly enriched uranium is defined by the International Atomic Energy Agency (IAEA) as uranium containing at least 20% of the isotope U-235. This threshold is significant because it marks the point at which a fissile material becomes directly usable in a nuclear explosive device; below 20%, the material is considered low-enriched uranium (LEU) and is not weapons-usable without further enrichment. HEU is often further categorized into "weapons-grade" (typically 90% or more U-235) and "reactor-grade" (20–90%), though the latter still poses proliferation risks.

The enrichment process increases the proportion of U-235, which constitutes only about 0.7% of natural uranium. The most common methods are gaseous diffusion and gas centrifugation, with the latter being more energy-efficient and widely used in modern facilities. Electromagnetic separation, used during the Manhattan Project, is now obsolete but still relevant historically.

HEU is the primary fissile material in nuclear weapons, where its high U-235 content allows for a compact, efficient explosive yield. The first atomic bomb, Little Boy, used about 64 kg of 80% enriched uranium. In reactors, HEU is used in some research reactors and in naval propulsion reactors for submarines and aircraft carriers, where its high energy density enables long intervals between refueling. The use of HEU in civilian research reactors has been a proliferation concern, leading to international programs to convert them to LEU fuel.

HEU also finds niche applications in radioisotope production for medical and industrial purposes, and in space exploration as a heat source for radioisotope thermoelectric generators (RTGs). However, these uses are limited and increasingly being replaced by LEU alternatives.

Because HEU can be used to build a nuclear weapon with relatively simple gun-type designs, its control is a cornerstone of nuclear non-proliferation. The Treaty on the Non-Proliferation of Nuclear Weapons (NPT) obligates non-nuclear-weapon states to accept IAEA safeguards on all nuclear materials, including HEU. The 2010 Nuclear Security Summit and subsequent initiatives have promoted the minimization and eventual elimination of civilian HEU stockpiles.

Global HEU stockpiles are estimated at around 1,400 metric tons, with the majority held by the United States and Russia. Since the end of the Cold War, over 60 countries have eliminated their HEU holdings, and many research reactors have been converted to LEU fuel. However, challenges remain, including securing HEU in poorly guarded facilities and preventing illicit trafficking.

One lesser-known fact is that HEU was used in the fuel of the Soviet Union's RORSAT satellites, which were nuclear-powered reconnaissance satellites; some of these satellites re-entered the atmosphere, scattering radioactive debris. Another is that the first controlled nuclear chain reaction, achieved by Enrico Fermi in 1942, used natural uranium, not HEU, but HEU was essential for the first atomic bomb.

HEU has also been used in the production of the isotope molybdenum-99, a key medical isotope, but efforts are underway to replace it with LEU-based production. Additionally, the term "highly enriched uranium" is sometimes confused with "depleted uranium," which is a byproduct of enrichment and has very low U-235 content, used in armor and munitions.

Fissile material

A material capable of sustaining a nuclear chain reaction with slow neutrons, such as U-235 or plutonium-239.

Low-enriched uranium (LEU)

Uranium with a U-235 concentration below 20%, not directly usable in weapons.

Weapons-grade uranium

HEU with a U-235 concentration of 90% or more, optimized for nuclear weapons.

Gaseous diffusion

An enrichment method that separates isotopes by passing uranium hexafluoride gas through porous barriers.

Gas centrifuge

A modern enrichment method using high-speed rotation to separate isotopes by mass.