Nuclear fission occurs when neutrons bombard atomic nuclei and split them, releasing massive amounts of energy. Fission occurs easily in only a few isotopes, or forms of elements, typically uranium and plutonium. (Unlike uranium, plutonium isn’t found in nature—we’ll return to this issue later.)
In nature, uranium is a mix of mostly two isotopes: uranium-235 (U-235) and uranium-238 (U-238). U-235 is especially important because it easily undergoes fission (unlike U-238) and is extremely rare, accounting for less than 1 percent of the world’s natural uranium.
So countries with nuclear ambitions—peaceful or not—need to first increase the proportion of U-235 in their uranium samples through a process called enrichment.
In most cases, nuclear energy production begins with uranium enrichment.
Uranium enrichment most commonly occurs in gas centrifuges. After uranium is converted into gas, it is fed into centrifuges, which rotate at high speeds, to separate the slightly heavier U-238 from U-235. Each round of rotation in a centrifuge lowers the proportion of U-238 and increases that of U-235 in the sample. Uranium can be enriched to various levels, which fall into two categories:
- low-enriched uranium (LEU), which has less than 20 percent U-235 and is often used for nuclear power or in non-power reactors, which produce materials for medical use, scientific research, and other purposes; and
- highly enriched uranium (HEU), which has 20 percent or more U-235 and is mainly used for military purposes: to develop nuclear weapons and in a few other specialized applications such as the reactors on nuclear-powered submarines.
Any level of HEU can be used for a weapon, but HEU enriched to at least 90 percent, sometimes called weapons-grade uranium, is the most common. The more enriched the uranium is, the less of it is needed for a weapon. That means warheads can be smaller and lighter, enabling missiles to cover greater distances and aircraft to deliver more weapons.