In nuclear energy, fission occurs when neutrons bombard atoms, specifically targeting their nuclei, and split them, releasing massive amounts of energy. Fission occurs easily in only a few isotopes, or types, of elements, typically uranium and plutonium. (Unlike uranium, plutonium isn’t found in nature--we’ll return to it later.)
In nature, uranium is a mix of mostly two isotopes, uranium-235 and uranium-238, and only the former easily undergoes fission. The trouble with U-235 is that it 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. That’s where enrichment comes in.
In most cases, nuclear energy production begins with uranium enrichment.
Uranium enrichment most commonly occurs in gas centrifuges; this increases the proportion of U-235 in a uranium sample to facilitate fission. Uranium can be enriched to various levels, which fall into two categories: low-enriched uranium (LEU) and highly enriched uranium (HEU). LEU has up to 20 percent U-235 and is often used for nuclear power or in “nonpower reactors,” which produce materials for medical use, scientific research, and other purposes. HEU has more than 20 percent U-235, and is mainly used for military purposes; this includes naval reactors (such as those used in nuclear-powered submarines) but generally means nuclear weapons.
Any level of HEU works for a weapon, but it’s weapons-grade uranium, enriched to at least 90 percent, that’s most common. This is because the higher the enrichment level, the less material that’s needed--clearly desirable for weapons, because smaller warheads fit more easily on missiles that can travel long distances.
Why is this important? Because of “breakout time.”
Breakout time refers to the theoretical amount of time required for a country with an existing enrichment plant to produce enough HEU for a nuclear weapon. (Other technologies are needed to build the weapon itself.) It takes significantly fewer resources and expertise to enrich LEU to the 90 percent needed for weapons-grade uranium than it does to enrich uranium in its natural state to between 3 and 5 percent U-235 (the enrichment level of LEU used in most nuclear power plants).
Because of this, once a country can enrich uranium at all, its breakout time is often just a matter of months.
And that’s a precarious prospect.