Iran’s nuclear program was a major topic in President Joe Biden’s July 13-16, 2022 trip to the Middle East. The most challenging part of producing nuclear weapons is making the material that fuels them, and Iran is known to have produced uranium that is near-weapons grade.

The Conversation asked Brandeis University professor Gary Samore, who worked on nuclear arms control and nonproliferation in the U.S. government for over 20 years, to explain why uranium enrichment is central to Iran’s nuclear ambitions and where the Iranian effort stands now.

What does it mean to enrich uranium?

Natural uranium contains two main isotopes, or forms whose atoms contain the same number of protons but different numbers of neutrons. It’s about 99.3% uranium-238 and 0.7% uranium-235. The uranium-235 isotope can be used to generate nuclear power for peaceful purposes, or nuclear explosives for military purposes.

Enrichment is the process of separating out and increasing the concentration of U-235 to higher levels above natural uranium. Generally speaking, lower levels of enriched uranium, such as uranium with 5% U-235, are commonly used for nuclear reactor fuel. Higher levels of enrichment, such as 90% U-235, are most desirable for nuclear weapons.

For military purposes, why are higher levels of enrichment important?

The higher the level of enrichment, the smaller the amount of nuclear material necessary to produce a nuclear weapon.

The International Atomic Energy Agency identifies 25 kilograms (55 pounds) of 90% enriched uranium as a “significant quantity” necessary for a simple nuclear weapon. But larger amounts of lower-enriched uranium can also work.

For example, the “Little Boy” atomic bomb that the U.S. dropped on Hiroshima, Japan, in 1945 used about 64 kilograms of uranium (141 pounds) enriched to an average of 80% U-235.

From a nuclear weapons design standpoint, smaller amounts of higher-enriched nuclear material are more desirable because that reduces the size and weight of the nuclear weapon and makes it easier to deliver. As a result, modern nuclear weapons based on uranium typically use uranium enriched to 90% to 93% U-235, which is known as weapons-grade uranium, for the primary fuel.