Iran has announced plans to enrich uranium beyond the levels allowed under the terms of the 2015 nuclear deal. But how is uranium enriched?

Fresh out of the mine, uranium ore contains about 1% uranium oxide. This is the starting material and it needs some processing. It is treated with chemicals, often strong acids, to extract the oxide and make yellowcake, a powder that contains about 80% uranium oxide.

Before the uranium can be used in nuclear reactors or atomic bombs, it has to be enriched. This is because natural uranium contains too little uranium-235, the form of uranium that is easily split to release energy in the process known as fission.

Natural uranium contains only 0.7% uranium-235, with the remainder mostly made up of uranium-238. The isotopes differ only in the number of neutrons found in their atoms. Uranium-235 atoms have 92 protons and 143 neutrons, whereas uranium-238 atoms have 92 protons and 146 neutrons.

To enrich uranium, yellowcake is first turned into a gas called uranium hexafluoride. This is pumped into centrifuges that spin so fast the ever-so-slightly heavier gas containing uranium-238 is forced to the outside, while the lighter gas containing uranium-235 stays in the middle.

In enrichment plants, thousands of centrifuges are connected in cascades. Each unit enriches the gas a little and then passes it on to the next centrifuge to enrich some more. The process produces two streams of gas: the enriched “product”, which is ultimately used to make fuel or bombs, and the “tails”, known as depleted uranium.

The uranium used in nuclear reactors is enriched to about 4% U-235. But for nuclear bombs it must be enriched to about 90%. Under the nuclear deal, Iran is permitted to enrich uranium to 3.67% but now intends to exceed that limit.

There is no technical barrier facing the Iranians. It is the early stages of enrichment that consume the most energy and the process becomes easier down the line. Industry data shows that more than half of the effort needed to enrich uranium to 90% is spent getting from 0.7% to 4%. When enrichment reaches 20%, the threshold for what counts as “highly enriched uranium”, and a level Iran has produced at Natanz in the past, about 90% of the work towards weapons-grade uranium is done.

The process gets easier because less material has to be moved around at higher levels of enrichment. A plant that enriches uranium to 4% with 5,000 centrifuges may need only 1,500 to reach 20% enrichment. From there, several hundred centrifuges are sufficient to reach the 90% needed for a nuclear bomb. Use 5,000 throughout and the rate of enrichment accelerates dramatically.

“It’s really hard at the start because you have very, very little of the uranium isotope you want. Natural uranium is almost all U-238 and initially getting that little bit of U-235 out is really difficult. But the more refined you make it the faster the refinement process happens,” said Anne Harrington, a lecturer in international relations at Cardiff University.

The more enriched the uranium, the less is needed for a weapon. At 20% U-235 enrichment, the critical mass is about 400kg, but at 90% enrichment the mass drops to about 28kg. The precise amounts depend on bomb design and that will be the bigger barrier should Iran want to become a nuclear state.