A guide to uranium enrichment
To make a nuclear bomb, a state needs uranium that contains high concentrations of a certain isotope: U235. Natural uranium contains a mere 0.7% of this isotope; weapons-grade uranium contains more than 90% (but it is possible to build a bomb with uranium enriched to less than 90%). This gap is why uranium needs to be “enriched”.
*The process has been simplified; technical and regulatory steps have been omitted. To make a nuclear bomb, further weapon design and engineering steps are needed that are not shown here.
The process begins by converting uranium ore into a gas called uranium hexafluoride (UF6). This gas is then fed into centrifuges that spin UF6 at high speed to concentrate the U235 isotope to the required level. To achieve this at scale, gas centrifuges are linked together forming “centrifuge cascades”.
The level of enrichment determines what the uranium can be used for. Below 20%—known as low-enriched uranium—it is suitable for commercial power plants (3–5%) or research reactors and medical isotope production (up to 20%).
Above 20% is highly enriched uranium, usable in some research reactors and naval propulsion. Weapons-grade is conventionally 90% or above, though lower concentrations can suffice with enough material.
The 20% threshold is a key proliferation dividing line. Iran’s enrichment to 60% raises alarm because it sits far above any civilian justification and within reach of weapons-grade.
By May 2025, Iran had a vast stockpile of highly enriched uranium at 60% and centrifuges which could further enrich to weapons-grade at 90%. These two factors significantly reduced how long it would take Iran to produce enough nuclear material for a bomb—its “breakout” time. But producing enough fissile material is only part of the challenge; weaponisation also requires making warheads and delivery systems, and integrating the two.
Iranian leaders insisted that they had no intention to weaponise their nuclear programme. They maintain they accelerated their activities after 2018 in response to unjustified Western sanctions and the US withdrawal from the 2015 nuclear deal, the Joint Comprehensive Plan of Action (JCPOA) that had limited Iran’s nuclear activities in exchange for the lifting of international sanctions.
By 2025, around two-thirds of Iran’s centrifuges were advanced models—IR-2m and above—and delivered the vast majority of enrichment output; the rest were the older IR-1 model, which is much less efficient.
The International Atomic Energy Agency
Western countries rely heavily on the IAEA for information on Iran’s nuclear programme and use its assessments to judge the threat it poses. A key Western demand has been to restore IAEA access in line with Iran’s obligation as a party to the Non-Proliferation Treaty (NPT).
Iran ratified the NPT in 1970. Under article IV, the treaty guarantees a non-nuclear-weapon state access to nuclear technology for peaceful purposes, subject to verification by the IAEA. Through monitoring and inspections of nuclear sites, the agency is meant to confirm that party states, like Iran, are not diverting nuclear material for any undeclared activities, like weaponisation.
Authorities began restricting the IAEA’s access to Iran’s nuclear activities in 2021, as part of their response to the US withdrawal from the 2015 nuclear deal. The agency issued regular reports on Iran’s nuclear activities up until May 2025; after the June 2025 strikes, Iran blocked IAEA access to the bombed facilities altogether and refused to provide the agency with updated reports regarding the status and location of its enriched uranium.
