Iran and the United States are negotiating the extension of a ceasefire in hopes of beginning formal discussions that will include Tehran’s nuclear program. A central objective for Washington is a guarantee that Iran cannot produce a nuclear weapon. The most sensitive element in those negotiations is not the damaged centrifuges or destroyed buildings but the inventory of enriched uranium Iran had accumulated prior to the June strikes.
Although Israel and the United States struck much of Iran’s enrichment infrastructure in June and left several facilities either destroyed or seriously damaged, analysts and officials are concerned that a large portion of the enriched uranium stockpile may have survived those attacks. That surviving stock is the primary focus of U.S. concern as diplomats prepare for nuclear talks.
On social media on Friday, the U.S. President demanded that Iran permit the recovery and destruction of enriched uranium that he said had been buried underground following earlier strikes. He called for that process to be coordinated with Iran and the U.N. nuclear watchdog.
What is highly enriched uranium?
Highly enriched uranium, or HEU, is one of the two fissile materials that can be used to make the core of a nuclear bomb, the other being plutonium. Plutonium is typically derived from spent reactor fuel through a process that requires large, conspicuous facilities. By contrast, uranium requires enrichment using centrifuges - equipment that can have a relatively small physical footprint compared with the infrastructure for plutonium extraction.
Uranium is considered highly enriched at 20% purity, while weapons-grade material is generally around 90% enrichment. Modern commercial reactors ordinarily use fuel enriched up to about 5%, and some specialized reactors use higher enrichment. The process becomes exponentially easier as enrichment levels rise - for example, advancing from 60% to 90% is less difficult than taking natural or low-enriched uranium up to 5%.
When enriched, uranium initially exists in a gaseous form and additional steps are required to convert it into metal suitable for use in a weapon. Those steps mean that enrichment alone does not equate to an immediate weapon, but enriched uranium at higher levels shortens the path to a weapon if further processing occurs.
How much enriched uranium did Iran hold before the June attacks?
The International Atomic Energy Agency (IAEA) estimated Iran’s holdings at the moment when the first Israeli bombs fell on June 13 as follows:
- 440.9 kg enriched to up to 60%
- 184.1 kg enriched to up to 20%
- 6,024.4 kg enriched to up to 5%
- 2,391.1 kg enriched to up to 2%
Using an IAEA metric, the report indicated that the roughly 440.9 kg at 60% enrichment could, if further enriched, be sufficient for around 10 nuclear weapons. The 184.1 kg at up to 20% would be enough for about one weapon, and the 6,024.4 kg at up to 5% could be further processed to yield material for some 12 weapons. How much of these quantities survived the June strikes remains unclear, because Iran has not informed IAEA inspectors about the post-attack disposition of the stocks and has not allowed inspectors to return to the sites where they were stored.
IAEA Director General Rafael Grossi has said the agency believes "a bit more than 200 kg" of the 60% material is kept in a tunnel complex in Isfahan that appears to have been predominantly spared by the June strikes. He also indicated that some of the material had been at the Natanz site.
Why is the 60% material the focus of concern?
The United States is particularly worried about the uranium enriched to around 60% because it represents the shortest and simplest route to weapons-grade material. As enrichment rises, each incremental increase toward 90% entails fewer and easier steps. That is why Washington has pressed for the removal or destruction of the high-enriched stocks.
Iran maintains that it does not seek nuclear weapons. The U.S. withdrawal from the 2015 nuclear deal - cited by U.S. officials as a turning point - allowed Iran to expand its atomic program beyond the limits set under that accord, which had restricted enrichment to 3.67%.
Can the material be moved?
Moving enriched uranium is possible and has precedent. Prior to the June attacks, Iran transferred enriched material between facilities under IAEA monitoring. Under the 2015 deal and its precursor, Iran shipped out or converted stocks of uranium enriched to up to 20% by diluting them or turning them into reactor fuel plates for export. Transferring highly enriched material internationally is described as sensitive but attainable with appropriate precautions. Mr. Grossi has said the 60% material can be moved, though it requires care.
Is Iran prepared to surrender or transfer the 60% material?
Reports indicate Iran’s supreme leader has issued a directive against sending the 60% stock abroad, according to two senior Iranian sources. Iranian officials have, however, floated the possibility of a compromise: transferring half of the 60% stock to a third country in return for uranium enriched to 5%, and diluting the remaining half within Iran to lower enrichment levels. Whether such an arrangement will be acceptable to the United States and verifiable to the IAEA is an open question that will be central to upcoming negotiations.
What remains uncertain?
Key uncertainties include the exact quantities of enriched uranium that survived the June strikes, the locations where those stocks now reside, and whether Iran will permit the IAEA sufficient access to verify any agreement on the disposition of the material. These ambiguities complicate both the diplomatic process and the technical work required to render the high-enriched material unusable for weapons production.
As talks progress, diplomats will need to balance the logistical and verification challenges of handling and possibly moving HEU against the strategic imperative that the United States has expressed - preventing Iran from having a readily accessible path to a nuclear weapon.