Eyes on the Prize: Iran’s Highly Enriched Uranium Is Out of Control

Dr Ian J Stewart, 14 June 2025

Much is currently being written about the consequences of Israel’s military action in Iran. The conflict certainly carries  monumental importance that deserves careful analysis. However, as of today, the 14^th of July, only one thing matters and world attention should focus on this question: What’s happening with Iran’s 60% enriched uranium? Right now, more than 400 kgs of material – enough for perhaps 10 weapons – is outside of international oversight. Iran could convert this material into 90% HEU within a couple of days. Attention needs to focus on ensuring this material is not diverted to weapons use. 

Highly enriched uranium is one of the two main types of material that can be used in nuclear weapons and was the material used in the bomb dropped on Hiroshima. There, 64 kilograms of uranium enriched to 80% were used in a crude nuclear weapon that was so simple that it didn’t even need to be tested. Today, Iran would likely favor a more complex design using a smaller quantity of more highly enriched uranium.

For the coming days, what matters most is the amount that is already enriched to 60%, which is a quantity of at least 408.6 kg.[1] That would be enough just under 10 quantities of 25 kgs of HEU as each quantity would be produced from 43.75 kg of 60% enriched uranium. 

While Iran’s broader stockpile of uranium matters in the weeks and months ahead, what matters in the days and weeks ahead is the 60% material.  Once uranium has already been enriched to 60 % U-235, taking it the rest of the way to 90 % requires only a few percent of the separative work needed to go from natural uranium (≈0.7 % U-235) to 60 %. Assuming Iran requires 25kg at 90% enrichment to make each nuclear weapon, the math does not look good. Iran could convert one cascade at Fordow to produce 90% HEU to produce 25kgs of 90% HEU in 21 days. Since Iran has 7 cascades, it could also produce 25 kgs of 90% HEU in as little as 3 days. [2]

It’s important to note that the exact amount of fissile material required and the level of enrichment are variable based on bomb design. Iran could opt for a design that requires a lower level of enrichment or a smaller quantity of material – or both. That in turn relates to weapon delivery considerations. Still, since Iran could put a nuclear weapon in a truck and drive it to Israel, these considerations should be secondary at this point. 

The problem is this. As of 13^th July, we do not know where Iran’s stocks of 60% enriched material are or what is being done to it. The majority of the material was apparently stored at the Esfahan nuclear facility, with smaller quantities presumably also stored or in process at the now destroyed Pilot Fuel Enrichment Plant at Natanz and the Fordow Enrichment Plant, where enrichment to 60% takes place. Prior to the 13^th, IAEA inspectors on the ground had regular access to that material. With the bombing campaign in full swing, the IAEA has not verified that the material is still there. Inspectors also have not verified that Iran is not enriching to 90% HEU at the Fordow site or that material has not been diverted from Fordow. Iranian officials appear to have indicated that material has been moved from Fordow. [3] It is possible that Iran has a cascade of 160 machines (or fewer) hidden away in a shed somewhere in the country that could produce 90% material in a few weeks or that Iran is rapidly building such a facility. 

Of course, enriching the material is not the final step in producing a nuclear weapon. The material would need to be converted into Uranium metal and cast into a weapons shape. To date, Iran has built expertise in converting uranium to metal at the Esfahan site. Presumably, if Iran was to make a dash for a weapon this work could not be done at Esfahan since Israel is likely to continue to pound the facility. Again, Iran may have a secret site squirreled away somewhere in the country for this purpose.

Consequently, a key question is whether Iran has a parallel set of nuclear facilities ready to enrich the uranium to 90%, convert it to metal and fashion it into a bomb. It is unlikely but not impossible that Iran has such facilities. The presence of such facilities would have had to escape the attention of western and Israeli intelligence agencies which have thus far had unprecedented insight into Iran’s nuclear program. For example, intelligence agencies identified the Fordow uranium enrichment facility before its construction was completed, announcing it to the world in 2009.[4] Such is the level of penetration that Iran may have been deterred from building such facilities for fear that their discovery would lead to the conclusion that Iran had resumed its nuclear weapons program. Still, the possibility cannot be ruled out. It is for this reason that the principal verification tool for nuclear weapons verification is to track the fissile material – the uranium. 

Thus, as of today, we are on a ticking clock to reestablish verification of Iran’s 60% highly enriched uranium. To be 100% confident that Iran is not turning this into a nuclear weapon, we must verify Iran’s stock of enriched uranium within the next two days. Beyond this, the level of confidence will decline by the day. After a period of a few weeks, we could have no confidence that Iran had not used the material to make at least one nuclear weapon. 

Of course, facts on the ground could overtake such an assessment. Key questions include whether the Fordow facility continues to be available to Iran or whether it is destroyed. Intelligence agencies may also be able to provide more insight or confidence one way or the other as time passes. It may also be that Israel’s assassination of Iranian nuclear scientists removes Iran’s immediate capability to produce nuclear weapons. The point is that we should not be reliant on such external factors.