By Robert Goldston, Guest Contributor and Member of the Council for a Livable World Board
On March 15, 2026, Iranian Foreign Minister Abbas Araghchi told Margaret Brennan of CBS Face the Nation, “I offered actually that we are ready to dilute those enriched material, or down-blend them, as they say, into lower percentage. So that was a, you know, a big offer, a big concession…”
How big a concession was it?
The International Atomic Energy Agency (IAEA) calculated in its September 2025 verification and monitoring report that on the eve of the June 2025 attacks by Israel and the United States, Iran had in its possession 440.9 kg of uranium enriched up to 60% of the explosive uranium isotope, U-235. This and earlier IAEA Verification and Monitoring Reports provide a unique basis for understanding the implications of this stockpile.
Here we use the concept of “separative work” to draw out the implications of the IAEA reports more than is available in press coverage. Separative work is the product of the number of centrifuges employed, the time over which they are employed, and their “separative power.” Separative work is measured in “Separative Work Units,” abbreviated SWU.
The separative work that was used to produce Iran’s stockpile of 60% enriched uranium is about 55,330 SWU. Such a stockpile represents a major commitment by Iran, not just a symbolic gesture, since the IAEA report also details Iran’s stockpile at all other levels of enrichment. This sums to a similar commitment of 54,540 SWU. Thus the 60% enriched uranium represents about half of all the embodied separative work in Iran’s possession.
The separative work required to further enrich Iran’s stockpile of 60% enriched uranium to so-called “weapons-grade,” or 90%, is only 564 SWU, about 1% of the 55,330 SWU committed already. While press coverage has indicated that it is a short technical step from 60% enrichment to 90%, perhaps it is not evident to the public that 99% of the work needed to enrich the full stockpile to 90% has already been performed.
The May 2025 IAEA verification and monitoring report provides redundant data to estimate the separative work capacity of Iran’s workhorse centrifuge “cascades,” each composed of 175 IR-6 centrifuges. Two different cases give 912 SWU per year.
Starting in February 2021, the IAEA was no longer able to verify and monitor centrifuge manufacturing in Iran. Furthermore, the IAEA reports that Iran has informed it that Iran intends to install more IR-6 cascades in its existing facilities and that it plans to construct a new facility, called the Isfahan (sic) Fuel Enrichment Plant, at the Nuclear Reactors Fuel Company site. Another, undeclared deep underground facility may already be under construction near the Natanz enrichment site. Thus, since the IAEA has lost verification and monitoring capability of centrifuge manufacturing in Iran for over four years, it is credible that Iran could already have one or more cascades of IR-6 centrifuges hidden in an unknown location.
The IAEA defines a “significant quantity” as “the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded.” This corresponds, per the IAEA, to highly enriched uranium (above 20% U-235) containing 25 kg of U-235. By these parameters, Iran’s supply of 60% enriched uranium, when processed to so-called “weapons-grade,” 90% enrichment, would provide the fuel for nine nuclear weapons. Importantly, starting from the 60% enriched material, a single cascade of 175 IR-6 centrifuges could produce the “weapons-grade” material needed for one nuclear weapon every 25 days. With more cascades, this would go proportionally faster. According to the IAEA’s definitions, highly enriched uranium hexafluoride gas from a centrifuge is “direct use material” that can be converted to finished uranium metal components for use in nuclear weapons in 1–3 weeks.
These calculations are of necessity simplified, and many issues are not assessed here, such as the detectability of Iranian enrichment and weaponization activities, and the degree to which final preparations have already been made, awaiting only the finished uranium metal components. We do not address the issue of use of the 60% enriched material without further enrichment. But two things are evident. One is that Iran’s stockpile of 60% enriched uranium can be of strategic significance. For example, if the world were to detect a single successful Iranian nuclear weapon test, the potential for eight more such weapons would present a formidable strategic deterrent. The second is that IAEA verification and monitoring provide unique information on the quantity and enrichment of Iran’s stockpile of highly enriched uranium, and on the capacity of its centrifuges and cascades to enrich this stockpile to “weapons grade.”
Thus, a renewed offer to retrieve Iran’s 60% enriched uranium “under the supervision of the Agency [IAEA],” as stated by Foreign Minister Araghchi, and to down-blend it to lower enrichment, along with a return to IAEA verification and monitoring as well as constraints on Iran’s future enrichment levels, would be extremely valuable and well worth bargaining for.
