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On 13 April, Iran announced its intention to enrich uranium to 60 per cent U-235. This was characterized by Iran as a response to a sabotage of its vast underground enrichment cascades at Natanz two days before. The move comes against the backdrop of sensitive negotiations happening in Vienna aimed at rescuing the 2015 Iran nuclear deal, the Joint Comprehensive Plan of Action (JCPOA), and bringing the United States back into compliance with the deal.
Iran had already been producing uranium enriched to just under 20 per cent (around 19.5 per cent) following a decision in December 2020, a deliberate step away from compliance with the JCPOA’s terms. Enrichment to 60 per cent, however, is a significant escalation in enrichment operations.
Once it has been enriched beyond 20 per cent, uranium enters a different nuclear materials safeguards accounting category: highly enriched uranium (HEU). Although under the Nuclear Non-Proliferation Treaty (NPT) it is legal for any country to produce HEU, the JCPOA limits Iran’s uranium enrichment to 3.67 per cent.
Iran’s decision has also inevitably drawn international attention because it brings the country so close to producing 90 per cent-enriched uranium, which is generally considered weapons-grade.
Uranium enriched to 60 per cent cannot be used to make a useful nuclear explosive device, and Iran has no other realistic use for this material.
Nevertheless, 60 per cent was not an arbitrary choice. Cascades of centrifuges are designed to enrich uranium in steps; Iran’s centrifuges are likely set up to enrich up to 20 per cent, from 20 to 60 per cent, and from 60 to 90 per cent. Assuming the 60 per cent-enriched uranium is stored in the form of uranium hexafluoride (UF6) gas—and there would be no point in Iran converting it to any other chemical form—the enrichment step from 60 per cent-enriched to weapons-grade uranium is very short.
This strongly suggests that Iran’s decision was intended to send a political message: ‘We have gone as far as we can go in response to provocations without producing weapons-grade uranium.’
Iran has informed the International Atomic Energy Agency (IAEA; GOV/INF/2021/22) that enrichment to 60 per cent will take place at the Pilot Fuel Enrichment Plant (PFEP) at Natanz. The PFEP is located above ground and is not part of the vast underground enrichment complex. It does not seem to have been damaged in the recent sabotage.
Subsequent developments reinforce the impression that the move is a largely ad hoc political gesture with no immediate benefit to an alleged weapons (or indeed civilian) programme. On 16 April 2021, Dr Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran (AEOI), announced on state television that Iran had begun enriching uranium to 60 per cent at the rate of 9 grams per hour, or about 1 kilogram every five days.
In the same statement, however, Salehi said the intention was to reduce that rate of production to 5 grams per hour and to ‘simultaneously’ produce 20 per cent-enriched uranium. The IAEA confirmed last week that Iran had indeed cut the number of centrifuges producing HEU.
The link with 20 per cent enrichment relates to a process called recombination. According to the IAEA (in GOV/INF/2021/22), Iran is producing the HEU in advanced IR-4 and IR-6 centrifuges. It then intends to feed the ‘tails’ (waste) of this enrichment stage into another IR-4 cascade to produce 20 per cent-enriched UF6. These tails will be somewhere between 5-per cent and 20-per cent enriched.
The likely reason why Iran cut the rate of production of HEU—almost immediately after it started—is to match the quantity of tails to the feed requirement of the 20-per cent enrichment stage. As I argued earlier this month in a SIPRI essay, 20 per cent-enriched uranium has a clear (civilian) purpose for Iran: producing advanced fuel for the Tehran Research Reactor.
If this is the case, it would seem to confirm that demonstrating the capability to produce 60 percent-enriched uranium is more important to Iran than actually producing that material.
Finally, it should be noted that the production of 60 per cent-enriched uranium is a far from irreversible step, and creates no significant new technical barrier to Iran returning to compliance with the JCPOA. However, what impact it has on the current negotiations to salvage the deal remains to be seen.