SWU math and IR-40

Reporting indicates that Iran will concede to a compromise on the IR-40 reactor at Arak. They will “redesign” the research reactor to cut output to one-tenth the planned size (assume from 40MW to 4MW). In terms of material:

The amount of plutonium the reactor will be able to yield will be reduced to less than 1 kg (2.2 pounds) from 9-10 kg (20-22 pounds) annually in its original design, he said. Western experts say 9-10 kg would be enough for 1-2 nuclear bombs and that Arak’s capacity should be scaled back.

While a nice concession, an important design aspect of the reactor should be noted. From all indications, the IR-40 design is based on the Russian Reaktor Bolshoy Moshchnosti Kanalnyy, a design that allows rapid changing out of fuel rods without shutting down operations. This makes these type of reactors ideal for plutonium production, as well as medical production (although 4MW or 40MW is still a huge for medical isotope production, when also considering the Tehran Research Reactor meets demand). As the core has already been put into place and the containment vessel constructed, how this will be limited to 1/10 the capacity will likely come through changing out fuel rods with additional pressure tubes. This change would limit the amount of material in the reactor, but can easily be undone by re-configuring the core.

Separately, Iran’s nuclear establishment in a recent report has stated that due to the down-blending of uranium, that Iran is now 18-42 months away from having enough enriched uranium to make a nuclear weapon. This math is dubious. Iran Watch has a nice overview, albeit dated, of Iran’s timetable for production. Based on their footnote i:

According to recent production data at the Natanz Fuel Enrichment Plant, Iran’s IR-1 centrifuges have achieved an average annual output of about .78 Separative Work Units, or SWUs, per machine.  A SWU is a standard measure of the effort required to increase the concentration of the fissionable U-235 isotope within natural uranium.  Based on the assumptions set forth above (see note d), Iran would need approximately 955 SWUs to bring 1,053 kg of low-enriched UF6 to weapon grade (author note: the amount of 3.5% required to get to 25kgs of HEU).  If each of Iran’s 9,000 centrifuges produces an average of .78 SWUs per year, their total output over one year would be 7,045 SWUs, or 587 SWUs per month. Thus, it would take about 1.7 months to produce 955 SWUs.

Now, this is just for 3.5% U-235, not for their stockpile of 20%, which has been dramatically cut from 209kg to 38kg . According to the most recent IAEA report, the level of 5% enriched UF6, the gas state necessary to enrich, is at 8475kg. Using the assumptions from above, this would mean that this stockpile of 5% could be enough for roughly 8 weapons in 13 months, but enough for one weapon in under two months.

This post is not meant to be accusatory or insinuate that Iran is destined to develop a nuclear weapon regardless of what action the international community takes. Rather, it is to highlight that the importance of understanding the technical aspects of international relations. The actions by Iran are being undertaken to signal cooperation, but as evident, they are keeping options open. Putting the genie back in the bottle in nuclear proliferation is difficult (impossible?), but steps can be taken in the right direction to reduce risk. The JPA and, hopefully any future deal, will move in that direction. But focusing on the capabilities of Iran may not yield the long-term benefits and stability that is hoped for. Rather, shaping the intentions of Iran, via incentives and reform, are more likely to meet the long-term objectives of the international community.



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