Dany noted in an extended Twitter conversation last week (on the question of whether Governor Romney’s advisers were right that Iran is on the brink of having a nuclear weapon) that the Iranian regime is on the brink of a nuclear weapons capability, citing figures for the amount of 20% enriched uranium it has produced at its declared facilities and the threshold amount required to convert that material into fuel for a nuclear weapon.
A commentator, Royal United Services Institute fellow Shashank Joshi, took issue with those figures, drawing invalid comparisons and accepting a debatable assumption as fact in the process.
There are two main issues here with Joshi’s post:
• Anyone attempting to compare enriched uranium figures must avoid the apples-versus-oranges pitfall. Referencing either gas or solid form figures are both acceptable so long as there is consistency. The figure Dany correctly cited (73 kg) is the amount of 20% enriched uranium Iran had produced at its two enrichment facilities as of the February 2012 International Atomic Energy Agency (IAEA) report. The IAEA reports Iran’s stockpiles in terms of enriched uranium hexafluoride (the gaseous form spinning in centrifuges). This gas would need to be converted into a solid once further enriched to weapons-grade levels for use in a nuclear weapon; 1 kg of enriched uranium gas is equal to approximately 0.67 kg enriched uranium (solid). Hence, the 109.2 kg 20% enriched uranium gas reported by the IAEA is equivalent to about 73.1 kg 20% enriched uranium (109.2 x 0.67 = 73.1). Joshi erroneously implied that the figure Dany gave was incorrect because he was comparing a gas-based figure used by other analysts to the solid-based figure used by Dany and included in our assessment.
• The exact amount of weapons-grade uranium Iran would need to build a nuclear weapon is dependent on numerous variables, but it is possible to accurately assess breakout capability by examining plausible lower- and higher-end thresholds for that amount. In writing that Dany’s reference to an 85 kg threshold of 20% enriched uranium for breakout “is clearly untenable,” Joshi dismisses the possibility that Iran could build a weapon with 15 kg of weapons-grade uranium (90%). The article that Joshi cites to support that conclusion actually concedes that the IAEA’s “significant quantity” threshold of 25 kg weapons-grade uranium “is not a perfect measure” and that it has been criticized for overestimating nuclear weapons fuel requirements. It is possible that Iran could build a weapon with a considerable explosive yield by converting 85 kg of 20% enriched uranium into 15 kg 90% weapons-grade uranium. This breakout can be achieved within months using the more-efficient interconnected cascades at the Fordow facility or within weeks if Iran uses the larger Natanz facility (details here). Our assessment includes estimates and timelines for both 15 kg and 25 kg requirements; however, Joshi only quotes the upper threshold timeline from the assessment and ignores the lower end in order to support his assertion.
The important thing to understand about the Iranian nuclear threat is that the regime has dramatically shortened the time it would need to produce fuel for an atomic weapon in the two-plus years that it has been enriching uranium at a higher level. It is on the brink because it can produce weapons-grade uranium—the most difficult component of a nuclear weapon to obtain—in short order.