Note: post amended with a correction at the bottom
Dan Senor, a foreign policy adviser to Mitt Romney, has declared that “”Iran is on the brink of getting a nuclear weapon”.
This is hardly unprecedented language. A few months ago, The Times (of London, that is) splashed “Iran close to Bomb after nuclear breakthrough” on its front page. That headline was changed for the online edition – but it still pronounced Iran “one step away from the Bomb”. The Telegraph was just as injudicious: “Iran on brink of nuclear weapon, warns watchdog”. Ditto The Daily Beast, which also put Iran “on the brink of nuclear weapons”.
This language might not surprise many people. After all, the proverbial “brink” is becoming a bit of a foreign policy joke. Yemen and Pakistan both seem to approach it asymptotically. But it does convey the requisite urgency and alarm.
Iran has 73+ kg of 20% LEU, needs 85[kg] for a bomb. [M]aking the bomb’s easy. I call that “brink”.
These figures don’t appear to be correct.
This seems like a good time to quote three chunks of an Arms Control Wonk post from two months ago:
All too often, pundits make serious mistakes when converting a real measurement like kilograms into the more evocative “bombs worth.” Few pundits make clear their assumptions on how much nuclear material is needed for a weapon in which context. Invariably, the result is to exaggerate either the danger faced or averted. The problem is especially bad when the topic is an amount of highly enriched uranium.
The concept of “significant quantity” of uranium is important in this regard:
IAEA safeguards standards define a significant quantity as “the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded. Significant quantities take into account unavoidable losses due to conversion and manufacturing processes and should not be confused with critical masses.” And the current IAEA glossary defines that threshold amount for HEU as 25kg of U-235 in HEU. (U235 is the stuff that goes boom.)
So the claim that Iran needs 85kg of 20% LEU is clearly untenable.  In fact, even
[o]ne hundred kilograms of 20 percent HEU contains 20 kg of U-235 — less than one significant quantity.
How much does Iran actually have, and how much does it need? On the basis of the February IAEA report, Daryl Kimball estimates that
[a]lthough Iran has now produced about 110 kilograms of 20% enriched uranium, it has dedicated 8 kilograms to make fuel assemblies for the TRR. That material would no longer be part of a ready stockpile of 20% enriched uranium that can be rapidly converted to weapons-grade. Iran would need at least 120 kilograms of 20% material in order to make enough weapons grade uranium for a single weapon. It would want sufficient material for more than just one weapon if it were to decide to produce them. [emphasis added]
Some estimates are more conservative. Iran Watch, part of the Wisconsin Project on Nuclear Arms Control, estimates the quantity “to produce a bomb’s worth of weapon-grade uranium metal” to be higher, at 140 kilograms.
Others are more generous. The ScienceWonk blog at the Federation of American Scientists (FAS) judges that:
A hundred kilograms of uranium enriched to 20% U-235 [just under what Iran is estimated to possess at the moment] will have about 20kg of U-235. But a fissionable mass of 94% pure U-235 weighs about 16kg so Iran might have enough uranium with further enrichment to the weapons-grade level to make a single nuclear weapon – maybe two. [emphasis added]
The AEI’s own blog says the following:
If Iran required 25 kg of weapons-grade uranium to fuel one bomb, it would take 4.7 months after September when it is projected to have 141 kg uranium enriched to 20%. [emphasis added]
In fact, there are three distinct questions about timelines.
The first is how much time it would take Iran to enrich a “significant quantity” of uranium to 20% (not long).
The second is how long it would take Iran to enrich that stockpile further to weapons-grade (obviously, longer). The answers to both of these questions depend on a complex set of assumptions about how and where they conducted the enrichment.
The third question is how long it would then take to fabricate a nuclear device, if they chose to do so (and, it bears repeating, that they are not thought to have made any such decision). There is a range of views here:
Greg Jones claims it’s easy:
the viewpoint that it will take Iran years to develop the non-nuclear components required for a nuclear weapon is hard to square with the actual historical experience of the nuclear weapon states. It is well-known that for past nuclear weapon programs, the key impediment was the need to acquire the fissile material (HEU or plutonium) for the weapon.
Jacques Hymans, writing in Foreign Affairs, says it’s hard:
The Iranians had to work for 25 years just to start accumulating uranium enriched to 20 percent, which is not even weapons grade. The slow pace of Iranian nuclear progress to date strongly suggests that Iran could still need a very long time to actually build a bomb — or could even ultimately fail to do so. Indeed, global trends in proliferation suggest that either of those outcomes might be more likely than Iranian success in the near future.
This is because of “Iran’s long-standing authoritarian management culture”:
In a study of Iranian human-resource practices, the management analysts Pari Namazie and Monir Tayeb concluded that the Iranian regime has historically shown a marked preference for political loyalty over professional qualifications. “The belief,” they wrote, “is that a loyal person can learn new skills, but it is much more difficult to teach loyalty to a skilled person.” This is the classic attitude of authoritarian managers. And according to the Iranian political scientist Hossein Bashiriyeh, in recent years, Iran’s “irregular and erratic economic policies and practices, political nepotism and general mismanagement” have greatly accelerated. It is hard to imagine that the politically charged Iranian nuclear program is sheltered from these tendencies.
Further reading: a presentation (PDF) by Maseh Zarif for AEI, some questions by ISIS about the intelligence on the Iranian program, former ambassador Thomas Pickering writing on negotiations, a wrap-up of Senate testimony on Iran, and a useful NYT timeline of US-Iran interaction on nuclear issues.
 Note, however: “Tehran claims it has enriched to only 19.75%, thereby avoiding the 20% level, which is notionally the divide between low-enriched uranium and HEU” [Op-Ed, Olli Heinonen and Simon Henderson]
Update/correction: Maseh Zarif correctly notes that I confused solid and gas figures for enriched uranium, and that I omitted to note that a lower-bound for one bombs-worth of enriched uranium was as little as 15kg (much lower than the IAEA’s 25kg “significant quantity).