“They are very close, they are six months away from being about 90 percent of having the enriched uranium for an atom bomb.”

--Israel Prime Minister Benjamin Netanyahu, on NBC’s “Meet The Press,” Sept. 16, 2012

Israeli officials have a long history of claiming that Iran is close to having a nuclear weapon–indeed, in 1992, Israeli officials suggested Iran was just a “few years” from a nuclear weapon. So with that track record, the latest assertion by the Israeli prime minister might be easy to ignore.

But in this case, Netanyahu is on the right track. In fact, a case could be made that Iran already is ahead of his timeline. Note that he did not say Iran would have a nuclear bomb—just that the Islamic Republic would have the material for a nuclear bomb.

The latest report from the International Atomic Energy Agency suggests that Iran already has more than enough uranium enriched to 20 percent that could converted into weapons-grade (90 percent) uranium for at least one nuclear weapon.

An interesting commentary at ArmsControlWonk goes through the numbers, as does an article by Gregory S. Jones for the Nonproliferation Policy Education Center. Jones notes that the the combined stockpile of Iran’s enriched uranium is less than one cubic yard--making it a difficult target for any possible nuclear strike. (Some of Jones’ ‘breakout” calculations for Iran, which have received publicity, have been disputed by other experts.)

By way of explanation, below is a graphic (which first appeared in The Washington Post in 2010) that demonstrates that the leap from 20 percent to 90 percent enriched is much quicker than going from 5 percent to 20 percent. But it would be a risky gambit. Most experts believe that Iran’s breakout would be detected by the IAEA and the United States, and there would be enough time to respond, including militarily, before Iran could make enough weapons-grade uranium for a bomb.

Jeffrey Lewis, who runs ArmsControlWonk and is director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies, added further context:

Iran is 90 percent of the way there.

Now, as my colleague noted, Iran is busy converting about half this stockpile into metal for fuel, so it would have to be reconverted to be further enriched.

So, how long will it take Iran to produce another 90 kilograms of 20 percent UF6?

Iran produced 43.8 kg since the last report, a period of about three months, so six months is a reasonable timeframe to produce about twice that much again.

Iran has been 90 percent of the way there before and probably will be again in a few months.

The important question is whether this is a significant measure of capability or an important red line. I don’t think so. Ninety percent is not one hundred percent, and close only counts in horseshoes. I believe that both Washington and Tehran are confident that the international community would detect an effort to use Natanz or Fordow [Iranian facilities] to enrich to weapons grade with enough time to intervene before Iran could produce a working nuclear device. (After all, the “90 percent” number counts only separative work, not the act of building and/or configuring the cascades.)

Iran is making progress toward acquiring a bomb option. This is a very serious challenge that should not be ignored. But for Iran to exercise that option -- and go the last 10 percent -- Tehran would need to reconfigure cascades and produce HEU [highly enriched uranium] over a long period of time during which IAEA inspections are certain to occur, to say nothing of the possibility that any order to do so would leak or be intercepted.

I do worry about secret locations and better centrifuges than the ones installed… If Iran attempted to reconfigure large numbers of cascades at Natanz or Fordow, the world would notice and Washington’s patience would run out.

Bibi’s “90 percent” talking point is simply a clever way to use the counterintuitive way that SWU [separative work units] and enrichment levels work to scare people into agreeing with him.

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