UN Photo, Credit: J. Carrier
UN Photo/J Carrier

Note from Jeffrey: I’m still on blog sabbatical in baby-land. Here’s another anonymous guest post.

Let’s be fair. It’s hard to simplify what’s not so simple. And it’s harder still to get your point across if few make the effort to understand. The Acme-brand bomb diagram brandished by Israeli Prime Minister Binyamin (Bibi) Netanyahu before the UN General Assembly on Thursday has left the multitudes scratching their heads, but it didn’t have to be that way. The media have done an altogether poor job of interpreting the message. For the most part, they haven’t even tried, reporting instead on the ensuing confusion. Or the smirk-inducing graphic-design choice. You know what? That’s shoddy work.

Now, with just a bit of background information, it’s not hard to gather what Bibi was saying about Iran and The Bomb. Which isn’t to say that it’s wise, in wider perspective, to “draw a red line” where the Israeli PM literally has done. It isn’t.

Let’s see if we can’t break this down.

What the cartoon bomb meant

Let’s start with the numbers. Here’s what Bibi said:

This is a bomb; this is a fuse. In the case of Iran’s nuclear plans to build a bomb, this bomb has to be filled with enough enriched uranium. And Iran has to go through three stages. The first stage: they have to enrich enough of low enriched uranium. The second stage: they have to enrich enough medium enriched uranium. And the third stage and final stage: they have to enrich enough high enriched uranium for the first bomb. Where’s Iran? Iran’s completed the first stage. It took them many years, but they completed it and they’re 70% of the way there. Now they are well into the second stage. By next spring, at most by next summer at current enrichment rates, they will have finished the medium enrichment and move on to the final stage. From there, it’s only a few months, possibly a few weeks before they get enough enriched uranium for the first bomb.

The Prime Minister then cited the reports of the International Atomic Energy Agency as the source of this information, and proceeded to draw a red line — yes, a literal red line — at the 90% mark.

A careful reader would tell you that the 70% and 90% figures in the cartoon don’t appear in the IAEA reports. Bibi isn’t wrong on this point, though; the information is indeed present, but framed differently. The IAEA measures Iran’s enriched uranium stockpile in terms of the amounts (mass) of material at different levels of enrichment – not how close the stockpile comes to “enough high enriched uranium for the first bomb.” Unlike the Prime Minister of Israel, the IAEA doesn’t presume that Iran’s nuclear program is military in character. The reports are instead phrased in terms of Tehran’s failure thus far “to establish international confidence in the exclusively peaceful nature of Iran’s nuclear programme.” It’s left to others to judge the relevance of the enriched uranium stockpile to the “possible military dimensions” of the Iranian program.

The latest IAEA report gives cumulative figures through August 2012 for how much uranium hexafluoride (UF6) Iran has produced at two levels of enrichment: up to 5% and up to 20%. Weapons-grade uranium is about 90% enriched (or 93%). That sounds like a big step up, but it isn’t really. In terms of separative work — the amount of processing needed to reach a particular level of enrichment — 5% enriched is about 70% of the way to weapons-grade, and 20% enriched is about 90% of the way there.

70% and 90%. If those numbers seem unfamiliar, consult the photo at the top of this post.

“Enrichment” is really “separation”

Jeffrey discussed why enrichment works this way a couple of years ago and again in the comments more recently [with a slight correction]:

 …20 percent is something like 9/10ths of the way to 90 percent.

To understand why, recall that “enrichment” is a misleading term — the method is separation or the removal of unwanted material, nothing is added. The simple example that I like starts with 1000 atoms of uranium. Only seven of them will be the fissile isotope Uranium 235. The rest are useless Uranium 238. (We can ignore the U234 in this example.)

To make typical reactor fuel, Iran or any other country removes 860 of the non-U235 isotopes, leaving a U235:U238 ratio of 7:140 (~5 percent).

To make fuel for the TRR, Iran removes another 105 non-U235 atoms from the 140, leaving a ratio of 7:35 (20 percent).

To make a bomb, Iran need[s] only to remove 27 of the remaining 35 atoms, leading [to] a ratio of 7:8 (~90 percent).

To make a bomb, Iran needs only to remove 34 of the remaining 35 U-328 atoms, leading to a ratio of 7:1 (~90 percent).

Which takes less work? Moving 965 atoms or 27 atoms 34 atoms? This simplification does omit the important concept of tails and some other details at very high levels of enrichment (say 95 percent and beyond). But those details aren’t enough to alter the nice smooth curve by Drell et al (on page 59) that shows cumulative separative work units to various levels of enrichment. Notice the sharp “knee bend” around 20 percent.

There is a reason that Iran’s enrichment to 20 percent is provoking a real crisis.

The Washington Post devised a graphic illustrating enrichment-as-separation back in February 2010. But for good measure, here’s Fig. 3.2 from Drell et al. (“Verification of Dismantlement of Nuclear Warheads and Controls on Nuclear Materials,” JASON Report JSR-92-331, Jan. 1993). Notice the “knee bends,” which really end around 20%:

There. That’s not too hard to grasp, I hope.

The assumptions behind the cartoon

Why draw the red line at the 90%-of-the-way mark — or, based on Netanyahu’s remarks, somewhere just before that mark? Let’s go back to the tape:

By next spring, at most by next summer at current enrichment rates, they will have finished the medium enrichment and move on to the final stage. From there, it’s only a few months, possibly a few weeks before they get enough enriched uranium for the first bomb.

Going by this statement, it appears that Bibi has taken a single “significant quantity,” or SQ, as the basis of his timetable. That’s a reference measure used in the context of IAEA safeguards. According to Ron Ben Yishai of Yediot Aharonot, 

For Netanyahu, the term “too late” refers to a situation whereby Iran will have sufficient weapons-grade material (or uranium enriched to 93% purity level) to produce one atomic bomb. Currently the Iranians do not have even one ounce of weapons-grade material, but they have already amassed about 200 kilograms (about 440 pounds) of 20% material.

As soon as Iran will have stockpiled some 260 kilograms (573 pounds) of uranium refined to 20% purity level they will be able to immediately move on to the final stage (enriching the same amount to 93%). With 260 kilos of 20% material it would take Iran a few weeks to two months to produce 26-28 kilos (57-62 pounds) of uranium enriched to 93%, which can be used to build one nuclear warhead.

That 26-28 kg of weapons-grade material translates to about 25 kg of U-235 content, or 1 SQ. For the reasons discussed here, 1 SQ is off-base; Iran is probably already beyond the point of having enough U-235 in its up-to-20%-enriched stockpile for a realistic first weapon. Furthermore, it’s not clear where Bibi gets his growth projection for UF6 enriched up to 20%; as Greg Thielmann has pointed out, Iran’s stockpile of this material actually diminished slightly between May and August 2012. Even as Iran produced more, much of it was being made into uranium oxide suitable for reactor fuel.

But these cavils are probably beside the point. The Prime Minister’s remarks betray a conviction that just as Iran produced a large amount of UF6 enriched up to 5% before starting to use some of it to make UF6 enriched up to 20%, it will in due course start producing UF6 enriched up to 90%. Bibi’s goal comes down to not to getting salami-sliced to weapons-grade uranium, as Joshua would put it. For that purpose, a line simply needs to be drawn at some distinct and recognizable point.

The liabilities of the Netanyahu theory

So what’s the problem? The short version is that committing to use force prior to an Iranian attempt to make weapons-grade uranium is a very dangerous idea. There’s basically no chance that bombing will stop the Iranian nuclear program. But it might spur Iran to take its bomb program off the back burner, speeding up the weapons timetable. As Joshua put it a couple of years back:

It’s often asserted, with an air of worldy maturity and sobriety, that a resort to arms will only provide a few years’ breathing room…. The truth is closer to the opposite.

Here’s how Jeffrey put it recently:

The benefit of a strike is an induced pause in the program — more or less what we have now[,] though imposed through force.  The question is whether an airstrike creates more delay than the current indecision of the Supreme Leader.  So far, I think, the best answer has been no…

It’s gratifying to see, in Sunday’s New York Times, that this message is finally starting to creep into broader awareness, a mere five years since the 2007 NIE.

With Jeffrey’s indulgence, I’ll return to this theme again before long.