A friend and colleague who prefers to remain anonymous has asked me to post the following. For reasons you can probably guess, I haven’t had time to take a hard look at the numbers.

By now everyone already knows about the spat between Israeli PM Netanyahu and the Obama administration over “red lines” for the Iranian nuclear program. The New York Times explains what was behind it:

“The Israelis are worried that once Iran accumulates a bomb’s worth of 20 percent-enriched uranium, it’s an easy dash to get weapons-grade nuclear fuel,” said David Makovsky, a senior fellow at the Washington Institute for Near East Policy, who met recently with Israeli officials. “Before they decide they’re on their own, I think they want to know that they and Washington see eye-to-eye that this is a red line that cannot be passed.”

In the annals of Israeli red lines for Iran’s nuclear program, “a bomb’s worth of 20 percent-enriched uranium” is only the latest threshold criterion, but it’s by far the most specific. (Compare it to “the point of no return” and “the zone of immunity.”) So, by this measure of breakout capability, just how close is Iran, anyway?

The answer is, they’re already there!

First, let’s establish a little more clearly what’s being discussed. Former Israeli deputy national security adviser Chuck Freilich recently explained it this way in the Jerusalem Post:

The Netanyahu government, convinced that sanctions have failed and that Iran is rapidly nearing both nuclear capability and the point of invulnerability to an Israeli attack, has clearly begun preparing public opinion for a military strike. The Obama administration, preoccupied with the elections, continues to cling to sanctions, stressing that the US’s unique military capabilities will still enable it to act for some time and thus that it is too early for a strike.

Technically, this is true, but Iran is just months from having sufficient fissile materials for its first bomb and if it disperses them, or actual bombs once operational, around the country, the US too will no longer have the ability to strike.

What this seems to describe is a diversion scenario, with a stockpile of 20% enriched uranium being raced out of storage to one or more hidden sites where it would be re-enriched to weapons grade and then fashioned into a bomb. This idea appears to be born of the conviction that Iran is racing to get to its first bomb at any cost, and all other objectives and considerations are negligible. Call it the worst-case scenario.

You could also call it overtaken by events.

Bring on the numbers!

The IAEA’s latest report on Iran, GOV/2012/37, says that Iran has produced, at last measure, “189.4 kg (+43.8 kg since the previous report) of UF6 enriched up to 20% U-235.” UF6 is the gas containing uranium that’s fed through the enrichment process, and would be used in the diversion scenario. U-235 is a fissile isotope of uranium, the stuff that would make a bomb (or a reactor) work. Slightly more than half of the cumulative total of the UF6, 96.3 kg, has already been fed into a conversion process to make reactor fuel, which is relatively benign, although any fuel not actually introduced into a reactor core or otherwise irradiated could be reconverted to UF6. Another 1.6 kg has been blended down, meaning it’s no longer enriched. A total of 91.4 kg is stored as UF6 and is therefore potentially available for rapid diversion.

For simplicity, let’s initially consider just the 91.4 kg of “up to 20% U-235″ that’s in storage at last count. How close is that to a bomb’s worth?

Just 67.618% of UF6 by mass is uranium, so setting aside impurities, there’s about 61.8 kg of uranium in there. Most of that mass isn’t U-235, the fissile isotope. How much is? The enriched uranium is being produced at two sites, PFEP and FFEP. According to GOV/2012/37, back in early April, as part of an investigation into certain irregularities discussed earlier at ACW, Iran disconnected the product cylinder at FFEP, allowing the IAEA to verify the enrichment level. It turned out to be 19.2% U-235. If the PFEP product is enriched to a little more than that, then the overall result could be slightly higher, but using the 19.2% figure for the overall calculation gives us 11.9 kg of U-235.

In practice, a bit less than that would be available for a weapon. Re-enrichment would involve the loss of some of the U-235 into the tails (i.e., waste) stream. Exactly how much would be up to the Iranians. Achieving a smaller “tails fraction” would require more time to complete the work of re-enrichment. Presumably, they wouldn’t select a tails fraction that would put that first bomb out of reach. But just to account for this issue, let’s round down to 11 kg of U-235.

And what if we throw in the converted product? As mentioned above, GOV/2012/37 says that 96.3 kg of the “up to 20% U-235″ UF6 has entered the conversion process for fuel. Fig. 8 conveniently states how much of that is already irradiated or is in the core at the Tehran Research Reactor, and is therefore unavailable for the scenario we’re contemplating. The total comes to 3.375 kg of uranium (not UF6). Following the same calculation above, we get 96.3 kg of UF6 * 0.67618 = about 65.2 kg of uranium. Subtracting the above-mentioned 3.375 kg of uranium leaves about 61.7 kg of uranium. This 61.7 kg * 0.192 = about 11.9 kg of U-235, coincidentally.

Not only would this total be subject to losses from re-enrichment, but it would face additional losses from conversion in both directions, since a certain amount of material gets “held up” in the system. (Ultimately, of course, these “process losses” would also apply to conversion to metal after the re-enrichment is done, and a bit would be left over after casting into hemispheres, too.) For convenience, let’s say that we’re left with just 9 kg of U-235 in UF6 form after re-enrichment.

Adding that to the estimated 11 kg sums to about 20 kg of fissile U-235.

How much is a bomb’s worth, anyway?

The IAEA considers 25 kg of U-235 to be one “significant quantity,” i.e., a bomb’s worth, but as Cochran and Paine have explained, and as Dalnoki-Veress, Lewis, and Pomper have already discussed extensively, this figure is much too high in almost all contexts. At this point, of course, it would be nice to know something about Iran’s bomb design. If anyone is sufficiently inclined, there might be enough information in GOV/2011/65 (the November 2011 report) and this article in Der Spiegel to take a stab at the question of how much fissile material it takes.

But to make a long story short, Cochran and Paine estimate that a relatively low-tech bomb program would need between 8 and 16 kg of U-235 for a pure fission weapon (an “A-bomb”), depending on how much explosive yield is desired. A medium-tech program, which is perhaps where Iran fits thanks in part to the foreign assistance described in GOV/2011/65, would need between 4 and 9 kg of U-235.

As a reminder, Iran already has about 11 kg of U-235 in the almost-20% U-235 UF6 that’s sitting in storage, and about 20 kg of U-235 available if you throw in other material that could be turned back into UF6. What’s more, GOV/2012/37, the source of the UF6 numbers above, is already out of date. As Table 3 of the report shows, more UF6 enriched to almost 20% U-235 is accumulating rapidly.

The bottom line is that Iran probably has enough material on hand today to support the breakout scenario that Israeli officials reportedly fear is coming within months.

On to the next artificial threshold of capability!