By Ivan Oelrich and Ivanka Barzashka
Last week, the New York Times and the Financial Times USA ran stories that implied that Iran had been hiding enriched uranium and had been caught red-handed during the most recent International Atomic Energy Agency (IAEA) physical inventory inspection. While supposedly based on the IAEA report (GOV/2009/8), the articles more closely followed the ISIS analysis of the report. [Jeffery Lewis, as usual, also has good analysis and comments on Arms Control Wonk.] The IAEA report itself raises few alarm bells. Yes, the Iranians are continuing to enrich uranium; yes, they claim it is exclusively for a civilian nuclear reactor program, a claim for which no one can provide credible assurances, and, yes, every day they enrich uranium, they are closer to having enough for nuclear weapon capability, once that political decision is made. But the IAEA report does not reveal any sudden jump in enrichment capability or even uranium inventory and it goes out of its way to say that the result of the inspection is consistent with what was previously declared by Iran, within “the measurement uncertainties normally associated with enrichment plants of similar throughput”. So what is the issue here?
The Financial Times headline, “Iran holds enough uranium for bomb” with the subtitles “UN report reveals leap in nuclear stocks” and “Capacity breaches Israel’s ‘red line’ limit,” and the New York Times headline, “Iran Has More Enriched Uranium than Thought” are both more provocative than warranted by the IAEA report itself. Both articles report that, in the most recent IAEA report from 19 February, the estimated inventory of low-enriched uranium (LEU) had jumped by a third. The New York Times said that the IAEA had “discovered” an additional 460 pounds (or 209 kg) of LEU. This number is wrong to begin with because the IAEA reported an additional 209 kg, not of uranium, but of uranium hexafluoride (UF6). UF6 is about 68 percent uranium, so there is only an additional 142 kg of uranium in 209 kg of UF6. We will come back to this.
Reasoning Behind the Headlines
“Discovered” is stretching. The origin of that 209 kg is the difference between the amount of enriched UF6 reported in IAEA documents GOV/2009/8 and GOV/2008/59 — two consecutive reports for Iran’s inventory as of November 2008. The IAEA report of 19 November 2008 states that from February 2007 to 7 November 2008 “…based on the operator’s daily accounting records, Iran had produced approximately 630 kg of low enriched UF6 [uranium hexafluoride]. All nuclear material at FEP [Fuel Enrichment Plant], as well as all installed cascades, remain under Agency containment and surveillance.”
The next IAEA report, released just last Thursday 19 February 2009, contained the results of the IAEA physical inventory verification (PIV) that took place between 24 and 26 November 2008. According to the PIV, Iran had produced 839 kg of low enriched UF6 from February 2007 to 17 November 2008.
The most important point is that these two “inventories” were very different animals. The 7 November inventory was based on adding up the entries in the operators’ logbooks and developing an estimate of overall LEU stockpile. For the PIV, the IAEA actually measured how much LEU the Iranians had on hand. Ideally, the two should match quite closely, so one would expect that the November LEU according to the two reports would be the same. If there is a discrepancy, then Iran may be hiding something.
Is there a discrepancy?
So if there is an extra 209 kg of UF6, why doesn’t the IAEA report scream fraud? Because according to the IAEA reports, all the quantities of UF6 produced balance out with the measured inputs so no uranium has gone missing and none that was hidden has suddenly appeared. All of the uranium and enrichment equipment was under constant IAEA monitoring so nothing was “discovered” except in a bookkeeping sense. The Iranians knew during the 7 November inspection that the IAEA measurement verification was coming a fortnight later. The Iranians are at least months away from getting significant quantities of highly enriched uranium (HEU), so it seems unlikely that they were trying to hide any LEU inventory for only a couple of weeks to get any head start on the IAEA inspectors.
The logbook inventory and the measurement inventory took place ten days apart. Could that account for more the difference? Most likely not. In the following measurement period, from 17 November 2008 to 31 January 2009 ( 75 days) the Iranians enriched an additional 171 kg of UF6 or 2.28 kg UF6/day. Similarly, from 30 August 2008 to 7 November 2008 Iran enriched an average of 2.17 kg UF6/day. So from November 7th to the 17th,we might expect them to enrich an additional 22 to 23 kg, or only about ten percent of the 209 kg difference we are concerned with. So the additional 10 days’ production cannot account for the difference.
It is more plausible that the Iranians, novices at the centrifuge business, have not developed sophisticated measurement and recording systems to track their own production. The IAEA reports do not contain enough detail to determine the source of the discrepancy but there are many possibilities. We do not know what containers the Iranians use to store their UF6 (the international standard is a steel cylinder called the 30B, which can hold up to 2.5 tons of UF6 and is too big for Iran’s program). UF6 is a solid at room temperature and pressure. It is stored by pumping the gas from the centrifuges into a cylinder, where the UF6 condenses into a white solid. Because the gas condenses on the inside, the gas pressure does not go up as more gas is pumped in. Knowing how much UF6 is in the tank is not as simple, therefore, as reading a pressure gauge –the mass of the cylinders must be measured. The storage containers can be weighed continually; that is, the cylinders essentially sit on a scale but perhaps the Iranians don’t do that. If cylinders are moved from the production area to a storage area only when full and are then weighed, jumps in measured inventory are expected.
The 17 November PIV was the second IAEA physical inventory since Iran began enrichment in February 2007. The first one was done in December 2007. The IAEA report (GOV/2008/4) announcing the results of the December 2007 PIV did not report any inconsistencies in inventory or Iran’s technical problems reporting LEU. The report was also the first time that enriched amounts of UF6 were disclosed and it commented that “the throughput of the facility had been well below its declared design capacity.” There are no prior published logbook enrichment amounts to compare the December PIV results to, probably because of the small cascade throughput. Consequently, we do not have evidence that Iran has ever understated production previously and obviously we have no comparison for historical logbook-to-inventory accuracy.
Not Enough for a Breakout…Yet.
By “breakout” they mean that, by starting with LEU, Iran could use its enrichment capacity to more quickly produce a bomb’s worth of the highly-enriched uranium needed for a bomb than it could if it were starting with natural uranium. While true in theory, the numbers presented by the IAEA indicate that Iran is not quite there.
Natural uranium has two isotopes (well, a trace of a third, but we can ignore that). In natural uranium, the U-238 is 99.3 percent and the U-235 is 0.7 percent. A reactor requires uranium that is 3-5 percent U-235 and a bomb requires uranium that is 90 or so percent U-235. By saying that the Iranians have enough LEU to further enrich to make a bomb, both articles are depending on simplistic calculations contained in the ISIS summary that look solely at the amount of U-235 in the LEU and see that it adds up to the 25 kg that is estimated to be required of a bomb. But just taking the quantity of LEU and multiplying by the U-235 concentration does not work because not all of the U-235 is recovered.
It is amazing how helpful even a little arithmetic can be. Taking the IAEA number, as of last November, the Iranians had fed 9956 kg of natural UF6 into their machines. Natural uranium is 0.71 percent U-235 so 9956 kg of UF6 contains 47.6 kg of U-235. During this time, the Iranians produced 839 kg of 3.5 percent LEU UF6. Multiply that out and we discover that the LEU contains 20.0 kg of U-235, which seems to imply there is not quite enough U-235 for a bomb, assumed to require 25 kg or so. (Note that if the calculations are redone using the masses, not of uranium, but of UF6, then the result is more than 29 kg, which would be enough for a bomb if it were pure uranium, not UF6. We suspect there is some confusion in the uranium-to-UF6 conversions.) But note that that also means that 27.8 kg of U-235 ended up in the waste or “tails,” which is to say that the Iranians are throwing away 58 percent of the U-235 that goes into the their machines. If the Iranians just recycled their LEU through the existing centrifuges, they would still throw away roughly the same fraction of U-235 so, while the U-235 is, indeed, present in the LEU, the Iranians cannot get it out in a pure form as easily as these articles suggest.
These numbers also indicate that the tails have a U-235 concentration of .45 percent, which is about double the normal commercial tails concentration. In normal centrifuge operation, natural uranium is fed into a set of centrifuges and the product is slightly enriched in U-235 and the waste is slightly depleted in U-235. The product of the first group of centrifuges is fed into another group for further concentration and so one until the desired concentration is reached. The waste stream is not just thrown away, however, and other centrifuges work to reconcentrate the waste to recover as much of the U-235 as practical. The high concentration of U-235 in the waste suggests that the Iranians have made a decision to get as much enriched product as quickly as possible with a given centrifuge capability. This is probably bad news for the rest of the world because it is consistent with getting a small amount of bomb material quickly, rather than a large amount of nuclear fuel eventually.
Another way to look at the problem is to consider that the Iranians have two constraints here: one is uranium and the other is enrichment capacity. The latest IAEA report states that the Iranians produced an additional 171 kg of 3.5 percent LEU UF6 between 17 November and 31 January. That is 116 kg of uranium equivalent over a period of 75 days. If the tails are still .45 percent U-235, that works out to 391 kg Separative Work Units (SWUs) or 5.2 kg-SWU/day.
If the Iranians had enough 3.5 percent LEU to feed into their machines to go for HEU, then, with 5.2 kg-SWU/day capacity, they could get 25 kg of 90 percent U-235 in 214 days if they left their tails at 1.2 percent U-235. However, they currently do not have enough uranium for this option. If they reduce the tails to 0.2 percent U-235, they will extract more U-235 and stretch their uranium supply, but then the enrichment to HEU of enough material for a bomb mass would take 384 days.
It’s Bad Enough, Don’t Make It Worse
We do not want to seem to be apologists for Iran. Their uranium enrichment program makes no economic sense. It could be consistent with a nuclear power fuel program but it is also consistent with a nuclear weapons program. It seems undeniable that Iran wants to at least maintain the option of developing a nuclear weapon. An Iranian nuclear weapon would be a danger to the world, and to Iran. We believe the rest of the world should work hard to avoid such a development but the world should develop policies based on the best analysis available. The hard facts are bad enough, there is no need for exaggeration.
To empower new voices to start their career in nuclear weapons studies, the Federation of American Scientists launched the New Voices on Nuclear Weapons Fellowship. Here’s what our inaugural cohort accomplished. The FAS Nuclear Notebook is one of the most widely sourced reference materials worldwide for reliable information about the status of nuclear weapons and has been published in the Bulletin of the Atomic Scientists since 1987. The Nuclear Notebook is researched and written by the staff of the Federation of American Scientists’ Nuclear Information Project: Director Hans […] [UPDATED] The Biden administration has decided to add a new nuclear gravity bomb to the US arsenal. The bomb will be known as the B61-13. New satellite imagery shows that preparations to deploy Russia’s new Sarmat intercontinental ballistic missile are well underway.
To empower new voices to start their career in nuclear weapons studies, the Federation of American Scientists launched the New Voices on Nuclear Weapons Fellowship. Here’s what our inaugural cohort accomplished.
The FAS Nuclear Notebook is one of the most widely sourced reference materials worldwide for reliable information about the status of nuclear weapons and has been published in the Bulletin of the Atomic Scientists since 1987. The Nuclear Notebook is researched and written by the staff of the Federation of American Scientists’ Nuclear Information Project: Director Hans […]
[UPDATED] The Biden administration has decided to add a new nuclear gravity bomb to the US arsenal. The bomb will be known as the B61-13.
New satellite imagery shows that preparations to deploy Russia’s new Sarmat intercontinental ballistic missile are well underway.