This is first part in a three part series on Iran’s nuclear capabilities that I am writing at the urging of Noah Shachtman from DefenseTech.

When some moron like Charles Krauthammer claims Iran is now just “months” away from a bomb, you can pretty much ignore him: He has no idea what he is talking about.

Overall, Iran is probably a little less than a decade away from developing a nuclear weapon. The key question here is how long it will take Iran to enrich a few tens of kilograms of uranium to more than 90 percent U-235.

Dafna Linzer reported that the US Intelligence Community does not believe that Iran could do so before “early to mid next decade”—a revision of previous assessments that Iran would “have the ability to produce nuclear weapons early in the next decade.”

Why so long? The answer is that Iran still has to build, install and operate its centrifuges to enrich uranium.

David Albright and Corey Hinderstein at the Institute for Science and International Security (ISIS) released an estimate that breaks down the steps for Iran to make fissile material for a bomb, along with a nifty satellite image (at right) of Iran’s Fuel Enrichment Plant (FEP) at Natanz.

Most references to Iran being “months” away from a bomb are really statements about how close Iran will be once it completes the FEP—something, as you will soon see, that will take a few years.

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But, first a little digression …

Iran plans to house about 50,000 centrifues in the Fuel Enrichment Plant (FEP) at Natanz in order to produce low enriched uranium for a notional civil nuclear power program. The output of a centrifuge is measured in “seperative work units”—a measure of the amount of work required to enrich a given amount (product) uranium. In math:

Separative work per unit of product = V(XP) – V (XW) – F/P *[V(XF) – V(XW)]

V(S) = (2*S – 100) * LOG (S/(100-S)]

F/P = (XP- XW)/(XF-XW), where

XF = feed assay (W/O)
XP = product assay (W/O)
XW = tails assay (W/O)
V = separation potential
S = XF, XP, or XW
F/P = feed to product ratio

URENCO understood that most of us find math a quaint endeavor in the age of the calculator, so they posted a sweet SWU calculator on their website. Now, you too can caclulate how much SWU is required to produce 25 kg of HEU (a few thousand depending on some technical factors).

Each of Iran’s centrifuges has an output between 2-3 SWU/year. Iran plans a that the full scale FEP at Natanz will house 50,000 centrifuges, giving the plant a capacity of 150,000 SWU/year—enough for annual reloads of LEU for the Bushehr reactor or, if configured differently, 25-30 nuclear weapons worth of HEU per year. (More on Natanz)

Of course, those are Iran’s plans. Iran probably only has about 700 centrifuges, as well as components for another 1,000 or so.

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So, the real question, however, is how quickly Iran could assemble and operate 1,500 centrifuges in a crash program to make enough HEU for one bomb (say 15-20 kg).

Albright and Hinderstein have created a notional timeline for such a program:

  • Assemble 1,300-1,600 centrifuges. Assuming Iran starts assembling centrifuges at a rate of 70-100/month, Iran will have enough centrifuges in 6-9 months.
  • Combine centrifuges into cascades, install control equipment, building feed and withdrawal systems, and test the Fuel Enrichment Plant. 1 year
  • Enrich enough HEU for a nuclear weapon. 1 year
  • Weaponize the HEU. A “few” months.

Total time to the bomb—about three years.

David and Corey state that this timeline is a worst case estimate that assumes Iran encounters no significant problems along the way:

This result reflects a worst case assessment, and thus is highly uncertain. Though some analysts at the IAEA believe that Iran could assemble centrifuges quicker, other analysts, including those in the US intelligence community, appear to believe that a date of 2009 would be overly optimistic. They believe that Iran is likely to encounter technical difficulties that would significantly delay bringing a centrifuge plant into operation. Factors causing delay include Iran having trouble making so many centrifuges in that time period or it taking longer than expected to overcome difficulties in operating the cascades or building a centrifuge plant.

The interesting question is what technical problems the US IC expects Iran to encounter. The thing about a crash program is that things, well, crash. In another paper, Albright and Hinderstein note some of the potential problems:

Iran might not be able to meet such a schedule for bringing a centrifuge plant into operation. The suspension of manufacturing and operating centrifuges could be reestablished, or Iran might have trouble making so many centrifuges. In addition, Iran does not appear to have accumulated enough experience to operate a cascade of centrifuges reliably. Iran had assembled 164 centrifuges into a cascade just before the suspension, but it did not acquire sufficient experience in operating the cascade to be certain it would perform adequately. Centrifuges can crash during operation, causing other centrifuges in the cascade to fail—in essence, destroying the entire cascade. Thus, Iran might need a year or more of additional experience in operating test cascades before building and operating a plant able to make HEU for nuclear weapons.

Yes, centrifuges spinning at supersonic speeds can crash. Especially if you don’t get the lead out.

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Well, not really lead—but molybdenum hexafluoride (MoF6) (Folks in the 18th century thought molybdenum was lead—hence the name derived from molybdos, or lead in Greek).

I’ve previously emphasized one technical problem—the inability of Iran to make relatively pure uranium hexafluoride (“hex”) to be fed into centrifuges for enrichment. (See Got Gas? Iran Stinks at Making UF6, Aug 13, 2005)

Before introducing UF6 into a centrifuge cascade, the Iranians must rid the gas of impurities like MoF6 or the impurities will plug cascade piping, crashing Iran’s centrifuges.

Richard Stone in this week’s Science Magazine further documents the problems that Iran is having purifying hex at its Uranium Conversion Facility (UCF) near Esfahan:

Creating purified UF6, which can be fed as a gas into centrifuges for isotope separation, would be a much bigger one. According to an official at the U.S. State Department, Iran has struggled to convert UF4 into UF6, a dangerous process involving highly toxic and corrosive fluorine gas. The official also claims that Iranian UF4 is tainted with large amounts of molybdenum and other heavy metals. These oxyfluoride impurities in UF6 “might condense” and thereby “risk blockages” of valves and piping, an IAEA specialist told Science.

Iran’s bad at making hex in part because the Clinton Administration convinced the Chinese to stop building the UCF—a major nonproliferation victory that Stone mentions. Stone cites an interview that Dr Mohammad Saeidi, AEOI deputy for planning and international affairs, discussing the deleterious impact of the Chinese cut-off. (See: Sticks and Stones: China, Iran and the UCF, Sep 05, 2005 . Contains the full-text of the interview, in the event you’re interested.)

Stone also mentions a series of stories by Mark Hibbs detailing Iran’s difficulty in using pulse columns to purify uranium. (Iran’s UF6 Is Crap, Sep 28, 2005 and Chinese mixer-settlers at UCF, Oct 20, 2005).

How long will it take Iran to get it’s act together on hex? Hibbs reported a wide variety of estimates among intelligence services:

Intelligence analysts do not agree on how long it will take Iran to solve current process chemical problems at its restarted Uranium Conversion Facility (UCF) at Isfahan.

These difficulties have thus far prevented Iran from producing uncontaminated uranium hexafluoride (UF6) feedstock for its gas centrifuge enrichment program. Last month, as Iran prepared to operate the plant, Vienna officials said that Iran would require “at least several months” to address its problems (NF, 15 Aug., 1).

According to Israeli government analysts now examining related technical issues, it may take Iran two or three months to begin producing pure UF6. According to U.K. government experts, however, Iran may need about 18 months to do that.

But government analysts do agree on one point: The higher the enrichment level sought by Iran from its gas centrifuges, the more critical it will be for Iran to first eliminate technical problems associated with producing pure UF6.

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Iran still faces a number of technical challenges before it can start churning out fissile material. Those challenges are going to years to solve.

Parts 2 and 3 will discuss whether Iran could mate a warhead to a missile and prospects for a strike against Iran’s nuclear facilities.