Iran's Nuclear Timetable

Updated September 6, 2012

Iran’s bank of rapidly spinning centrifuges has produced a growing stockpile of low-enriched uranium, able to fuel nuclear reactors, but able also to fuel nuclear weapons if further enriched. Enrichment raises the concentration of the uranium isotope U-235, which fissions in first-generation nuclear weapons.

Based on the amount of low-enriched uranium Iran has stockpiled, and the amount it is believed to be producing each month, the Wisconsin Project estimates that by mid-August, Iran had accumulated enough uranium hexafluoride (UF6) enriched to 3.5 percent U-235 to fuel five nuclear weapons -- assuming Iran decided to further enrich the low-enriched material to weapon-grade (90 percent or more U-235) and process the material into a form suitable for use in a weapon. This estimate accounts for the fact that Iran has been converting some of its low-enriched uranium to research reactor-grade (just under 20 percent U-235). Similarly, Iran has converted about half of the 190 kg of 20 percent enriched UF6 it has produced through mid-August 2012 into fuel assemblies.

As Iran increases its stockpile of low-enriched uranium, and in particular its stockpile of research reactor-grade uranium, it will consolidate its status as a "virtual" nuclear weapon state.

Iran's progress towards this status is estimated a below. These estimates are based upon the theoretical performance of Iran’s existing centrifuges and upon how these centrifuges appear to have performed in the past:

Bomb potential of Iran’s low-enriched uranium stockpile

• Amount of uranium hexafluoride (UF6) enriched to 3.5 percent U-235 produced through early August 2012:
      6,878 kg b
  

• Amount of this material on hand as of August 2012:
      5,310 kg c

• Average daily production rate of this low-enriched UF6 at the Natanz Fuel Enrichment Plant:
      6.8 kg d

• Amount of this low-enriched UF6 needed to produce a bomb’s worth of weapon-grade uranium metal:
      1,053 kg e

• Number of separative work units (SWUs)f needed to accomplish the above:
      955 g

• Number of first-generation implosion bombs this low-enriched uranium stockpile could fuel, if further enriched:
      5 h

• Time needed to convert this low-enriched uranium to one bomb’s worth of finished uranium metal enriched to 90 percent U-235:
      3 - 12 months i

• Date by which Iran's low-enriched uranium stockpile probably was sufficient to fuel one first-generation implosion bomb, if further enriched:
      February 2009 j

• Approximate number of first generation IR-1 centrifuges being fed with UF6 at the Natanz Fuel Enrichment Plant, as of the last reported visit by IAEA inspectors:
      9,156 k

• Average number of SWUs each centrifuge appears to be producing:
     .77 l

• Number of months theoretically needed for these 9,156 centrifuges operating at such a capacity to produce the 955 SWUs theoretically needed to produce weapon-grade fuel for one bomb:
     1.6 m

• Number of additional centrifuges installed or being instaled at the Natanz Fuel Enrichment Plant as of the last reported visit by IAEA inspectors:
     6,000 n
  

Bomb potential of Iran’s research reactor-grade uranium stockpile

• Amount of uranium hexafluoride (UF6) enriched up to research reactor grade (approximately 20 percent U-235) produced through August 2012:
     189.4 kg o

• Approximate amount of this material on hand as of mid-August 2012:
     91.4 kg p

• Amount of this 20 percent enriched UF6 theoretically needed to produce a bomb’s worth of weapon-grade uranium metal:
     140 kg q

• Average monthly production rate of this 20 percent enriched UF6 at the Natanz pilot plant:
     4.5 kg r

• Number of first generation IR-1 centrifuges being fed with UF6 at the Natanz pilot plant as of the last reported visit by IAEA inspectors:
     328 s

• Average monthly production rate of this 20 percent enriched UF6 at the Fordow plant since the last IAEA report:
     9.9 t

• Number of first generation IR-1 centrifuges being fed with this UF6 at the Fordow fuel enrichment plant as of the last reported visit by IAEA inspectors:
     696 u

• Number of additional first generation IR-1 centrifuges installed at Fordow as of the last reported visit by IAEA inspectors:
     1,444 v

• Number of SWUs needed to accomplish the enrichment of a bomb's worth of this 20 percent enriched UF6 to weapon-grade:
     292 w

• Number of months theoretically needed for the 696 IR-1 centrifuges operating at Fordow to accomplish the above:
     6.5 x

• Number of months theoretically needed to accomplish the enrichment to weapon-grade if the number of centrifuges devoted to production at Fordow were:
     doubled: 3.3 months
     tripled: 2.2 months
     quadrupled: 1.6 months y

• Date by which this quadrupling of centrifuges may occur:
     November 2012 z

Number of centrifuges deployed over time at the Natanz Fuel Enrichment Plant

Date of IAEA inventory	Centrifuges being fed with UF6	Other centrifuges (installed or being installed)
2/17/2007	0	656
5/13/2007	1,312	820
8/19/2007	1,968	656
11/3/2007	2,952	0
12/12/2007	2,952	?
5/7/2008	3,280	2,624
8/30/2008	3,772	2,132
11/7/2008	3,772	2,132
2/1/2009	3,936	1,968
6/1/2009	4,920	2.296
8/12/2009	4,592	3,716
11/2/2009	3,936	4,920
1/31/2010	3,772	4,838
5/24/2010	3,936	4,592
8/28/2010	3,772	5,084
11/5/2010	4,816	3,610
11/16/2010	0	~ 8,426
11/22/2010	~ 4,592	~ 3,834
2/20/2011	~ 5,184	~ 2,816
5/14/2011	~ 5,860	~ 2,140
8/28/2011	~ 5,860	~ 2,140
11/2/2011	~ 6,208	~ 1,792
2/19/2012	8,808	348
5/19/2012	8,818	512
8/21/2012	9,156	~ 6,000

Comments

• This assessment assumes that Iran would use 16 kg of weapon-grade uranium (~90 percent U-235) in the finished core of each nuclear weapon. Sixteen kilograms are assumed to be sufficient for an implosion bomb. This was the amount called for in the implosion device Saddam Hussein was trying to perfect in the 1980’s, and the design for such a device has circulated on the nuclear black market, to which Iran has had access. Some experts believe that Iran could use less material, assuming Iran would accept a lower yield for each weapon. According to these experts, Iran could use as few as seven kilograms of this material if Iran’s weapon developers possessed a “medium” level of skill, and if Iran were satisfied with an explosive yield slightly less than that of the bomb dropped on Hiroshima, Japan. aa If Iran chose to use an amount smaller than 16 kg, the time required to make each weapon would be less than estimated here. Or, in the amount of time estimated here, Iran could make a greater number of weapons. Iran could decide not to use such a smaller amount of weapon-grade uranium if Iran wanted to have more confidence that its weapons would work, or if it wanted to reduce the size of its weapons by reducing the amount of high explosive required.
  

• Uncertainties about the number of centrifuges that Iran is operating make it difficult to draw a conclusion about the performance of individual machines. An increase or decrease in the production rate could be attributed to the fact that more machines were operating when IAEA inspectors were not present at the plant, rather than because the machines were operating more efficiently.

• Following start-up, centrifuge cascades must be operated for a time without product withdrawal. This process is called passivation.
  

NOTES
(a) The following estimates are based on information in quarterly reports by the International Atomic Energy Agency (IAEA), which is responsible for nuclear inspections in Iran.
(b) According to the IAEA, Iran had an inventory of 4,871 kg of low-enriched UF6 as of October 16, 2011, based on production from the beginning of operations in February 2007. Iran estimates that it produced a further 2,005 kg of this material between October 17, 2011 and August 6, 2012, for a total stockpile of 6,876 kg (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(c) According to the IAEA, Iran has used some of its stockpiled low-enriched UF6 (1,566.8 kg) for the production of 20% enriched uranium gas. Therefore, Iran had approximately 5,309.2 kg of low-enriched UF6 left as of August 2012 (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(d) Iran estimates that it produced 2,005 kg of low-enriched UF6 over 295 days, from October 17, 2011 to August 6, 2012, for an average daily production rate of 6.8 kg (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(e) This is assuming uranium tails of 1% U-235, a feed assay of 3.5% U-235, a product assay of 90% U-235, a 20% loss of material during processing, and that 16 kg of finished uranium metal enriched to 90% are needed for a bomb. See the SWU calculator published by URENCO, a European uranium enrichment consortium: web.archive.org/web/20021226100607/www.urenco.de/trennarbeit/swucal_e.html.
(f) The Separative Work Unit is the standard measure of the effort required to increase the concentration of the fissionable U-235 isotope. See www.urenco.com/Content/89/Glossary.aspx.
(g) Based on the assumptions set forth above (see note d), Iran would need approximately 955 SWUs to bring 1,053 kg of low-enriched UF6 to weapon grade. See the SWU calculator published by URENCO, a European uranium enrichment consortium: web.archive.org/web/20021226100607/www.urenco.de/trennarbeit/swucal_e.html.
(h) If 1,053 kg of low-enriched uranium are required to produce a bomb’s worth of weapon-grade uranium (see note e), the 5,310 kg of low-enriched uranium in Iran’s stockpile as of August 2012 might be sufficient to fuel at about five first-generation implosion bombs. This number takes into account the conversion of about one fifth of Iran’s low-enriched UF6 stockpile to 20% enriched uranium gas.
(i) The IAEA estimates the conversion time for low-enriched uranium to weapon-grade uranium metal to be approximately 3-12 months (www-pub.iaea.org/MTCD/publications/PDF/nvs-3-cd/PDF/NVS3_prn.pdf).
(j) According to the IAEA, Iran had produced about 1,010 kg of low-enriched UF6 by late January 2009. Given the average daily production rate of this material at the time, Iran's stockpile probably contained the requisite 1,053 kg by the following month. (http://www.iranwatch.org/international/IAEA/iaea-iranreport-021909.pdf)
(k)As of August 21, 2012, Iran claimed to be operating 54 cascades (9,156 centrifuges) in Production Hall A of the Natanz Fuel Enrichment Plant (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(l) Iran's IR-1 centrifuge is estimated to have an annual enrichment capacity of about 2 SWU. Iran, however, has been achieving a lower ouput. For instance, between October 2010 and October 2011, during which time Iran is estimated to have been operating between 4,800 and 6,200 machines (for an average of 5,500), an estimated 1,736 kg of low-enriched UF6 were produced. Assuming a product assay of 3.5% U-235 and tails of .4% U-235, this amounts to about 4,268 SWUs over one year, or about .77 SWU per machine.
(m) If each of Iran’s 9,156 centrifuges produces an average of .77 SWUs per year, their total output over one year would be 7,050 SWUs, or 588 SWUs per month. Thus, it would take just over 1.6 months to produce 955 SWUs.

(n) According to the IAEA, as of August 21, 2012, Iran had installed the following in Production Hall A of the Natanz Fuel Enrichment Plant: 18 cascades with empty centrifuges casings in Unit 25, 15 cascades with empty centrifuge casings in Unit A27, and two additional cascades in that Unit that are either fully or partially installed. One cascade in Unit A27 is empty (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(o) Iran is producing 20% enriched UF6 at both its Natanz pilot plant and its Fordow enrichment plant, allegedly for the purpose of fueling the Tehran Research Reactor. The IAEA has verified that Iran produced 73.7 kg of this material between February 9, 2010 and September 13, 2011. Since then, Iran has produced an estimated 50.4 kg of 20% enriched UF6 at Natanz (through August 21, 2012) and an estimated 65.3 kg at Fordow (through August 12, 2012), for a total of 189.4 kg (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(p) According to the IAEA, Iran converted some of this material (97.9 kg), including to produce reactor fuel . Therefore, Iran had 91.4 kg of 20% enriched UF6 left by mid-August 2012.
(q) This is assuming uranium tails of 1% U-235, a feed assay of 19.75% U-235, a product assay of 90% U-235, a 20% loss of material during processing, and that 16 kg of finished uranium metal enriched to 90% are needed for a bomb core. See the SWU calculator published by URENCO, a European uranium enrichment consortium: web.archive.org/web/20021226100607/www.urenco.de/trennarbeit/swucal_e.html.
(r) Between September 14, 2011 and August 21, 2012, Iran estimates that it produced 50.4 kg of 20% enriched UF6, for an average monthly production rate of 4.5 kg.
(s) Since July 2010, Iran has been enriching uranium in two interconnected cascades of 164 centrifuges each (328 machines) at the Natanz pilot plant.
(t) Between May 13, 2012 and August 12, 2012, Iran estimates that its total amount of 20% enriched UF6 increased from 35.5 kg to 65.3 kg, indicating that it produced approximately 29.8 kg during that three-month time span, for an average monthly production rate of 9.9 kg. This marks an increase in the production rate at the plant since earlier this year. The IAEA reported in May 2012 that Iran produced an estimated 21.7 kg of 20% enriched UF6 during the three-month period between February 11, 2012 to May 13, 2012, for an average monthly production rate of 7.2 kg.
(u) Since mid-December 2011, Iran has been enriching uranium in two interconnected cascades of 174 centrifuges each (348 machines) at the Fordow plant. Iran began enriching uranium in two additional cascades of 174 centrifuges each in late January 2012 (http://www.iranwatch.org/iaea-iranreport-052512.pdf).
(v) As of August 18, 2012, in addition to the centrifuges enriching uranium Iran had installed eight cascades of 174 centrifuges each and one cascade with 52 centrifuges (http://www.iranwatch.org/international/IAEA/iaea-iranreport-083012.pdf).
(w) Based on the assumptions set forth above (see note q), Iran would need approximately 292 SWUs to bring 140 kg of 20% enriched UF6 to weapon grade. See the SWU calculator published by URENCO, a European uranium enrichment consortium: web.archive.org/web/20021226100607/www.urenco.de/trennarbeit/swucal_e.html.
(x) If 292 SWUs are needed to bring a bomb’s worth of 20% enriched UF6 to weapon-grade, and if the 696 IR-1 centrifuges in Iran’s Fordow enrichment plant were to achieve the same average production rate as those in the main enrichment plant at Natanz (.77 SWU per machine), then it would take less than seven months to achieve 292 SWUs at the Fordow plant.
(y) If 292 SWUs are needed to bring a bomb’s worth of 20% enriched UF6 to weapon-grade, and if Iran’s centrifuges at Fordow were to produce approximately 1,072 SWUs per year, or 89 SWUs per month, then it would take about 3.3 months to achieve 292 SWUs; if Iran's centrifuges were able to produce 1,608 SWUs per year, or 134 SWUs per month, then it would take 2.2 months to achieve 292 SWUs; if Iran’s centrifuges were able to produce approximately 2,144 SWUs per year, or 179 SWUs per month, then it would take 1.6 months to achieve 292 SWUs. The above calculations assume that the each centrifuge would achieve the same average production rate as those in the main enrichment plant at Natanz (.77 SWUs).
(z) Iran completed the installation of about 1,000 centrifuges since May 2012. To fully outfit the Fordow plant, a further 644 centrifuges must be installed. Based on Iran’s past installation rate, the plant could be fully outfitted by November 2012.
(aa) Thomas B. Cochran and Christopher E. Paine, “The Amount of Plutonium and Highly Enriched Uranium Needed for Pure Fission Nuclear Weapons,” (Washington, DC: Natural Resources Defense Council, revised April 13, 1995).

New on Iran Watch

* The World Intellectual Property Organization releases a report on technical assistance provided to countries subject to UN sanctions (PDF)

* IAEA resolution expresses "serious concern that Iran continues to defy [its] requirements and
obligations" and "urges Iran to comply fully and without delay" (PDF)

* IAEA report reveals increased enrichment capacity at Natanz and Fordow, impasse in the Agency's investigation of nuclear weapon-related work

* Congress passes HR. 1905, the Iran Threat
Reduction and Syria Human Rights Act of 2012 (PDF), expanding sanctions on Iran