Iran's Nuclear Timetable

Updated November 10, 2011

 

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 December 2008, Iran had accumulated enough U-235 to fuel one bomb -- assuming Iran decided to further enrich the low-enriched material to weapon-grade. The Project further estimates that by the end of 2009, Iran had enough U-235 to fuel a second bomb; that Iran had enough of this material for a third bomb by August 2010; that Iran had enough of this material for a fourth bomb by April 2011; and that Iran had enough of this material for a fifth bomb by November 2011 -- in each case assuming that Iran decided to raise the level of U-235 in its low-enriched uranium stockpile (3.5 percent U-235) to weapon-grade (90 percent or more U-235).

As Iran increases its stockpile of low-enriched uranium, and its stockpile of uranium enriched to 20 percent U-235, it will consolidate its status as a "virtual" nuclear weapon state.

Iran's progress towards this status is estimateda 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 U-235 contained in Iran’s stockpile of low-enriched uranium:
      116 kg b

• Amount of this U-235 produced each month:
      3.45 kg c

• Amount of this U-235 required to fuel a first-generation implosion bombd:
      21.6 kg e

• Date by which Iran probably had stockpiled the above:
      December 2008 f

• Number of additional months needed to convert this low-enriched uranium to weapon-grade g:
      From 3 to 12 h

• Date by which Iran probably had enough U-235 to fuel a second bomb:
      December 2009 i

• Date by which Iran probably had enough U-235 to fuel a third bomb:
      August 2010 j

• Date by which Iran probably had enough U-235 to fuel a fourth bomb:
      April 2011 bb
  

• Date by which Iran probably had enough U-235 to fuel a fifth bomb:
      November 2011ff

Moving from low-enriched to weapon-grade uranium

• Amount of uranium hexafluoride (UF6) enriched to 3.5 percent U-235 now on hand:
      4,922 kg k

• Average daily production rate of this low-enriched UF6:
      4.8 kg l

• Amount of this low-enriched UF6 needed to produce a bomb’s worth of weapon-grade UF6:
      914 kg m

• Number of separative work units (SWUs)n needed to accomplish the above:
      840 o

• 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:
      6,208 p

• Average number of SWUs each centrifuge appears to have produced over the past six months:
     .7 q

• Number of months needed for these 6,208 centrifuges operating at such a capacity to produce the 840 SWUs theoretically needed to fuel a bomb:
     2.3 cc
  

• Approximate number of centrifuges installed at Natanz as of the last reported visit by IAEA inspectors:
     8,000 r

• Number of SWUs these 8,000 centrifuges could produce per year, based on their recent rate of production:
     5,600s

• Number of months needed for these 8,000 centrifuges operating at such a capacity to produce the 840 SWUs theoretically needed to fuel a bomb:
     1.8 t
  

Moving from research reactor-grade to weapon-grade uranium

• Approximate amount of uranium hexafluoride (UF6) enriched to research reactor grade (approximately 20 percent U-235) on hand as of November 1, 2011:
     80 kg u

• 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 v

• Average monthly production rate of this UF6:
     4.7 kg w

• Amount of this 20 percent enriched UF6 theoretically needed to produce a bomb’s worth of weapon-grade UF6:
     120 kg x

• Date by which Iran could produce this 120 kg, assuming its present rate of production:
     June 2012 dd

• Number of SWUs needed to accomplish this enrichment to weapon-grade:
     260 y

• Number of months theoretically needed for the 328 centrifuges, operating at their present capacity, to accomplish the above:
     12 z

• Number of months theoretically needed to accomplish the enrichment to weapon-grade if the number of centrifuges devoted to production were:aa
     doubled: 7 months
     tripled: 4.5 months
     quadrupled: 3.5 months
     raised to 3,000: 1.5 months
  
  

Number of centrifuges deployed over time

 

Comments

• According to the IAEA’s May 2011 report, a seal in the feed and withdrawal area of the Natanz Fuel Enrichment Plant was broken. This could theoretically have allowed Iran to remove uranium from the process stream, an action that could go undetected until the next IAEA inventory, due to take place in October or November 2011.

• This table assumes that Iran would use 16 kg of weapon-grade uranium (~93 percent U-235) in the finished core of each nuclear weapon. That is an estimate used by the Wisconsin Project based on Iran’s perceived technical capability and on the desired yield of its weapons. However, according to some 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 slighty less than that of the bomb dropped on Hiroshima, Japan. ee 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 small 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 produced a total of 4,922 kg of low-enriched UF6 as of November 1, 2011. (see note k) (http://www.iranwatch.org/international/IAEA/iaea-iranreport-110811.pdf). Of that amount, 3,327 kg is uranium. This 3,327 kg of uranium enriched to 3.5% contains 116 kg of U-235..

(c) In recent months, Iran is estimated to have produced about 4.8 kg of low-enriched UF6 each day (see note l), for an average monthly production rate of 146 kg, about 98 kg of which is uranium. Enriched to 3.5%, this 98 kg contains about 3.45 kg of U-235.

(d) 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. The critical mass of a sphere of U-235 metal is only 15 kg with a Beryllium reflector. See Gunter Hildenbrand, Nuclear energy, nuclear exports and the proliferation of nuclear weapons, AIF Conference on International Commerce and Safeguards for Civil Nuclear Power, March 1977. For a schematic diagram of an implosion bomb, see: www.wisconsinproject.org/bomb-facts/images/nw-1.jpg. Some experts believe that Iran could use less material, assuming Iran would accept a lower yield for each weapon (see Comments).

(e) Because of losses during the enrichment and weaponization processes, Iran would need about 914 kg (see note m) of UF6 enriched to 3.5% U-235, of which about 618 kg would be uranium, in order to achieve 16 kg of weapon-grade uranium. This 618 kg of uranium enriched to 3.5% U-235 contains 21.6 kg of U-235. 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) Assuming 19.9 kg of U-235 on hand as of November 17, 2008, a requirement of 21.6 kg for a first bomb, and a production rate, at the time, of 1.6 kg of U-235 each month, Iran would have had enough in December 2008.

(g) Once enriched to weapon-grade, this material would still need to be converted from gas to metal and then machined into a form suitable for a bomb.

(h) 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). However, if it would take approximately 840 SWUs to produce 16 kg of U-235 from a stockpile of 3.5% enriched uranium (see note o), and if Iran is capable of producing approximately 500 SWUs per month, then a conversion time at the lower end of this range is probable.

(i) Assuming 41.7 kg of U-235 on hand as of November 1, 2009, a requirement of 21.6 kg for a first bomb, and a production rate, at the time, of 2 kg of U-235 each month, Iran would have accumulated the requisite 21.6 kg for a second bomb by the end of December 2009.

(j) On August 6, 2010, Iran was estimated to have accumulated 66.3 kg of U-235 in its stockpile of low-enriched UF6 (based on the IAEA’s inventory and Iran’s estimates). Assuming 21.6 kg of this U-235 is needed per bomb, the 66.3 kg would be enough for three bombs (http://www.iranwatch.org/international/IAEA/iaea-iransafeguardreport-090610.pdf).

(k) According to the IAEA, Iran had an inventory of 3,135 kg of low-enriched UF6 as of October 17, 2010, based on production from the beginning of operations in February 2007. Iran estimates that it produced a further 1,408 kg of this material between October 18, 2010 and August 13, 2011 (a period which included a production freeze that lasted about one week), and a further 379 kg between August 14, 2011 and November 1, 2011, bringing its total stockpile to 4,922 kg (http://www.iranwatch.org/international/IAEA/iaea-iranreport-110811.pdf).

(l) Iran appears to have produced 817 kg of low-enriched UF6 over 171 days, from May 14, 2011 to October 31, 2011, for an average daily production rate of 4.8 kg (http://www.iranwatch.org/international/IAEA/iaea-iranreport-110811.pdf).

(m) This is assuming uranium tails of 1% U-235, a feed assay of 3.5% U-235, a product assay of 93% U-235, a 5% loss of material during bomb manufacture, and that 16 kg of this product 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.

(n) 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.

(o) Based on the assumptions set forth above (see note m), Iran would need approximately 840 SWUs to bring 914 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.

(p) According to the IAEA, as of November 2, 2011, Iran was operating 37 cascades (6,208 centrifuges) in Production Hall A of the Natanz Fuel Enrichment Plant (http://www.iranwatch.org/international/IAEA/iaea-iranreport-110811.pdf).

(q) Iran is operating its IR-1 centrifuges at below their estimated capacity. For instance, between May 14, 2011 and November 1, 2011, during which Iran is estimated to have been operating an average of 6,000 machines, an estimated 817 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 2,008 SWU (4,286 SWUs over one year), or about .71 SWU per machine.

(r) According to the IAEA, as of November 2, 2011, Iran had installed 54 centrifuge cascades in three of the eight units in Production Hall A. Twelve of these cascades have been modified to contain 174 centrifuges each; the remaining cascades are designed to contain 164 centrifuges each (http://www.iranwatch.org/international/IAEA/iaea-iranreport-110811.pdf).

(s) Iran’s IR-1 centrifuge is widely estimated to have an annual enrichment capacity of about two SWUs. Iran, however, has been achieving a lower output (see note q). Based on the average rate of production achieved in recent months, (.7 SWU per machine), the 8,000 centrifuges installed at Natanz could produce about 5,600 SWUs per year.

(t) If 840 SWUs are needed to bring a bomb’s worth of Iran’s stockpiled low-enriched UF6 to weapon-grade, and if Iran’s centrifuges were to produce approximately 5,600 SWUs per year, or 466 SWUs per month, then it would take nearly two months to achieve 840 SWUs.

(u) Iran is producing 20% enriched UF6 at its Natanz pilot plant, allegedly for the purpose of fueling its 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 about six kg of 20% enriched UF6 through November 1, 2011, for a total of 80 kg.

(v) Since July 2010, Iran has been enriching uranium in two interconnected cascades of 164 centrifuges each (328 machines) at the Natanz pilot plant.

(w) Between May 22, 2011 and August 20, 2011, Iran produced about 14.1 kg of 20% enriched UF6, for an average monthly production rate of 4.7 kg.

(x) This is assuming uranium tails of 1% U-235, a feed assay of 19.75% U-235, a product assay of 93% U-235, a 5% loss of material during bomb manufacture, and that 16 kg of this product 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.

(y) Based on the assumptions set forth above (see note x), Iran would need approximately 260 SWUs to bring 120 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.

(z) If 260 SWUs are needed to bring a bomb’s worth of 20% enriched UF6 to weapon-grade, and if the 328 IR-1 centrifuges in Iran’s pilot enrichment plant were to achieve the same average production rate as those in the main enrichment plant (see note q) of approximately 226 SWUs per year, then it would take just over one year to achieve 260 SWUs.

(aa) If 260 SWUs are needed to bring a bomb’s worth of 20% enriched UF6 to weapon-grade, and if Iran’s centrifuges were to produce approximately 460 SWUs per year, or 38 SWUs per month, then it would take about seven months to achieve 260 SWUs; if Iran's centrifuges were able to produce 690 SWUs per year, or 58 SWUs per month, then it would take over four months to achieve 260 SWUs; if Iran’s centrifuges were able to produce approximately 920 SWUs per year, or 77 SWUs per month, then it would take over three months to achieve 260 SWUs; and if Iran devoted 3,000 centrifuges to this work, producing 175 SWUs per month, it would take about 1.5 months to achieve 260 SUWs. The above calculations assume that the each centrifuge would achieve the same average production rate as those in the main enrichment plant at Natanz (.7 SWUs).

(bb) On May 13, 2011, Iran was estimated by the IAEA to have accumulated 97 kg of U-235 in its stockpile of low-enriched UF6. Assuming 21.6 kg of this U-235 is needed per bomb, the 97 kg would be enough for four bombs by April 2011 (http://www.iranwatch.org/international/IAEA/iaea-iranreport-052411.pdf).

(cc) If each of Iran’s 6,208 centrifuges produces an average of .7 SWUs per year, their total output over one year would be 4,345 SWUs, or 362 SWUs per month. Thus, it would take 2.3 months to produce 840 SWUs.

(dd) Starting with 80 kg on November 1, 2011, Iran would need to produce an additional 40 kg to achieve 120 kg. With a monthly production rate of 4.7 kg, Iran would require about 8.5 months to produce the 40 kg.

(ee) 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).

(ff) On November 1, 2011, Iran was estimated by the IAEA to have accumulated 116 kg of U-235 in its stockpile of low-enriched UF6. Assuming 21.6 kg of this U-235 is needed per bomb, the 116 kg would be enough for five bombs by November 2011.

 

 

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