This report provides an estimate of how soon Iran could have fueled a nuclear weapon before the implementation of the new nuclear agreement reached in 2015. It is phrased in the present tense from the standpoint of a reader looking forward from the autumn of 2015, shortly after the agreement was reached. The data below, which are based on reports from the International Atomic Energy Agency, describe Iran’s uranium stockpile, its centrifuges, and the rate at which its nuclear capacity had grown.[a]
- By using the approximately 9,000 first generation centrifuges operating at its Natanz Fuel Enrichment Plant as of October 2015, Iran could theoretically produce enough weapon-grade uranium to fuel a single nuclear warhead in less than 2 months. This timetable is longer if Iran operates fewer centrifuges, or feeds the machines with natural uranium rather than low-enriched uranium.
- During a visit to Natanz on October 28, 2015, the IAEA observed that Iran had stopped feeding natural uranium into centrifuge cascades in order to perform an annual physical inventory at the plant.
- Iran’s stockpile of low-enriched uranium is currently sufficient, after further enrichment, to fuel approximately seven nuclear warheads.
- The restrictions on Iran’s centrifuges and its enriched uranium stockpile required by the Joint Comprehensive Plan of Action for ten years would extend the time it would take Iran to “breakout” and make fuel for one nuclear weapon to one year; the breakout time would decline after ten years, when restrictions on advanced centrifuge operations are eased. Iran has begun implementing centrifuge restrictions, dismantling 4,112 IR-1 centrifuges and 160 IR-2m centrifuges from Natanz and 258 IR-1 centrifuges from Fordow as of November 15, 2015.
- Because Russia has a ten-year contract to fuel Iran’s only power reactor at Bushehr, Iran has no present need for enriched uranium to generate civilian nuclear energy.
- Iran could fuel approximately 25 first generation implosion bombs if it had the ability to enrich the uranium needed to supply the Bushehr reactor annually.
Bomb potential of Iran’s low-enriched uranium
- Total amount of uranium hexafluoride (UF6) enriched to approximately 3.5 percent U-235 produced as of November 2015:
- Amount of this material ready for further enrichment (i.e., stored in gaseous form) as of November 2015:
- Amount theoretically needed to produce a bomb’s worth of weapon-grade uranium metal:
- Number of first generation implosion bombs this 8,306 kilograms could fuel, if further enriched:
- Time needed to convert this uranium to one bomb’s worth of finished uranium metal enriched to 90 percent U-235:
3 – 12 months[f]
- Date by which Iran’s uranium stockpile probably was sufficient to fuel one first generation implosion bomb, if further enriched:
- Approximate number of first generation IR-1 centrifuges being fed with UF6 at the Natanz Fuel Enrichment Plant, as of October 2015:
- Number of months theoretically needed for these 9,000 centrifuges operating at their estimated capacity to produce enough enriched uranium for one bomb:
Civilian need for this uranium
- Approximate amount of low-enriched uranium needed annually to fuel Iran’s sole civilian power reactor at Bushehr:
21 metric tons[j]
- Percent of this uranium Russia will supply under a ten-year fuel contract:
- Number of years it would take the roughly 9,000 operating IR-1 centrifuges at Natanz to produce one year’s worth of fuel for Bushehr:
- Approximate number of separative work units (amount of enrichment work)[m] Iran would need to generate in order to produce one year’s worth of fuel for Bushehr:
- Approximate number of IR-1 centrifuges Iran would need to operate in order to produce this level of work annually:
- Approximate number of first generation implosion bombs Iran could fuel if able to enrich the uranium needed to supply Bushehr annually:
Iranian Nuclear Breakout Times Under Different Scenarios
- 9,000 centrifuges (the number running in October 2015):
- 6,104 centrifuges (the number allowed to be installed under the Joint Comprehensive Plan of Action):
- 5,060 centrifuges (the number allowed to operate under the Joint Comprehensive Plan of Action):
- 9,000 centrifuges:
- 6,104 centrifuges:
- 5,060 centrifuges:
- Before using uranium in a warhead, it must be enriched to weapon-grade (90 percent or more U-235) and processed into a metallic shape sufficient to explode in a chain reaction.
- 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.[u] 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.
- Iran has converted 337.2 kg of 20 percent enriched uranium gas (or 227.6 kg of uranium) into oxide form, producing 162.3 kg of uranium, some of which has been used to produce fuel for the Tehran Research Reactor. If it is not irradiated in the reactor, this material could be returned to gaseous form and enriched to weapon grade. However, it would not be sufficient to fuel more than one nuclear weapon and it is unclear how long it would take to convert and further enrich the material.
- 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.[v] A change in production rate could also be attributed to a decision by Iran to lower the output of its centrifuges.[w]
- Following start-up, centrifuge cascades must be operated for a time without product withdrawal. This process is called passivation.
Growth of enrichment capacity at the Natanz Fuel Enrichment Plant
|Date of IAEA Inventory||IR-1 Centrifuges Being Fed |
|Other IR-1 Centrifuges Installed|
|17 Feb 2007||0||656|
|13 May 2007||1,312||820|
|19 Aug 2007||1,968||656|
|3 Nov 2007||2,952||0|
|12 Dec 2007||2,952||?|
|7 May 2008||3,280||2,624|
|30 Aug 2008||3,772||2,132|
|7 Nov 2008||3,772||2,132|
|1 Feb 2009||3,936||1,968|
|1 Jun 2009||4,920||2,296|
|12 Aug 2009||4,592||3,716|
|2 Nov 2009||3,936||4,920|
|31 Jan 2010||3,772||4,838|
|24 May 2010||3,936||4,592|
|28 Aug 2010||3,772||5,084|
|5 Nov 2010||4,816||3,610|
|16 Nov 2010||0||~ 8,426|
|22 Nov 2010||~4,592||~3,834|
|20 Feb 2011||~5,184||~2,816|
|14 May 2011||~5,860||~2,140|
|28 Aug 2011||~5,860||~2,140|
|2 Nov 2011||~6,208||~1,792|
|19 Feb 2012||8,808||348|
|19 May 2012||8,818||512|
|21 Aug 2012||9,156||270|
|10 Nov 2012||9,156||1,258|
|19 Feb 2013||~8,990||~3,680|
|15 May 2013||~8,990||~4,565|
|24 Aug 2013||9,156||6,260|
|9 Nov 2013||~8,800||~6,620|
|10 Feb 2014||~9,000||~6,420|
|14 May 2014||~9,000||~6,420|
|13 Aug 2014||~9,000||~6,420|
|15 Oct 2014||~9,000||~6,420|
|8 Feb 2015||9,156||6,264|
|17 May 2015||9,156||6,264|
|22 Aug 2015||9,156||6,264|
|28 Oct 2015||0||11,308|
|Date of IAEA Inventory||IR-2m Centrifuges Being Fed with UF6||IR-2m Centrifuges Installed|
|19 Feb 2013||0||180|
|15 May 2013||0||689|
|24 Aug 2013||0||1,008|
|9 Nov 2013||0||1,008|
|10 Feb 2014||0||1,008|
|14 May 2014||0||1,008|
|13 Aug 2014||0||1,008|
|15 Oct 2014||0||1,008|
|8 Feb 2015||0||1,008|
|17 May 2015||0||1,008|
|22 Aug 2015||0||1,008|
|28 Oct 2015||0||848|
[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. These quarterly reports are available here: http://www.iranwatch.org/authoring-agency/iaea-report.
[b] Iran continues to produce low-enriched UF6, which is allowed under the interim accord. In all, Iran has produced 16,141.6 kg of this material, which includes 115.6 kg that was produced from the downblending of UF6 enriched to 20%. According to the IAEA, Iran has slowed the rate of production of low-enriched UF6, which was estimated to be an average of 5.9 kg each day (down from an average of 7.9 kg each day).
[c] Prior to the interim nuclear accord, Iran had used some of its stockpiled low-enriched UF6 (~3,400 kg) for the production of 20% enriched uranium gas. Under the accord, this production has been suspended and Iran has downblended the 20% material to low-enriched UF6 or converted it to oxide form. Iran is also using centrifuges that had been producing 20% enriched uranium gas for the production of low-enriched UF6. In both cases, this has led to an increase in Iran’s stockpile of low-enriched UF6. However, Iran has also begun converting a portion of its low-enriched UF6 stockpile accumulated since January 2014 (~4,334 kg) into oxide form, thus reducing the size of the stockpile. As a result of these activities, Iran had approximately 8,306 kg of low-enriched UF6 as of November 2015, according to the IAEA.
[d] 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 Separative Work Unit (SWU) calculator published by URENCO, a European uranium enrichment consortium: web.archive.org/web/20021226100607/www.urenco.de/trennarbeit/swucal_e.html.
[e] If 1,053 kg of low-enriched uranium are required to produce a bomb’s worth of weapon-grade uranium (see note d), the 8,306 kg of low-enriched uranium in Iran’s stockpile as of November 2015 might be sufficient to fuel about seven first generation implosion bombs.
[f] 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).
[g] 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.
[h] As of October 18, 2015, Iran was operating 54 cascades of approximately 9,000 first-generation IR-1 centrifuges (9,156 according to the IAEA) in Production Hall A of the Natanz Fuel Enrichment Plant. Enrichment at the plant was suspended on October 28, 2015, as part of an annual inventory. The November 2015 IAEA report does not indicate that enrichment resumed after that date, and if so, how many centrifuges were operating.
[i] According to production data at the Natanz Fuel Enrichment Plant, Iran’s IR-1 centrifuges have achieved an average annual output of about .79 Separative Work Units, or SWUs, per machine. A SWU is a standard measure of the effort required to increase the concentration of the fissionable U-235 isotope within natural uranium. 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. If each of Iran’s roughly 9,000 operating centrifuges produces an average of .79 SWUs per year, their total output over one year would be 7,110 SWUs, or 592 SWUs per month. Thus, it would take about 1.6 months to produce 955 SWUs.
[j] A typical 1,000 MWe pressurized light water reactor of the type Iran is operating at Bushehr requires about 21 tons of low-enriched uranium fuel each year. See the nuclear fuel cycle simulation system published by the IAEA (http://infcis.iaea.org/NFCSS/NFCSSMain.asp?RightP=Calculation&EPage=2&Refresh=0&ReactorType=1).
[k] Russia and Iran signed a nuclear fuel agreement in 1995. Under the agreement, Russia committed to supplying fuel for Bushehr for ten years and Iran committed to returning the spent fuel to Russia. Reportedly, the original 1992 nuclear cooperation agreement between Russia and Iran stipulated that Russia would supply fuel for the Bushehr reactor “for the entire lifespan of the nuclear power plant.” See Mark Hibbs, “Iran’s Russia Problem,” July 7, 2014. (http://www.iranfactfile.org/2014/07/07/us-russia-split-wont-sink-iran-talks/)
[l] Iran has achieved a low-enriched UF6 production rate of about 2,900 kg/year (7.9 kg/day), which contains 1,960 kg of uranium. At this rate, to produce 21 metric tons of reactor-grade uranium would take 10.7 years.
[m] The separative work unit, or SWU, is the standard measure of the effort required to increase the concentration of the fissionable U-235 isotope. See http://www.urenco.com/index.php/content/89/glossary.
[n] See the nuclear fuel cycle simulation system published by the IAEA (http://infcis.iaea.org/NFCSS/NFCSSMain.asp?RightP=Calculation&EPage=2&Refresh=0&ReactorType=1).
[o] This result assumes that the efficiency level of each centrifuge stays at .79 SWU per machine (see note i).
[p] To produce a bomb’s worth of highly enriched uranium, it takes approximately 4,000 SWUs. If Iran were able to produce 100,000 SWUs annually, and directed that power toward making nuclear weapon fuel, it could make enough fuel for about 25 bombs.
[q] As of November 2015, Iran was operating 54 cascades of approximately 9,000 first-generation IR-1 centrifuges (9,156 according to the IAEA) in Production Hall A of the Natanz Fuel Enrichment Plant. To produce a bomb’s worth of highly enriched uranium from natural uranium, it takes approximately 4,000 Separative Work Units, or SWUs. According to production data at the Natanz Fuel Enrichment Plant, Iran’s IR-1 centrifuges have achieved an average annual output of about .79 SWUs per machine. Thus, 9,000 centrifuges could produce fuel for one nuclear weapon from natural uranium in an estimated 6.8 months.
[r] According to the Joint Comprehensive Plan of Action released July 14, 2015, Iran will be limited to 6,104 IR-1 centrifuges installed under the agreement (5,060 at Natanz and 1,044 at Fordow). If these 6,104 centrifuges achieved an average annual output of .79 SWUs, Iran could produce fuel for one nuclear weapon from natural uranium in an estimated 10 months.
[s] According to the Joint Comprehensive Plan of Action released July 14, 2015, Iran will be limited to operating 5,060 IR-1 centrifuges under the agreement (all at Natanz). If these 5,060 centrifuges achieved an average annual output of .79 SWUs, Iran could produce fuel for one nuclear weapon from natural uranium in an estimated 12 months.
[t] See assumptions and calculations above.
[u] 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).
[v] See “Implementation of the NPT Safeguards Agreement and Relevant Provisions of Security Council Resolutions in the Islamic Republic of Iran (GOV/2014/43),” International Atomic Energy Agency, September 5, 2014.
[w] See “ISIS Analysis of Safeguards Report,” Institute for Science and International Security,” February 19, 2015 (http://isis-online.org/uploads/isis-reports/documents/ISIS_Analysis_IAEA_Report_19February2015_Final.pdf).