Iran's Nuclear Timetable: The Weapon Potential

October 4, 2021

Publication Type: 

  • Articles and Reports

Weapon Program: 

  • Nuclear

Author: 

Valerie Lincy and Gary Milhollin

This timetable estimates how soon Iran could produce the fuel for a small nuclear arsenal. It assumes Iran would try to build an arsenal of five warheads of the implosion type – the goal Iran set for itself when it began to work on nuclear weapons decades ago. With its thousands of gas centrifuges, some operating and some in storage, Iran can enrich uranium to a grade suitable for nuclear reactor fuel or to a higher grade suitable for nuclear weapons. On January 5, 2020, Iran announced that it would no longer observe any limit (such as that set by the nuclear accord of 2015) on the use of its centrifuges, or on the possession of uranium they enrich. Since then, Iran has expanded its stockpile of enriched uranium, increased the enrichment level of that stockpile, and brought more advanced centrifuges into operation.

The potential is estimated as of August 2021, the date of inspection contained in the latest public report by the International Atomic Energy Agency (IAEA). Because Iran has reduced its cooperation with the Agency, it is no longer able to verify Iran's stockpile of enriched uranium. The Agency’s reports are only able to estimate its contents. The analysis below is based on those estimates.

Summary
 

Although Iran's enriched uranium stockpile contains sufficient uranium to fuel one nuclear warhead, and almost enough to fuel five, with further enrichment, Iran's known capacity does not pose an imminent nuclear weapon threat. With its known capacity, Iran cannot make a sudden dash to a nuclear arsenal within a practical length of time. Nor would a dash to a single bomb be practical. Such a bomb would have to be tested[1] (consuming all the nuclear material the dash produced), the dash would probably be detected before it could succeed, and would invite retaliation Iran could not deter.

Iran has, however, made rapid progress in the testing and deployment of more powerful centrifuge models. Iran has installed several cascades of these new models in production lines where they have steadily increased both the size and enrichment level of Iran's uranium stockpile. This progress increases the risk of secret sites – permitting them to be smaller and easier to hide. Iran has used such sites to carry out illicit activity in the past and they continue to pose the greatest nuclear weapon risk. That risk has increased further recently because of Iran’s decision to limit inspections by the IAEA, block IAEA access to recorded data from centrifuge production plants, and refuse to cooperate with the Agency’s investigation of four suspicious sites.

These decisions by Iran, together with its steps to allow only limited and late cooperation with the IAEA, to increase its enriched uranium stockpile, and to perfect more powerful centrifuges, are vital parts in the long nuclear game Iran has been playing for decades.

Nuclear Weapon Potential of Iran's Centrifuges and Low-Enriched Uranium
 

By August 2021, Iran was operating 29 cascades of IR-1 centrifuges and five cascades of more powerful IR-2m centrifuges at the Natanz Fuel Enrichment Plant (FEP), as well as 1,044 IR-1 centrifuges at the Fordow Fuel Enrichment Plant (FFEP). Iran also had several thousand IR-1 centrifuges in storage at Natanz, had installed an additional cascade of IR-1 centrifuges and an additional cascade of IR-2m centrifuges at Natanz, and had been testing several other more powerful centrifuge models in smaller numbers at the Natanz pilot plant. Some of these more powerful models are adding to Iran’s enriched uranium stockpile. By deploying them in larger numbers, Iran would be able to produce nuclear weapon fuel more quickly. 

Iran's centrifuges have not produced uranium usually defined as weapon-grade, which is uranium enriched to 90% in the isotope U-235. All of Iran’s production has been at lower grades. Thus, the lower-grade uranium would have to be enriched further to reach 90%. The estimates below assume that, in a dash to make weapons, Iran would rely on its IR-1 and IR-2m centrifuges now operating, and would use first its accumulated stockpile of enriched uranium[2] and then its stockpile of natural uranium to produce nuclear weapon fuel. Iran's enriched uranium stockpile already contains sufficient uranium to fuel one nuclear warhead, and almost enough to fuel five, with further enrichment.[3] The estimates also assume that the IR-1 centrifuges currently operating will perform at the same rate they have in the past and that the IR-2m would perform at 80% of their estimated nominal output.[4]

Estimated minimum time it would take Iran’s IR-1 and IR-2m centrifuges presently operating in production mode to enrich enough uranium for
One bomb: At least 1.6 weeks[5]
Five bombs: At least six months[6]

These estimates are the minimum theoretical times it would take Iran’s known installed centrifuges, operating continuously at their proved capacity, to accomplish the required amount of work. The time actually needed in practice would be greater and Iran likely would not proceed in the manner used to calculate these estimates. The estimates assume that only the IR-1 and IR-2m centrifuges, which have been successfully operating in production mode for some time, would be used. The six month estimate for five bombs can be expected to fall sharply in the coming months, as Iran brings more centrifuges into production mode and raises the enrichment level of its uranium stockpile. 

It is important to consider that the enriched uranium produced would be in a gaseous compound, uranium hexafluoride (UF6). It would take additional time to convert the uranium in the gas to metallic form, and then to purify, cast, and machine the metal into bomb components. According to the IAEA, Iran began work on uranium metal production in early 2021. The uranium metal, however, would only be a threat if Iran had already perfected all the other parts needed for a working bomb, such as the high explosives and firing circuit, and had made sure the parts would work together to achieve a nuclear explosion. There is ample evidence in the public domain that Iran has tried to achieve that goal (see Weaponization below), but no conclusive evidence that it has succeeded. 

The Risk of Secret Sites
 

Intelligence agencies have long been unanimous in one prediction:  If Iran makes nuclear weapons, it would do so at secret sites. The reasons are clear. If, in a dash to make weapons, Iran were to divert known (and therefore inspected) sites, material, or equipment to bomb making, it would risk detection before success, would violate the Nuclear Nonproliferation Treaty and would make itself an international pariah. It would also invite an attack on the very sites, material and equipment it diverted. No country has ever chosen to make an illicit diversion and dash to weapons, probably for the reasons just stated. 

The data below reveal that as Iran develops more powerful centrifuges, it would need ever smaller sites to enrich bomb quantities of uranium. And the smaller the site, the more difficult it will be to detect. For example, operating at 80% of its nominal capacity, Iran’s IR-2m centrifuge, of which Iran has at least 1,000, could enrich the same amount of uranium as the IR-1 centrifuge in approximately one-fifth the space. Iran’s enrichment plant at Fordow, which was publicly exposed in 2009, was built clandestinely by Iran to house about 3,000 centrifuges. For this reason, the estimates below use 3,000 centrifuges as the possible size of a secret enrichment plant.

Estimated minimum time it would take 3,000 of Iran’s IR-2m[7] centrifuges operating at an assumed 80% of nominal capacity and starting with natural uranium to enrich enough uranium for
One bomb: Four months[8] 
Five bombs: One year and eight months[9]

These centrifuges would require only about 32,000 square feet, equal to approximately twice the size of the ice surface of a professional hockey rink.[10] Alternatively, Iran could decide to split these 3,000 IR-2m centrifuges equally among three smaller sites of approximately 11,000 square feet each. That would decrease the size of each site and therefore the likelihood of detection. Each site would be about two-thirds the size of the ice surface of a professional hockey rink.[11] By August 2021, Iran was operating approximately 870 IR-2m centrifuges in production mode at the Natanz Fuel Enrichment Plant.[12]

Also by August 2021, Iran was feeding a cascade of 153 IR-4 centrifuges and a cascade of 164 IR-6 centrifuges in production mode at the Pilot Fuel Enrichment Plant,[13] and announced plans to install two cascades of IR-6 centrifuges in production mode at the Fordow Fuel Enrichment plant.[14] According to Iran, the IR-6 produces about 10 SWU per year, ten times as much as the IR-1. If so, it could enrich the same amount of uranium in a fraction of the space. Iran’s claim to a capacity of 10 SWU has been strengthened recently by Iran’s plan for Fordow, where two cascades of IR-6 machines will produce the feed for the IR-1 centrifuges enriching to 20% U-235.[15] To produce enough feed for this configuration, each IR-6 machine would have to produce at least 6.6 SWU.[16]

Estimated minimum time it would take 3,000 of Iran’s model IR-6[17] centrifuges operating at an assumed 80% of claimed capacity and starting with natural uranium to enrich enough uranium for
One bomb: Two months[18] 
Five Bombs: Ten months[19]
 

These IR-6 centrifuges would require approximately the same space as the model IR-2m centrifuges above, or approximately twice the size of the ice surface of a professional hockey rink. The space requirements above reveal that as Iran develops more efficient centrifuges, it could rely on ever smaller sites to enrich bomb quantities of uranium.

The Status of Weaponization Efforts

The analysis above assumes that Iran would use 16 kg of highly enriched uranium metal (about 90% 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 a design for such a device that 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.[20] If Iran chose to use an amount smaller than 16 kg, the time required to make the fuel for 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 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.

According to an investigation by the IAEA into "possible military dimensions" of Iran's nuclear program, Iran had a coordinated nuclear weapon program between 1999 and 2003. Specifically, the IAEA found that Iran developed several components of a nuclear weapon and undertook related research and testing. The investigation revealed Iran's efforts in the following areas:

  • computer modeling of implosion, compression, and nuclear yield;
  • high explosive tests simulating a nuclear explosion using non-nuclear material in order to see whether an implosion device would work;
  • the construction of at least one containment vessel at a military site, in which to conduct such high explosive tests;
  • studies on detonation of high explosive charges, in order to ensure uniform compression in an implosion device, including at least one large scale experiment in 2003, and experimental research after 2003;
  • support from a foreign expert in developing a detonation system suitable for nuclear weapons and a diagnostic system needed to monitor the detonation experiments;
  • manufacture of a neutron initiator, which is placed in the core of an implosion device and, when compressed, generates neutrons to start a nuclear chain reaction, along with validation studies on the initiator design from 2006 onward;
  • the development of exploding bridgewire detonators (EBWs) used in simultaneous detonation, which are needed to initiate an implosive shock wave in fission bombs;
  • the development of high voltage firing equipment that would enable detonation in the air, above a target, in a fashion only making sense for a nuclear payload;
  • testing of high voltage firing equipment to ensure that it could fire EBWs over the long distance needed for nuclear weapon testing, when a device might be located down a deep shaft; and
  • a program to integrate a new spherical payload onto Iran’s Shahab-3 missile, enabling the missile to accommodate the detonation package described above.

Information obtained by Israeli intelligence and revealed in April 2018 indicates that Iran sought to preserve this program after 2003 by dividing its nuclear program between covert and overt activities and retaining an expert team to continue work on weaponization. This "atomic archive" includes blueprints, spreadsheets, charts, photos, and videos – apparently official Iranian documents – that provide additional detail about Iran's efforts to develop a working nuclear weapon design that could be delivered on a ballistic missile.

Need for Enriched Uranium?
 

Iran has no need to enrich large quantities of uranium for reactor fuel, which is the stated aim of its centrifuge enrichment program. Russia is fueling Iran’s only power reactor (at Bushehr) and stands ready to do so indefinitely at a cost much lower than Iran would incur by enriching the uranium itself.[21]

If Iran did try to make the fuel itself, it is unlikely that Iran could field enough centrifuges to do so within the next ten years, or even longer. A standard sized power reactor (1,000 MWe) such as Iran’s reactor at Bushehr requires about 21 metric tons of low-enriched uranium fuel per year, which would require generating nearly 100,000 SWU.[22] Iran’s IR-1 centrifuges now produce about one metric ton per year. Thus, Iran’s program would have to increase its capacity about twenty-one fold to have any plausibility as a civilian effort.

In an October 2015 letter to then-President Hassan Rouhani, Iran’s Supreme Leader Ali Khamenei called upon the government to develop a plan for the country’s nuclear industry to achieve an annual uranium enrichment capacity of 190,000 SWU within 15 years. In order to accomplish this, Iran would have to manufacture, install, and operate almost 240,000 additional IR-1 centrifuges, based on their historic output. Or, Iran would have to perfect, manufacture, and deploy in production mode a lesser number of more powerful centrifuges. It is uncertain how long it would take Iran to accomplish either of these steps, but either would take many years.

Iran's Violations of Nuclear Accord
 

Following the U.S. withdrawal from the 2015 nuclear accord in May 2018, Iranian leaders threatened to stop implementing some of Iran’s commitments under the accord. Approximately one year later, Iran began doing so. The table below summarizes the steps Iran has taken since July 2019.

Date Iran's Violations of the 2015 Accord
July 2019 Begins enriching uranium above the 3.67% U-235 limit set by the accord, to a level of up to 4.5% U-235.
August 2019 Exceeds the cap of 300 kg of UF6 on its stockpile of low-enriched uranium set by the accord.
September 2019 Expands its centrifuge research and development beyond the limits set by the accord, both in the number and type of more powerful centrifuge it operates.
November 2019 Resumes uranium enrichment at locations beyond those mandated by the accord, including the Fordow plant and the Natanz pilot plant.
January 2020 States it will no longer limit the number of centrifuges in operation, which had been capped at 5,060 IR-1 centrifuges operating at the Natanz Fuel Enrichment Plant. 
July 2020 Announces plans to transfer more powerful IR-2m, IR-4, and IR-6 centrifuges from the Natanz pilot plant to the Natanz Fuel Enrichment Plant. The accord limits Iran to the use of IR-1 centrifuges at the Fuel Enrichment Plant.
October 2020 Installs IR-2m centrifuges and begins installing IR-4 centrifuges at the Natanz Fuel Enrichment Plant.
November 2020 Begins uranium enrichment in a cascade of 174 IR-2m centrifuges at the Natanz Fuel Enrichment Plant.
January 2021 Begins enriching uranium to the level of 20% U-235 at the Fordow plant and begins uranium enrichment in a second cascade of 174 IR-2m centrifuges at the Natanz Fuel Enrichment Plant.
February 2021 Begins installing IR-6 centrifuges at the Fordow plant and uses a facility in Isfahan to produce uranium metal, which the accord prohibits for 15 years.
February 2021 Stops implementing transparency measures, including the Additional Protocol to Iran's Comprehensive Safeguards Agreement and additional transparency and access measures allowed under the accord.
April 2021

Begins enriching uranium up to 60% U-235.

May 2021 Installs equipment to produce uranium metal in quantity.

 

Footnotes: 

[1] In a dash, Iran would be expected to use its uranium to fuel a bomb using an implosion design, such as the bomb dropped on Nagasaki, Japan; such a bomb would have to be tested to prove it worked, as was the Nagasaki bomb. A gun-type device such as the one dropped on Hiroshima without being tested, would require more than twice as much uranium.

[2] The IAEA estimated, but was unable to verify, that as of August 30, 2021, Iran's uranium stockpile contained 2372.9 kg of uranium in the form of uranium hexafluoride (UF6), 10.0 kg of which was enriched "up to" a level of 60% in the fissionable isotope U-235, 84.3 kg of which was enriched "up to" a level of 20% U-235, 1774.8 kg of which was enriched "up to" a level of 5% U-235, and 503.8 kg of which was enriched "up to" a level of 2% U-235. The U-235 isotope makes up about .7% of natural uranium; its concentration can be increased, or enriched, using centrifuges. 

[3] Twenty kilograms of uranium enriched to 90% U-235 are assumed to be sufficient for one bomb. The uranium would need to be further processed into finished metal bomb components, which could cause about a 20% loss of material.

[4] According to pre-2016 production data from Natanz, Iran's IR-1 centrifuges have achieved an average annual output of about .8 separative work units, or SWUs, per machine. The IR-2m is based on Pakistan's P-2 centrifuge and is assumed in these estimates to have an operational output of 4 SWU (and a nominal output of 5 SWU). See Alexander Glaser, "Characteristics of the Gas Centrifuge for Uranium Enrichment and Their Relevance for Nuclear Weapon Proliferation (corrected)," Science and Global Security, Vol. 16, Nos. 1-2 (2008), p. 9. The 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.

[5] Iran's stockpile of enriched uranium is held at various levels and, as of August 30, 2021, was almost, but not quite not sufficient in U-235 to fuel five nuclear warheads. Thus, these calculations assume that Iran would first use this enriched stockpile and then its stockpile of natural uranium, and that a total of 100 kg of uranium enriched to 90% U-235 would be needed to fuel an arsenal of five nuclear weapons.  

Iran's Estimated LEU Stockpile (feed) If enriched to weapon grade (product)

SWUs required

Number of nuclear weapons

10.0 kg up to 60% U-235 (~54%)

6 kg 90% U-235

30 SWU

 

84.3 kg up to 20% U-235 (~18%)

14 kg 90% U-235

220 SWU

One weapon (250 SWU)

1774.5 kg up to 5% U-235 (~4.5%)

70 kg 90% U-235

2866 SWU  
503.8 kg up to 2% U-235 (~1.8%) 4.5 kg 90% U-235 314 SWU  

950 kg natural uranium (.711% U-235)

3.3 kg 90% U-235 562 SWU

5 weapons (4,027 SWU)

  • An estimated 4,027 SWU would be needed to produce the 100 kg of 90% U-235 for an arsenal of 5 nuclear weapons. 

  • These theoretical calculations are generated using a SWU calculator published by URENCO, a European uranium enrichment consortium.

  • Where the feed is enriched uranium, the tails are assumed to be 1%; the tails are estimated at .4% for natural uranium feed.

  • The IAEA describes the enrichment level as "up to" a percentage, therefore a lower number is used for these calculations, included parenthetically.

With an output of .8 SWU annually, Iran’s 29 cascades of IR-1 centrifuges at FEP (assumed to contain about 168  machines per cascade) would generate about 3,900 SWU per year, Iran’s 1044 IR-1 centrifuges at FFEP would produce about 835 SWU per year, and Iran’s five cascades of IR-2m centrifuges at FEP (assumed to contain about 174 machines per cascade) would generate about 3,480 SWU per year assuming an operational capacity of 4 SWU per machine and tails of 1%. The IR-2m is based on Pakistan's P-2 centrifuge and is assumed in these estimates to have a nominal output of 5 SWU. Thus, at their combined capacity of about 8,215 SWU per year it would take at least 1.6 weeks for these operational centrifuges to produce the 250 SWU. 

[6] As set forth in footnote 5, Iran would require a total of about 4,027 SWU to produce enough enriched uranium to fuel five bombs, which would take its operational capacity of 8,215 SWU about six months to produce. This time will diminish as Iran raises the enrichment level of its stockpile and deploys more centrifuges.

[7] In August 2021 Iran was operating five cascades of IR-2m centrifuges in production mode. It has at least 1,000 such centrifuges and may be producing more. The IR-2m is based on Pakistan's P-2 centrifuge and is assumed in these estimates to have an operational output of 4 SWU (and a nominal output of 5 SWU). See Alexander Glaser, "Characteristics of the Gas Centrifuge for Uranium Enrichment and Their Relevance for Nuclear Weapon Proliferation (corrected)," Science and Global Security, Vol. 16, Nos. 1-2 (2008), p. 9.

[8] 3,000 IR-2m centrifuges, each with an operational output of 4 SWU, would produce approximately 12,000 SWU in one year. If about 4,000 SWU are needed to produce the 20 kg of 90% U-235 to fuel one bomb (assuming tails of .3% and a feed assay of .7% U-235) then it would take at least 4 months to produce the 4,000 SWU.   

[9] The same 3,000 IR-2m centrifuges, producing an assumed 12,000 SWU per year, would produce the 20,000 SWU needed to fuel 5 bombs in approximately one year and eight months.

[10] Each centrifuge is assumed to require about one square meter (10.7 square feet) of space, the amount used in Iran’s enrichment plant at Natanz. The ice surface of a National Hockey League rink is 200 feet long and 85 feet wide.

[11]  1,000 centrifuges at 10.7 square feet each would require about 11,000 square feet.

[12] "Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015) (GOV/2021/39)," International Atomic Energy Agency, September 7, 2021, paragraph 32.

[13] "Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015) (GOV/2021/39)," International Atomic Energy Agency, September 7, 2021, paragraph 35.  

[14] "Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015) (GOV/2021/10)," International Atomic Energy Agency, September 7, paragraph 36. 

[15] "Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015) (GOV/2021/10)," International Atomic Energy Agency, February 23, 2021, paragraph 25. 

[16] The 1,044 IR-1 centrifuges at Fordow generate about 835 SWU annually, if operated at their historic production rate of .8 SWU each. If this amount of work is used to enrich feed at about 4% enrichment to a level of about 20% enrichment, which Iran plans to do at Fordow, Iran would require 435 kg of about 4% feed to produce 82 kg of 20% product annually. To produce the 435 kg of about 4% feed from natural uranium, as Iran expects the IR-6 centrifuges to do, would require 2,295 SWU. Dividing the 2,295 SWU by the number of IR-6 machines in the two cascades yields about 6.6 SWU per machine for two cascades of 174 machines (the number used at Fordow for the IR-1 machines) or about 7 SWU for two cascades of 164 machines (the number used at Natanz for the IR-6 machines in production mode).

[17] On August 28, 2021, Iran had 164 IR-6 centrifuges operating in a production capacity at the Natanz pilot plant, according to the IAEA and had begun installing two cascades of these machines at the Fordow plant. Iran has claimed that these centrifuges are ten times more powerful than the IR-1. The IR-6 is assumed in these estimates to have an operational output of 8 SWU (80% of the nominal output of 10 SWU). See Kiyoko Metzler, "UN Atomic Watchdog Raises Questions of Iran’s Centrifuge Use," Associated Press, May 31, 2019.

[18] 3,000 IR-6 centrifuges each producing 8 SWU per year would produce in one year 24,000 SWU, or 2,000 SWU per month. Thus, it would take two months to produce the 4,000 SWU needed to fuel one bomb.

[19] 3,000 IR-6 centrifuges would produce the 20,000 SWU needed to fuel five bombs in about ten months. 

[20] See 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).

[21] 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,” Carnegie Endowment for International Peace, July 7, 2014. 

[22] 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).