Iran's Nuclear Timetable: The Weapon Potential

June 17, 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 February 2021, the date of inspection contained in the latest public report in which the International Atomic Energy Agency (IAEA) could verify Iran's stockpile of enriched uranium. It does not reflect the damage done to Iran's centrifuges following an attack at the Natanz enrichment plant on April 11. In May, for example, the IAEA reported that at Natanz Iran was feeding uranium into only fifteen cascades of IR-1 centrifuges, whereas Iran had been operating thirty before the attack. Israeli and U.S. officials have reportedly said that Iran may need at least nine months to return enriched uranium production at the plant to previous levels.

Summary
 

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 given no sign that it is contemplating a dash. It has not re-installed the thousands of first-generation centrifuges it has in storage, which would be necessary. Instead, it has expanded testing and deployment of more powerful centrifuge models. This suggests that Iran’s primary goal at present is to build and operate better centrifuges. Likewise, Iran has steadily increased both the size and enrichment level of its uranium stockpile, but it has not done so in a manner indicating preparations for a dash.

Thus, the main nuclear weapon risk in Iran is work at secret sites, which Iran has used to carry out illicit work in the past. That risk will increase as Iran limits access by the International Atomic Energy Agency (IAEA), which will make it more difficult for Agency inspectors to visit suspicious sites. That risk will also increase as Iran develops more powerful centrifuges, allowing sites to be smaller and easier to hide. Perfecting such centrifuges is a vital step in the long nuclear game Iran has been playing for decades.

Nuclear Weapon Potential of Iran's Centrifuges
 

By February 2021, Iran was operating 5,060 IR-1 centrifuges and 348 more powerful IR-2m centrifuges at the Natanz Fuel Enrichment Plant, as well as 1,044 IR-1 centrifuges at the Fordow Fuel Enrichment Plant. Iran also has approximately 12,000 IR-1 centrifuges in storage at Natanz, is installing several hundred more IR-2m centrifuges at Natanz, and has 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 weapon-grade uranium, usually defined as uranium enriched to at least 90% in the isotope U-235, which explodes in fission bombs. Almost all of Iran’s production has been at reactor grade or lower (enriched not higher than 5% U-235). Thus, the reactor-grade uranium would have to be enriched further to reach weapon grade. The estimates below assume that, in a dash to make weapons, Iran would rely on its IR-1 centrifuges 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 reactor-grade uranium to fuel one nuclear warhead, with further enrichment. The estimates also assume that the IR-1s currently operating will perform at the same rate they have in the past.[3]

Estimated minimum time it would take Iran’s 6,104 IR-1 centrifuges presently operating in production mode to produce the fuel for
One bomb:[4] At least 2 months[5]
Five bombs: At least 1.6 years[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. It assumes that only the IR-1 centrifuges, which have been successfully operating in production mode for some time, would be used. The time actually needed in practice would be greater.

In addition, the enriched uranium produced would be in a gaseous compound (UF6). It would take additional time to convert the uranium in the gas to metallic form, and then to cast and machine the metal into bomb components. According to the IAEA, Iran began uranium metal production earlier this year. The uranium metal 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. 

Nuclear weapon potential of Iran’s low-enriched uranium
 

Iran would need about 590 kg of LEU with an enrichment of 4% (suitable for use in nuclear reactors) to fuel one bomb.[7] By February 16, 2021, Iran had substantially more than this amount, with a total of about 1,890 kg of uranium in the form of UF6 enriched between 2% and 5%, according to the IAEA.[8] Iran has been adding to the stockpile but does not yet have a sufficient amount of this material to fuel a small arsenal of five bombs. Accumulating this amount would take about one year from February, 2021.[9]

Enriching uranium to reactor grade accomplishes most (about two-thirds) of the work needed to reach weapon grade. Thus, a dash to weapons could succeed more quickly by starting with reactor-grade uranium than by starting with natural uranium. For that reason, a substantial stockpile of reactor-grade uranium in gaseous form is a strategic risk. Iran had accumulated such a stockpile, more than 16,000 kg, before reducing it under the 2015 nuclear accord. Such reactor-grade uranium would still need to be further enriched to weapon grade.

Estimated minimum time it would take Iran's 6,104 centrifuges, starting with sufficient reactor-grade uranium, to enrich the uranium further to weapon grade for
One bomb: At least 2.2 months[10]
Five bombs: At least 11 months[11]

 

To fuel a small arsenal, reactor-grade uranium needs to be enriched further to about 90% U-235, as explained above. Such enrichment would take at least an additional eleven months, using the IR-1 centrifuges Iran presently deploys. To shorten the time, Iran could add centrifuges. 

In January 2021, Iran began enriching uranium up to 20% U-235. This step is of concern because enriching to this level accomplishes roughly 90% of the work required to reach weapon grade. Once Iran accumulates a sufficient stockpile of 20% uranium, it could, in a matter of months, enrich it further to produce the fuel for five nuclear warheads. So far, Iran has only dedicated 1,044 IR-1 centrifuges at Fordow to producing 20% enriched uranium. At their reported capacity, it would take these centrifuges a minimum of nearly fourteen months to produce enough 20% uranium for one nuclear warhead[12], and a minimum of about 5.7 years to produce enough for five warheads.[13] Iran could, however, add more centrifuges at Fordow.

In addition, even after enriching to weapon grade, the uranium in gaseous form would have to be converted to metal, and the metal cast and machined into bomb components, as described above. 

The Risk of Secret Sites
 

Intelligence agencies have long been unanimous in one prediction:  If Iran makes nuclear weapons, it will 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 will 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, at its nominal capacity, Iran’s IR-2m centrifuge, of which Iran has about 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[14] centrifuges operating at nominal capacity and starting with natural uranium to fuel
One bomb: 3.2 months[15]
Five Bombs: One year and four months[16]

 

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.[17] 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.[18] By February 2021, Iran had deployed 348 IR-2m centrifuges in production mode at the Natanz Fuel Enrichment Plant, and was planning to add more.[19]

In February 2021, Iran was installing a cascade of IR-6 centrifuges in production mode at the Natanz Fuel Enrichment Plant[20] and announced plans to install two cascades of IR-6 centrifuges in production mode at the Fordow Fuel Enrichment plant.[21] 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.[22] To produce enough feed for this, each IR-6 machine would have to produce at least 6.6 to 8.6 SWU.[23]

Estimated minimum time it would take 3,000 of Iran’s model IR-6[24] centrifuges operating at claimed capacity and starting with natural uranium to fuel
One bomb: 1.6 months[25]
Five bombs: Eight months[26]

 

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 will need 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.[27] 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.[28] 

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 separative work units, or SWU.[29] 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 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 nuclear accord in May 2018, Iranian leaders threatened to stop implementing some of its commitments under the accord. Approximately one year later, it began doing so. The table below summarizes the steps Iran has taken since July 2019.

Date Iran's Violation
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.

 

 

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] According to the IAEA, as of February 16, 2021, Iran's uranium stockpile contained 2,933 kg of uranium in the form of uranium hexafluoride (UF6), 17.6 kg of which was enriched up to a level of 20% in the fissionable isotope U-235, 1,890 kg of which was enriched between 2% and 5% U-235, and 1,025.5 kg was enriched up to 2% U-235. The U-235 isotope makes up about .7% of natural uranium; its concentration can be increased, or enriched, using centrifuges. 

[3] 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 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.

[4] 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.

[5] IIran's stockpile of enriched uranium is held at various levels. Further enriching Iran’s small stockpile of 17.6 kg of uranium enriched “up to” 20% U-235 would require about 50 SWU and would produce about 2.8 kg of uranium enriched to 90% U-235, assuming the 17.6 kg used as feed had an average enrichment level of 15% and left 1% tails. Iran would require 510 kg of its 1,890 kg stockpile of uranium enriched to between 2% and 5% to produce the remaining 17.2 kg of uranium enriched to 90% U-235 needed for a bomb. Assuming an average enrichment level of 4% and 1% tails, Iran would require about 760 SWU to produce the 510 kg. Thus, Iran would need to produce a total of about 810 SWU. This theoretical calculation is generated using a SWU calculator published by URENCO, a European uranium enrichment consortium. With an output of .8 SWU annually, Iran’s 6,104 IR-1 centrifuges generate about 4,880 SWU per year or about 400 SWU per month. Thus, it would take at least 2 months to produce the 810 SWU. 

[6] Iran's stockpile of enriched uranium is not currently sufficient in U-235 to fuel five nuclear warheads. Thus, this calculation assumes that Iran would first use its enriched stockpile, which is held at various levels of enrichment, 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.
- As of February 16, 2021, Iran's small stockpile of 17.6 kg of uranium enriched "up to" 20% U-235, if assumed to have an average enrichment level of 15% U-235, would require about 50 SWU to produce  about 2.8 kg of uranium enriched to 90% U-235, assuming tails of 1%.
- Iran’s stockpile of 1,890 kg of uranium enriched to between 2% U-235 and 5% U-235, if assumed to have an average enrichment of about 4%, would require about 2,810 SWU to produce about 63.7 kg of uranium enriched to 90%, assuming tails of 1%.
- Iran’s stockpile of about 1.025.5 kg of LEU enriched "up to" 2%, if assumed to have an average enrichment level of 1.5%, would require about 440 SWU to produce at least 5.8 kg of uranium enriched to 90%, assuming tails of 1%. 

In total, Iran’s stockpile of enriched uranium would require a total of about 3,300 SWU to produce about 72.3 kg of uranium enriched to 90%. Iran would then have to use its stockpile of natural uranium to produce the remaining 27.7 kg of uranium enriched to 90% needed to fuel an arsenal of five bombs. Iran would require about 4,720 SWU to produce these 27.7 kg, assuming tails of .4%. Thus, Iran would have to produce a total of about 8,020 SWU, which would take its 6,104 IR-1 centrifuges about 1.6 years at their historic production rate of 4,880 SWU per year. This time will diminish as Iran accumulates more enriched uranium.

[7] About 590 kg of uranium enriched to an average of 4% U-235 would be sufficient feedstock to fuel one bomb after further enrichment, assuming uranium tails of 1% and that 20 kg of 90% U-235 are sufficient for one bomb. This theoretical calculation is generated using the SWU calculator published by URENCO.

[8] On February 16, 2021, according to the IAEA, Iran had a total of 2,933.1 kg of enriched uranium in the form of UF6, including: 1,025.5 kg enriched up to 2%; 1,890 kg enriched between 2% and 5%; and 17.6 kg enriched up to 20%.    

[9] Fuel for five bombs would require about 2,970 kg of uranium with an average enrichment of about 4%., assuming 1% tails, and that a total of 100 kg of uranium enriched to 90% U-235 are needed. In addition to the 1,890 kg enriched between 2% and 5% that Iran had on hand on February 16, 2021, 1,080 kg would still be needed. To produce this amount, Iran would need 5,700 SWU, assuming .3% tails. If the 6,104 IR-1 centrifuges perform at their historic production rate of .8 SWU per machine, this would take a little over one year. This time estimate does not reflect the use of more powerful centrifuges, which Iran is bringing online, or the use of uranium enriched up to 2% or the small amount of uranium enriched up to 20%.

[10] It is assumed here that only the IR-1 centrifuges already in production mode would be used in a dash to make nuclear weapons. Iran would need about 880 SWU to produce 20 kg of uranium enriched to 90% using a feed assay of 4% U-235, assuming 1% tails. At their proven production rate of .8 SWU per centrifuge, Iran’s 6,104 IR-1s could produce about 4,880 SWU per year, or about 400 SWU per month. Thus, it would take at least 2.2 months to make 880 SWU.

[11]  Iran would need to generate about 4,400 SWU to make the 100 kg of 90% enriched uranium needed to fuel an arsenal of five bombs, with a feed assay of 4% U-235, assuming 1% tails. If Iran's 6,104 IR-1 centrifuges generated about 4,880 SWU per year, this would take at least 11 months.

[12] At their recorded production rate of .8 SWU/year, Iran’s 1,044 centrifuges at Fordow are estimated to produce about 835 SWU per year. This amount of work, applied to an assumed feed of 4% enriched uranium, would create about 82 kg of 20% uranium per year, or about 6.8 kg per month. At this rate of production, it would take nearly 14 months to produce the 94 kg of 20% material (assuming tails of .3%) that Iran would need for one bomb.

[13] Iran would need about 470 kg of 20% enriched uranium to fuel five bombs. These 470 kg divided by the estimated yearly production rate of 82 kg yields 5.7 years.  

[14] TIn February 2021 Iran was operating two 174-machine cascades of IR-2m centrifuges in production mode. It has about 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 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.

[15] 3,000 IR-2m centrifuges, with a nominal output of 5 SWU, would produce approximately 15,000 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 3.2 months to produce the 4,000 SWU.

[16] The same 3,000 IR-2m centrifuges would produce the 20,000 SWU needed to fuel 5 bombs in approximately one year and four months.

[17] 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.

[18] 1,000 centrifuges at 10.7 square feet each would require about 11,000 square feet.

[19] "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 21.

[20] "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 19.

[21] "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. 

[22] "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. 

[23] 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 8.6 SWU for two cascades of 133 machines (the number used previously at Natanz for the IR-6 machines in a research mode).  

[24] On November 2, 2020, Iran had about 136 IR-6 centrifuges operating in a research capacity at the Natanz pilot plant, according to the IAEA. By February 2021, Iran had begun installing two cascades of these machines at the Natanz commercial plant. Iran has claimed that these centrifuges are ten times more powerful than the IR-1. Therefore, the IR-6 is assumed in these estimates to have a nominal output of 10 SWU. See Kiyoko Metzler, "UN Atomic Watchdog Raises Questions of Iran’s Centrifuge Use," Associated Press, May 31, 2019.

[25] 3,000 IR-6 centrifuges each producing 10 SWU per year would produce in one year 30,000 SWU, or 2,500 SWU per month. Thus, it would take 1.6 months to produce the 4,000 SWU needed to fuel one bomb.

[26] 3,000 IR-6 centrifuges would produce the 20,000 SWU needed to fuel five bombs in about 8 months. 

[27] 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).

[28] 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.

[29] 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).