Iran's Nuclear Timetable: The Weapon Potential

April 28, 2020

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. The data below estimate the weapon potential of these centrifuges and of Iran's growing stockpile of enriched uranium. The data come from inspection reports by the International Atomic Energy Agency (IAEA).

Summary
 

With its known capacity, Iran cannot make a sudden dash to a nuclear arsenal within a practical length of time. Nor will it be able to do so for a few years. 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 installed the thousands of centrifuges it has in storage, which would be necessary. Instead, it has expanded testing of more powerful centrifuge models. This suggests that Iran’s primary goal at present is to build better centrifuges.

Thus, the main nuclear weapon risk in Iran is work at secret sites, which Iran has relied on to carry out illicit work in the past. That risk will 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
 

Iran is operating 5,060 IR-1 centrifuges at the Natanz Fuel Enrichment Plant and 1,044 IR-1 centrifuges at the Fordow Fuel Enrichment Plant in production mode. Iran also has approximately 12,000 IR-1 centrifuges and 1,000 more powerful IR-2m centrifuges in storage at Natanz and has been testing several other more powerful centrifuge models in smaller numbers at the Natanz pilot plant.

The operating centrifuges have thus far produced only low-enriched uranium (LEU), which is suitable for nuclear reactors but not nuclear weapon fuel. The estimates below assume that, in a dash to make weapons, Iran would first use its accumulated stockpile of LEU[2] and then its larger stockpile of natural uranium to produce nuclear weapon fuel. The estimates also assume that the IR-1s currently operating will perform at the same rate as 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 2.3 months[5]
Five bombs: At least 3.5 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. The time actually needed in practice would be greater. In addition, the enriched uranium produced would be in a gaseous compound. 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. And even then, the uranium 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 685 kg of reactor-grade uranium to fuel one bomb.[7] Iran most likely had more than that amount as of February 19, 2020[8] and has been adding to the stockpile.[9] Such uranium would need to be further enriched to weapon-grade.

Estimated minimum time it would take Iran’s 6,104 IR-1 centrifuges to make enough reactor grade uranium to fuel
One bomb: Possibly zero time[10]
Five bombs: At least 2.5 years[11]

 

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 much faster 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, over 16,000 kg, before the conclusion of the nuclear accord.

The periods of time above, however, do not pose such a risk. It will take Iran at least 2.5 years to accumulate a dangerous amount of reactor grade uranium with the centrifuges now installed in production mode. To reduce that time Iran would have to remove from storage and install many thousand more centrifuges. Such an act would alarm many countries, and could invite an attack. If Iran did add more centrifuges, it would still have to enrich the reactor grade uranium further to weapon grade, which would take additional time, as illustrated below for the centrifuges it presently deploys.

Estimated minimum time it would it take these centrifuges, starting with sufficient reactor grade uranium, to enrich the uranium further to weapon grade for
One bomb: At least 2.3 months[12]
Five bombs: At least one year[13]

 

To fuel a small arsenal, reactor grade uranium needs to be enriched further to about 90% U-235 (the isotope of uranium that explodes in fission bombs). Such enrichment would take a full year, however, with the centrifuges Iran presently deploys. To shorten the time, Iran would have to install thousands more centrifuges, which would alarm many countries and could invite an attack, as stated above. 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. Such an extended period of time precludes a dash to an arsenal.

The Risk of Secret Sites
 

Western 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 in storage, 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]

Estimated minimum time it would take 3,000 of Iran’s model IR-6[19] centrifuges operating at claimed capacity and starting with natural uranium to fuel
One bomb: 1.6 months[20]
Five bombs: Eight months[21]

 

These 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.[22] 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.[23] 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 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.[24] 

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 100,000 separative work units, or SWU.[25] 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 years and would be detected.

ANNEX ONE
 

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 300 kg cap on its stockpile of low-enriched uranium in gaseous form 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. As of mid-February, Iran had not installed additional centrifuges at Natanz.

 

ANNEX TWO
 

Past growth of enrichment capacity at the Natanza Fuel Entrichment Plant (2007-2014)

Iran's past efforts to expand its uranium enrichment capacity at Natanz in the period before the nuclear accord was struck may be useful in estimating how long it would take Iran to reconstitute this capacity.

Date of IAEA Inventory IR-1 Centrifuges Being Fed with UF6 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
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

 

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 19, 2020, Iran's stockpile was comprised of 996.6 kg of uranium in the form of uranium hexafluoride (UF6), some of which was enriched up to 4.5% in the fissionable isotope U-235 and some of which was at a lower level of enrichment. U-235 makes up about .7% of natural uranium; its concentration can be increased, or enriched, using centrifuges. Uranium enriched to 90% or more U-235 can be used to fuel nuclear weapons.

[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 90% U-235 in the form of UF6 are assumed to be sufficient for one bomb. This assumes uranium tails of 1% U-235, a feed assay of 3.6% U-235, and a product assay of 90% U-235. The product would need to be further processed into finished uranium metal bomb components, which would cause about a 20% loss of material.

[5] Iran would need 940 SWU to produce 20 kg of uranium enriched to 90% U-235 from nuclear reactor grade feed enriched to 3.6%. 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 make a combined 4,883 SWU per year or 407 SWU per month. Thus, it would take approximately 2.3 months to produce the 940 SWU. The reactor grade feed would come from Iran’s existing stockpile, which contains 728 kg of low enriched uranium assumed to be at an average enrichment of 3.6% U-235. The 728 kg would be sufficient to fuel one bomb.  

[6] Iran's stockpile of LEU, assuming an average enrichment level of 3.6% U-235, contains only enough uranium to fuel one bomb. Fuel for the four additional bombs would need to be enriched from natural uranium, which would require about 4,000 SWU per bomb. This assumes uranium tails of .3%, a feed assay of .7% U-235 and a product assay of 90% U-235. If Iran's IR-1 centrifuges make a total of 4,883 SWU per year, it would take at least 3.3 years to make 16,000 SWU. Adding the 3.3 years needed for four bombs to the 2.3 months needed for one bomb equals at least 3.5 years.

[7] This amount of uranium enriched to 3.6% 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 Feb. 19, 2020, Iran had a total of 1020.9 kg of low-enriched uranium, 996.5 kg of which was in the form of UF6. This 996.5 kg has been enriched to various levels: 268.5 kg up to 2% and the balance of 728 kg enriched up to either 3.67% or 4.5%. It is assumed for the estimates made here that these 728 kg are enriched to an average level of 3.6%, although the actual level is unknown and the enrichment level for at least some of it may be lower. Therefore, it is possible, but unlikely, that the 728 kg includes less than 685 kg enriched to an average level of 3.6%. 

[9] In addition to the 6,104 IR-1 centrifuges operating at Fordow and the Natanz Fuel Enrichment Plant, Iran is using a smaller number of centrifuges set up at its Natanz pilot plant to grow its stockpile of low-enriched uranium. In R&D lines 2 and 3, Iran is operating a total of 120 centrifuges (IR-2m, IR-4, IR-5, IR-6 and IR-6s) and in R&D lines 4, 5, and 6, Iran is operating a total of 400 centrifuges (IR-4, IR-2m, and IR-6). These centrifuges are more powerful than the IR-1, but their enrichment rate is not known.

[10] The 728 kg of LEU in Iran's stockpile as of February 19, 2020 likely contains more than the 685 kg of 3.6% U-235 required to produce the 20 kg of uranium enriched to 90% needed for one bomb.

[11] Fuel for five bombs would require a total of 3,425 kg of LEU enriched to 3.6% U-235, assuming natural uranium feed, .3% tails, and that a total of 100 kg of U-235 are needed. This would require 4,700 SWU. Iran had 728 kg on February 19, 2020, thus an additional 2,697 kg of LEU would still be needed. Assuming that only the 6,104 IR-1 centrifuges are used and that they perform at their historic production rate of .8 SWU per machine, they would cumulatively produce 4,883 SWU, generating 1,079 kg of LEU annually (or 3 kg each day). To produce the additional 2,697 kg of LEU needed at a rate of 3 kg per day would take 900 days, or about 2.5 years. This period would be reduced slightly if the centrifuges now used for research contribute increasing amounts of LEU to the stockpile.

[12] It is assumed that only the IR-1 centrifuges already in production mode would be used in a dash to make nuclear weapons. Iran would need 940 SWU to produce 20 kg of uranium enriched to 90% using a feed assay of 3.6% U-235, and assuming 1% tails. At their proven production rate of .8 SWU per centrifuge, Iran’s 6,104 IR-1s could produce 4,883 SWU per year, or 407 SWU per month. Thus, it would take about 2.3 months to make 940 SWU.

[13] Iran would need to generate 4,700 SWU to make the 100 kg of 90% enriched uranium needed to fuel an arsenal of five bomb, with a feed assay of 3.6% U-235 and assuming 1% tails. If Iran's 6,104 centrifuges generate 4,883 SWU per year, this would take about one year.

[14] Iran has not operated the IR-2m centrifuge in production mode. It has about 1,000 such centrifuges in storage and several hundred installed in a research capacity at the Natanz pilot plant. 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 4,000 SWU are needed to produce the 20 kg of 90% U-235 to fuel one bomb (assuming uranium tails of .3% and a feed assay of .7% U-235) then it would take 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] Iran has about 100 IR-6 centrifuges operating in a research capacity at the Natanz pilot plant, according to the IAEA. 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.

[20] 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.

[21]  3,000 IR-6 centrifuges would produce the 20,000 SWU needed to fuel five bombs in about 8 months. 

[22] This assumes uranium tails of 1% U-235, a feed assay of 3.6% U-235, a product assay of 90% U-235, and a 20% loss of material during processing.

[23] 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).

[24] 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.

[25] 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).

Attachment: