Iran’s Nuclear Potential before the Implementation of the Nuclear Agreement

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]

Highlights:

• 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:
  16,142 kg[b]

• Amount of this material ready for further enrichment (i.e., stored in gaseous form) as of November 2015:
  8,306 kg[c]

• Amount theoretically needed to produce a bomb’s worth of weapon-grade uranium metal:
  1,053 kg[d]

• Number of first generation implosion bombs this 8,306 kilograms could fuel, if further enriched:
  7[e]

• 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:
  February 2009[g]

• Approximate number of first generation IR-1 centrifuges being fed with UF6 at the Natanz Fuel Enrichment Plant, as of October 2015:
  9,000[h]

• Number of months theoretically needed for these 9,000 centrifuges operating at their estimated capacity to produce enough enriched uranium  for one bomb:
  1.6[i]

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:
  100[k]

• 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:
  10.7[l]

• 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:
  100,000[n]

• Approximate number of IR-1 centrifuges Iran would need to operate in order to produce this level of work annually:
  126,500[o]

• Approximate number of first generation implosion bombs Iran could fuel if able to enrich the uranium needed to supply Bushehr annually:
  25[p]

Iranian Nuclear Breakout Times Under Different Scenarios

• 9,000 centrifuges (the number running in October 2015):
  6.8 months[q]

• 6,104 centrifuges (the number allowed to be installed under the Joint Comprehensive Plan of Action):
  10 months[r]

• 5,060 centrifuges (the number allowed to operate under the Joint Comprehensive Plan of Action):
  1 year[s]

• 9,000 centrifuges:
  1.6 months

• 6,104 centrifuges:
  2.4 months

• 5,060 centrifuges:
  2.9 months

Comments

• 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