Earlier this month, Iran began to enrich uranium to 19.75 percent, a fateful step that will bring Iran far closer to being able to fuel a nuclear weapon.(a) Iran justified this action by claiming that the enrichment was necessary to produce fuel for its Tehran research reactor (TRR),(b) which is expected to run out of fuel in a year or two.

Iran is capable of achieving an enrichment level of 19.75 percent. It could choose to do so either by further enriching part of its existing stockpile of low-enriched uranium (currently in excess of two tons of 3.5 percent enriched material) or by using some of the tons of natural uranium gas it also has on hand. The former step would take far less time than the latter.

It is important to understand that by enriching its uranium stockpile from natural levels (.711 percent U-235) to 3.5 percent -- a grade commonly used for power reactor fuel -- Iran has already accomplished 70 percent of the work needed to produce bomb-grade fuel. The further enrichment to 19.75 percent would accomplish 90 percent of the work.

The world has been anxious to dissuade Iran from taking this step. The obvious reason is that it would increase Iran's nuclear weapon potential. France, Russia and the United States underscored the risk in a February 12 letter to the International Atomic Energy Agency (IAEA), in which they termed Iran's decision an "escalation," and said it raises "new concerns about Iran's nuclear intentions."(c) This concern appears quite reasonable in light of the fact that Iran does not presently have the ability to manufacture the uranium into fuel elements useable in its reactor.

A draft agreement brokered by the IAEA last October would have sent about 1,200 kg of Iran's stockpiled low-enriched uranium to Russia and France for conversion into fuel for the reactor. The deal also would have reduced the size of Iran's enriched uranium stockpile, and therefore removed, for a short time, Iran's ability to fuel a bomb in a "breakout" scenario. Iran accepted the agreement "in principle," but since then has backpedaled to the point where the agreement appears dead. Iranian officials have effectively scuttled the deal by suggesting changes: Iran could send out its low-enriched uranium in batches equal to the fresh fuel it receives; or Iran could move small batches of uranium to Kish Island, an Iranian free trade zone, as it receives fresh reactor fuel from abroad. These changes were quickly rejected by the United States and its partners.

Iran's decision to reach higher enrichment raises two questions: How long would it take to do it, and how much closer would the higher enrichment bring Iran to bomb-grade fuel? The estimates below are based upon the theoretical performance of Iran's existing centrifuge cascades. Accidents, sabotage, or other misfortunes could result in large delays.

The quickest way for Iran to achieve 19.75% enrichment would be to further enrich some of the 3.5% enriched material now in its stockpile.

• Iran could decide to further enrich 1,200 kg of this material, which is the amount that Iran would have exported under the draft fuel exchange agreement described above.
  • Enriching this amount would yield about 176 kg of 19.75% enriched uranium hexafluoride (containing 119 kg of uranium, of which 23 kg would be U-235).(d)
  • Accomplishing the above would require approximately 970 SWUs.(e)

• Or Iran could decide to further enrich 1,950 kg of this material, which is the amount Iran recently moved to the pilot fuel enrichment plant at Natanz.
  • Enriching this amount would yield about 285 kg of 19.75% enriched uranium hexafluoride (containing 192 kg of uranium, of which 38 kg would be U-235).(f)
  • Accomplishing the above would require approximately 1,575 SWUs.(g)
  • Note: As of mid-February, according to the IAEA, the Natanz pilot plant had only one cascade of 164 centrifuges capable of enrichment up to 19.75%.(h) Thus, this plant is not presently capable of large scale enrichment work.

• The most logical known location for Iran to perform this enrichment is the Natanz Fuel Enrichment Plant (FEP), where over 8,000 centrifuges are currently installed. The work would require the following quantities of time:
  • If Iran were to operate one unit of 2,952 first-generation (P-1/IR-1) centrifuges at the FEP, and the centrifuges perform only at a low level of .5 SWU/machine, it would take these centrifuges about eight months to produce the 970 SWUs necessary raise the enrichment of the 1,200 kg of low-enriched uranium hexafluoride from 3.5% to 19.75%.
  • If Iran were to operate the same number of machines more efficiently, at 1 SWU/machine, Iran could probably raise the enrichment to 19.75% within four months.
  • If Iran were to dedicate two units of 5,904 centrifuges at the FEP to the production 19.75% material, it could take between two and four months (depending on centrifuge efficiency) to raise the enrichment of the 1,200 kg of low-enriched uranium hexafluoride.
  • Note: As of late January, of the over 8,000 P-1 centrifuges installed at the FEP, less than half were being used for the production of 3.5% enriched uranium.(i)

After producing 19.75% enriched uranium hexafluoride, Iran could then decide to raise its enrichment to weapon-grade in a "breakout" scenario.

• Raising the enrichment of the 176 kg of this material mentioned above would yield about 35 kg of 93% enriched uranium hexafluoride (containing about 23 kg of uranium, of which 22 kg would be U-235).(j) This amount would be sufficient to fuel one nuclear weapon.
  • Accomplishing the above would require approximately 380 SWUs.(k)
  • Such a step would take about three months if Iran were to use one unit of P-1 centrifuges with an efficiency of .5 SWU/machine, or less than two months with an efficiency of 1 SWU/machine.

• Raising the enrichment of the 285 kg of material mentioned above would yield about 58 kg of 93% enriched uranium hexafluoride (containing 39 kg of uranium, of which 36 kg would be U-235).(l)
  • Accomplishing the above would require approximately 615 SWUs.(m)
  • If Iran were to use one unit of P-1 centrifuges, such a step would take between 2.5 and 5 months, depending centrifuge efficiency.

• For use in a nuclear weapon, the material would then have to be converted from gas to metal and machined into a form suitable for a bomb core.