Iran In Brief

Moving 20 Percent Enrichment to Fordow:  Slow Motion Breakout Continues?

Moving 20 Percent Enrichment to Fordow:  Slow Motion Breakout Continues? photo

June 8, 2011

By David Albright, Paul Brannan, and Andrea Stricker

On June 8, Iran’s vice president and head of the Atomic Energy Organization of Iran (AEOI), Fereydoun Abbasi, announced that Iran would install 164-machine cascades of advanced centrifuges at the previously hidden Fordow enrichment plant and triple its enrichment output of 19.75 percent low enriched uranium (LEU) by the end of the year.  By moving its 19.75 percent LEU production to Fordow and tripling its output of 19.75 percent LEU, Iran positions itself to stockpile a large amount of 19.75 percent LEU more quickly in a facility better protected against military strikes.  A year after starting, Iran would have enough 19.75 percent LEU to more quickly break out and produce enough weapon-grade uranium for a nuclear weapon, if it chose to do so. 

Iran’s announcement indicates that as few as one centrifuge cascade of advanced centrifuges could produce the 19.75 percent LEU at Fordow.  ISIS is interpreting that the threefold increase in this case refers to the greater enrichment output of the advanced centrifuges compared to the IR-1 centrifuges at Natanz.

Based on its output at the pilot enrichment plant at Natanz, Iran’s monthly output of this LEU would increase threefold to almost 12 kilograms per month.  Iran has already produced about 60 kilograms of 19.75 percent LEU at its pilot plant at Natanz.  With increased production, Iran could accumulate about 200 kilograms of LEU one year after starting the cascade at Fordow, assuming the cascade at Natanz stops producing this material, as Iran has indicated will happen.  Two hundred kilograms of 19.75 percent LEU are enough material, if further enriched, to make sufficient weapon-grade uranium for one nuclear weapon.

A calculation illustrates this result and provides reasonable timelines for producing enough weapon-grade uranium for a nuclear weapon at either a declared facility like Natanz or Fordow, or at a secret enrichment plant.  This calculation assumes that an advanced centrifuge has an average enrichment output three times that of the IR-1 centrifuge, or about 2.9 separative work units (swu) per year per machine, and that 584 centrifuges in a total of six cascades are enriching in two steps, up to 60 percent, and then from 60 percent to 90 percent1.  In six months of enrichment, the feed needed would be 192 kilograms of 19.75 percent LEU, with a product of 31 kilograms of weapon-grade uranium, more than enough for one nuclear weapon.  If twice that number of centrifuges were used, or about 1,200 centrifuges, this amount of weapon-grade uranium could be produced in three months.  Similarly 2,400 centrifuges could produce enough for a bomb in one and half months.  The times could be longer if large numbers of the centrifuges fail or the cascades do not work properly.  But this calculation assumes that the cascades are relatively inefficient in producing enriched uranium compared to the ideal case.2

All of this supports a possible on-going effort by Iran to slowly acclimatize the international community to conditions that would make a breakout to nuclear weapons more feasible.  Although Iran claims that it needs 19.75 percent LEU to operate its Tehran research reactor and additional ones it plans to build, it does not yet have the capability to build these new reactors and it has produced several years’ worth of enriched uranium for the Tehran research reactor.  If Iran proceeds with its plan, it will accumulate a large stockpile of 19.75 percent LEU at Fordow, and this stock and the centrifuges producing it would be heavily fortified inside the Fordow mountain facility and rendered less vulnerable to aerial strikes.  Iran could quickly move its stock of 19.75 percent LEU elsewhere for enrichment to weapon-grade in a small, easily hidden centrifuge facility or kick out IAEA inspectors and quickly enrich to weapon-grade, though it may risk a ground strike.


1 Four cascades of 114 centrifuges take material from 19.75% to 60% and two cascades of 64 centrifuges go from 60% to 90%, for a total of 584 centrifuges.  The tails assay is about 4.5% for the first step and about 20% for the step to 90%.  The cascades are relatively inefficient, achieving about 35% of ideal separative power in the first step and about 40% of ideal separative power in the second step.

2The calculation treats the cascades as relatively inefficient.  The first step achieves about 35% of ideal separative power, and the second step achieves about 40% of ideal separative power. 

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