Memo on the Potential Value of Stricter Limits on Iran’s Stockpile of Low-enriched UF6

In the negotiation over Iran’s nuclear program there currently appears to be an unbridgeable gap between Iran’s minimum requirement for enrichment capacity, the equivalent of the approximately 10,000 IR-1 centrifuges currently operating at Natanz, and the U.S. upper limit, which appears to be considerably lower. But there is another variable which also determines how quickly Iran could produce enough 90% enriched uranium for a nuclear explosive if it broke its commitment to stay below 5% enrichment. This variable is the size of Iran’s stockpile of up-to-5%-enriched uranium. Having a large stockpile of low-enriched uranium to feed into its centrifuge cascades shortens by a factor of three, e.g. from six to two months, the time that it would take to produce enough 90% enriched uranium for a bomb.

In this memo, first circulated in late September, Frank von Hippel and Alex Glaser show that it would be possible to reduce Iran’s current stockpile of 5,000 kg of low-enriched UF6 to about 200 kg made possible by using a smaller (12-inch) cylinder for enriched uranium. This would make it possible to recover the factor of three in breakout time and might make it possible for the P5+1 to raise their upper limit on Iran’s centrifuge capacity.

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