ISIS Iran Report

How enriching uranium is like making apple cider

March 3, 2009

We want to say a few words on the issue of producing weapon-grade uranium from a stock of low enriched uranium.  Some have suggested that 1,010 kg of low enriched uranium is not sufficient, arguing that based on Natanz’s operation to date, too much uranium-235 is left in the tails to produce enough weapon-grade uranium for a bomb.  This is incorrect.  It is important to understand that the tails assay—the quantity of uranium left in the centrifuge cascade’s waste stream—is routinely varied by the cascade operator.  When enriching from natural uranium, a centrifuge operator such as Iran is likely to opt for a relatively high tails assay, about 0.4 percent, and leave about half the uranium-235 in the tails.  If Iran were to use its existing stock of LEU, however, as feed for producing weapons-grade uranium, it would choose a relatively low tails assay, which would maximize the uranium-235 in the enriched uranium product and minimize the loss of that enriched uranium in the tails.  The operator faces the opposite situation starting with natural uranium, where there is plenty of natural uranium and the operator would prefer to maximize the output of low enriched uranium at the expense of the uranium-235 in that uranium.  It is a misunderstanding therefore to assert that the same fraction of uranium 235 would be left in the tails whether the operator is going from natural to 3.5 percent enriched uranium or from 3.5 percent in stages to weapon-grade uranium.  Equating the two situations ignores choices the operator must make to balance material constraints against the enrichment effort. 

The situation is somewhat analogous to pressing apples for cider.  One can vary the cider press (based on the cost of fruit, or the effort to press the apples) to yield either more cider with less cider left in the waste (expensive apples) or less cider with more cider left in the waste (plentiful apples). 

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