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The more uranium 235 your nuclear fuel has, the smaller you can make your reactor—or your bomb, which is why the material is subject to such strict regulations.

The Iranian images of the plant released last year show the assembly of the centrifuges — tall, thin machines that enrich, or concentrate, uranium’s rare component, uranium 235.

The deal allows research, which now has improved their breakout time down to a day or two, by means of rapid centrifuge enrichment of uranium 235.

By December 6, the spectrographers had accumulated their first measurable, if invisible, quantity of uranium 235—far from pure but at more than triple the concentration found in nature.

It has enriched uranium to one part uranium 235 in five parts, not to one part in 20.