The common belief on the far right is that Iran could just wave a magic wand to instantly turn 4.5% enriched Uranium (fuel for the VVER-1000 power reactor under construction at Bushhehr) and turn it into Weapons Grade Highly Enriched Uranium. This is not true. The length of a centrifuge cascade determines the final product. Each centrifuge supplies only a small enrichment (Separative Work Units). Centrifuge Cascades are run as a continuous process. They are difficult to start and stop. With the IAEA keeping track of the centrifuge cascades and auditing the feed material as well as the product, it would be impossible for Iran to turn the 4.5% enriched Uranium into 90% bomb material. To do so it would have to throw out the IAEA, stop the cascades, reconfigure, and restart; or do batch processing with repeated stops and starts. There would be sufficient notice to take remedial measures before sufficient HEU for bombs could be produced.
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from the article: U.S. awaiting formal response from Iran on nuclear draft deal