The answer to a different question (Where did Fermi get the U235 for the first nuclear pile) about U235, was that Fermi used natural uranium for his reactor. This explains, in particular, the origin of the Pu239 for “Thin Man”.

But how did the US obtain in just a few years enough weapon-grade enriched uranium for “Little Boy” ? Just centrifuges ?

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    $\begingroup$ Check out the wiki article for the Oak Ridge site of the Manhattan Project (en.wikipedia.org/wiki/Clinton_Engineer_Works). They used a variety of methods, but the major one was by gas diffusion. $\endgroup$ Jan 6, 2022 at 19:07
  • $\begingroup$ @cesaruliana - no, the major one was the Calutrons, using magnetic separation of ion beams. Some liquid diffusion capability came on line towards the end, enriching the feedstock for the Calutrons. Significant gas diffusion did not occur until after the war. Lawrence, based on his experience with his cyclotron, invented the process. $\endgroup$
    – Jon Custer
    Jan 7, 2022 at 17:51
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    $\begingroup$ @JonCuster you're right, I misread my own reference. Gas diffusion output was entered into magnetic separation, and I did not pay attention to the fact that the most effective stage was the latter, not the first. Thank you for the correction $\endgroup$ Jan 7, 2022 at 19:21


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