> the hardest thing about producing an atomic bomb is spinning up enough U235. To produce highly enriched uranium requires huge numbers of gas centrifuges. Thi

> the hardest thing about producing an atomic bomb is spinning up enough U235. To produce highly enriched uranium requires huge numbers of gas centrifuges. This is true in the modern era. However it should be noted that the Manhattan Project did not use gas centrifuges to produce their enriched uranium. Centrifuges were apparently considered but ultimately gaseous diffusion was used. Gaseous diffusion requires larger facilities and more energy, but I suppose the process doesn't require such tight tolerances as modern highspeed centrifuges.