[knorby]

Well, there were basically a few different strategies to produce U-235 during the war. For Hiroshima, most of the Uranium came from a very temporary but quicker to setup using an electromagnetic process at Y-12 in Oak Ridge, and that ended with the war. The gaseous diffusion plant at K-25, also in Oak Ridge, came online in 1945 and ended up being the main enricher for much of the cold war; it was a construction marvel, and the fact it was built in 2 years is still remarkable. The US definitely would have been ready to produce many more in not too long for the war had it been needed.

All of these processes were brand new at the time and mostly untested. Plutonium enrichment at the Hanford site used a reactor design that was validated well into the construction of the site. The Manhattan project already had a lot of risks, and they weren't going to attempt chaining projects for something unproven. Real experiments came after the war.