Why uranium enrichment was slow/near impossible in the beginning of Manhattan project but few years later it became trivial?

Question

So I’ve watched Oppenheimer and a recurring theme in the movie was how tedious task it was to get fission material (demonstrated as filling a large fish bowl by marbles). All they manage to collect in two year period was just enough for two bombs. but fast forward few years US have a complete arsenal of bombs to flat the earth. I understand they must’ve innovated a method and the exact method maybe classified, what i’m interested in is knowing what was the obstacle(s) for this and rough idea of how they might’ve overcome it.

In: 450

Answer 1

There was no way to make just the amount of Uranium and plutonium they needed for just one or two bombs. The process is too slow so they had to make it at a huge scale to get the amounts they needed. They had to build huge industrial factories for it. The biggest in the world at the time with cities of 40000 people working on them (Hanford,WA & Oak Ridge, TN). Equal to building the entire US auto industry in 3 years. This was the slow part. Once they had that they knew that the US wasn’t just going to stop. It was never about building g just a couple of bombs. Nobody has ever created a new tech and then just put it back in the box. So once they had the system in place they could crank out enough U and Pu to make a bomb a week, and then a bomb a day. And then…
Side story, one of the engineers working on the methods of pre-gabbing the houses for the workers was a guy named Levit. After the war, he started cranking out quick, cheap houses for US Army veterans to buy with their new. G.I. Bill loans. They were called Levitown‘s and became the blueprint for the new suburbs that changed American cities from then on. We call these planned urban developments.
Also, the architecture firm that won the contest to create the urban plan for Oak Ridge was a little company called SOM (Skidmore, Owings and Merrill). Which is now one of the largest architecture firms in the world, and designed the Borge Khalifa in Dubai for example. (The tallest building in the world) the Oak Ridge project was the one that put them on the map, but you won’t find it on their website…
The atom bomb project didn’t just make bombs, it became the template for completely changing the whole way Americans lived. By the 60’s every American city was completely changed.