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

We had never done it before. We had to build up every part of it from scratch. Uranium had never really been useful until then, so there was no infrastructure to collect, store and refine it. All those things had to be created. We also didn’t know where a lot of uranium was, so we had to find it. When we did get it our refining processes were not efficient so we lost a lot the uranium that we could’ve kept today.

Once we had these systems and the procedures we developed in place, it became much easier to do. That doesn’t mean its easy to do, mind you. Its just easier than it was.