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

I think you misunderstand the size of the US nuclear arsenal in that time.
A few years after WW2 we did not have enough to “flatten the earth”.
We had 300 in 1950, and most were small.

To this day the industrial centrifuging of uranium is a long and tedious process, far from trivial.