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

Uranium enrichment is far from trivial, but we are better at it than we were. We use centrifuges to enrich uranium instead of gaseous diffusion but it is highly specialised equipment… even the oil on your hand from touching it is sufficient to ruin a centrifuge as the balance specification needed is so tight. Multiple centrifuges and many passes are required to enrich uranium to the level needed to make a weapon. A weapon you are almost certainly never going to use.

That the US had lots of bombs so quickly is a matter of priority rather than of difficulty.