The Minsky option...
We have discussed the possibilities of the Bomb either being delayed in development (no WWII = no Manhattan Project, etc.) or actively supressed, but what about a simpler option that doesn't require ASB?
In the field of AI, there is a great story about Marvin Minsky, one of the giants of the field, who concluded that neural networks were not practical. Without getting too deeply into the weeds, Minsky was utterly wrong (and to his credit, acknowleged this much later), but such was his reputation and the force of his argument, that no serious research occurred in the field for decades. Minsky wasn't trying to supress anything, but that was the effect of his error...
Now AI uses reasonably cheap hardware, and testing the propositions is easy and can be done by almost anyone. The propositions behind most nuclear physics are insanely expensive to test, particularly if they are somewhat subtle. Keep in mind that the most expensive part of the Manhattan project was not building the bomb (and by the way Astrodragon, the Uranium bombs were MUCH cheaper to build than Plutonium ones...I believe you have your cost estimates reversed), but figuring out whether it could be built in the first place, and if so, what would critical mass be. Hence if there was reason to believe that nuclear bombs were a dead end, it is possible that (absent some incredibly compelling motivation) anyone would be willing to commit the resources necessary to find out that this assumption was mistaken and that a bomb COULD be built.
To make an (already far too long) story short...what if one of the 'giants' of nuclear physics simply made a (convincing) error and concluded that nuclear bombs couldn't be built? Heisenberg came close to this (and it crippled the German program, though there is some reason to believe that this was more misdirection on his part than a mistake), and it isn't impossible to imagine Fermi or Mertiner making the same sort of error. Remember, the mistake could be a subtle one, and the impact might be that the belief was reactors would work, but bombs wouldn't, or something of that sort.
This gives us a world with the laws of physics and geology untouched, no conspiracy, and no bombs for a very long time, if ever....