How far back can nuclear weapons development be delayed?

As the tin says. What PODs in research, development, or even political changes can set back the development of nukes as late as possible?
 
With a low intensity conflict instead of WW2 atomic bombs would have been delayed significantly.

Agreed. Pull the Reich's teeth in '36 over the Rhineland. Hitler gets the boot and Germany goes from threat to irritant. The Japanese aren't going to take on the US, UK and France without them being distracted in Europe. Stalin may have been a SOB but he's not going to start a war in Europe. Expect low level work at least by 1950 though
 
This depends massively on when your Jonbar hinge occurs. the development of nuclear energy and weapons is the result of a long chain of events. For example:

Tokyo 2nd October 1995
Some wrongness is subtle, perceptible only to a Time Lord or time traveller of long experience. Some is as blatant as a still steaming, radioactive crater where a city should be.
The wrongness of Tokyo that October day wasn’t quite that that obvious. But to James Trenchard it was clear here and now. something was very wrong.
He froze at the mouth of the alley where Hex's
mostly-reliable TARDIS had materialised and attempted to blend into the shadow while he surveyed the city. He’s been to Tokyo before, “back” in 1999, now four years in the future.
He turned to Neko, clad in the image of a tall Asian woman, joined him, moving as silently as the felines that shared her DNA.
"Something’s wrong. Very wrong. This isn’t Tokyo as it should be".
"Are you sure?".
"Yes. The buildings aren’t right, Tokyo is a city of skyscrapers, maybe not as high as your time but not like this. And there should be far more people". He paused and stared at a small group of men who'd just come into view.
"Plus the soldiers are a give-away".
"You mentioned a poison gas incident, could they be providing additional security?".
"No, those troops aren’t Japanese".
"You may be right", she said, pointing at a poster on a light-pole to one side of the alley's mouth listing
Occupation Regulations.
"Bugger".
"Back to the TARDIS and Hex?".
"Yes, and quickly, we seem to be attracting attention".

In that particular case a quick jaunt to catch the premiere of Neon Genesis Evangelion shows that Something is Very Wrong. Specifically that an intern meteorologist (and graduate physics student) had an accident in 1895 on Ben Nevis, caught in a thunderstorm and killed. Unfortunately for continuity (and a lot of people to whom "Operation Downfall" would thus become rather more than a paper study) 'CTR' (Charles Thomas Rees Wilson) hadn't invented the Wilson cloud chamber[1] yet. Hence a lot of late nineteenth and early twentieth century history is derailed by a few years and the Manhattan Project never happens. With post-WW2 budget constraints the first atomic bomb isn't detonated until 1958.




[1] As Ernest Rutherford called it "the most original and wonderful instrument in scientific history". It was crucial in the study of ionising radiation and nuclear physics allowing the tracks of particles to be seen and photographed
 
The origins of the US Nuclear program was for Atomic 'Boilers' for ships, a controlled fission chain reaction in a pile. Only later did British prodding get them pointed toward the uncontrolled chain reaction, a bomb
 
With a 20th Century PoD it's going to be difficult to push it back that far. The fact is that while the Manhattan Project was a major undertaking, it would have been well within the resources of any of the Great Powers at the onset of the century, and would have offered tantalising prospects for enhancing the power of any such nation. It's worth remembering that the US only went for the bomb due to it already believing that it was possible, which was established well before any massive investments were made into the infrastructure. Maybe without WW2 the bomb gets pushed back a few years, but I doubt it would be more than a decade. It is simply too in reach technologically and financially, and too tempting for a world full of competing states.
 
The origins of the US Nuclear program was for Atomic 'Boilers' for ships, a controlled fission chain reaction in a pile. Only later did British prodding get them pointed toward the uncontrolled chain reaction, a bomb
In fact the the Naval Research Laboratory continued to work in parallel with (and in competition to) the Manhattan group until 1946. As you say it was aimed at nuclear energy for maritime propulsion, specifically for submarines. (Shades of Rickover).

The NRL developed the isotopic separation system using liquid thermal diffusion of uranium hexafluoride. Back in March 1939 Fermi had met with the Navy (and also the US Army) about the idea of nuclear energy, both explosive and as a power source. This meeting got Dr. Ross Gunn to apply for to the Chief of the Bureau of Engineering (Admiral Harold Bowen) for funding. Gunn had been interested in nuclear energy since the first information on fission had been published in 1937.
There's a fascinating PoD here: what if the Navy had been put in charge of the Manhattan Project? After all they were already working on the concept been the MP was formed. And FDR had Navy connections.

Even when the MP was formed it was the NRL that actually produced the initial batched of uranium hexafluoride for the first experiments in isotope separation. They weren't replaced (by Harshaw) until October of 1941. Likewise they built the initial thermal diffusion test plant and were (in autumn of 1942) ready to built a working pilot plant.
But in December 1942 Groves put a stop to most of the NRL work. Gunn was furious. However limited Navy research continued at Philadelphia Navy Yard until again the MP intervened in June of 1944 and reduced the plant to a training facility.
 
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