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