WI: Manhattan Project Disaster

[Hilarious Bolloc]

Absolutely - I should perhaps point out that I'm not saying this would be likely, and perhaps it may be "just-this-side-of-the-border" of ASB.
I would say, though that this is hypothetically about the worst that could happen to a major population centre given the OP's stipulations.

 

[Carl Schwamberger]

There were some accidents at the labs where the Uranium/Plutonium were shaped into bits for tests and the first bombs. Some of the scientists & techs we're irradiated, one fatally. Post 1945 there was a accident with one of the reactors at Haniford. Three technicians were killed.

 

[Aber]

OTL did get close to a major accident:

https://en.wikipedia.org/wiki/Demon_core

 

[Hilarious Bolloc]

Certainly, worse incidents could conceivably have happened at Los Alamos or Hanford (theoreticallly) even than these. Sensibly, these areas were sited away from major population centres.