After it was discovered that radiation was the result of atoms randomly decaying over long periods of time, there was speculation about making them release all their energy at once. There were sci-fi stories in the 'teens and 20s about discovering a ray that could be shined on radioactive material to make it go foof. There was no concept of neutrons, critical mass, or slamming hemispheres together. The idea was that the bomb residue would be nonradioactive because it all decayed.