When colleagues read the paper, they asked him to slow down. Nuclear fission goes on all the time in uranium and plutonium. Fission releases energy and neutrons. The neutrons cause more fission. The process is gentle because only a few neutrons hit anything.
Atoms are too far apart.
Neutrons are too small. Imagine two magnetic bullets fired toward each other on parallel paths, a half-inch apart. Bullets move too fast to be drawn into a collision. They'd have to move slowly to collide. In a nuclear reactor, engineers surround uranium with water.