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. Neutrons enter the water and are slowed down.