Fallout is the left over radiation hazard from a nuclear explosion. The name is because radioactive material "falls out" of the atmosphere into which it is spread during the explosion. These materials continue to undergo radioactive decay for minutes, days or centuries. "Fallout" commonly refers to the radioactive dust created when a nuclear weapon explodes. All nuclear explosions make fission products, which are the broken, radioactive atoms from a fission reaction. Neutrons from the explosion also make some nearby materials radioactive. 