Nuclear spallation

 Nuclear spallation is a process that occurs naturally in Earth's atmosphere.

It can also occur under man-made conditions in a process by which a particle accelerator may be used to produce a beam of neutrons. A mercury, tantalum, lead or other heavy metal target is used, and 20 to 30 neutrons are expelled after each impact. Although more costly that traditional methods (chain reaction of nuclear fission in a nuclear reactor).