The nuclei of uranium-235 atoms are bombarded by neutrons in order to induce a nuclear fission reaction. Some of these nuclei split into two smaller nuclei, releasing heat and radiation. This radiation contains other neutrons that can then bombard other atoms, which are in turn undergo fission. This process is known as a chain reaction. In nuclear power plants, this chain reaction is fully controlled. In a nuclear weapon, the concentration of fissile material is significantly higher to achieve the critical mass required to trigger the chain reaction spontaneously and release colossal amounts of energy in a few fractions of a second.