Nuclear fission: A chain reaction

Fission occurs when a neutron runs into the nucleus of a large atom (in this case Uranium=235 Atoms). The atom splits into two smaller atoms. In addition two or three neutrons fly off at a high speed. They in turn run into more U-235 atoms. The U-235 Atoms split, and the process continues.  Fission can occur in an atom bomb.  Fission also occur reactors.  However, in a reactor not all atoms that absorb neutrons split. When a U-238 atom absorbs a neutron something very different happens.  The atom undergoes a transformation.  The nucleus ejects a powerful electron - called a beta ray - and a neutron switches into a proton.  With this change the atom becomes Neptunium-239.  Another electron is ejected, and another neutron a proton, and the atom  becomes Plutonium-239.