Nuclear reactors create energy by nuclear fission, a process that splits the nucleus of an atom in two. An element with a heavy nucleus, most commonly uranium-235 (U-235), is hit by a free neutron. This divides the heavy nucleus into two lighter nuclei. The action of this split releases energy in the form of heat and radiation. Two to three other neutrons are released during this process. These neutrons then hit more U-235 nuclei and split them. This reaction continues on and on, forming what is called a chain reaction.