Only elements of high atomic numbers can be fissioned. When a heavy element fissions, the new elements formed have a more stable configuration because of the greater binding energy per nucleon. In a nuclear reactor, the chain reaction can be controlled. In an atomic bomb, the chain reaction is not controlled. Go to top .
Breeder reactors produce more fuel than is consumed.
Go to top . Moderators Understandings and Fundamental Concepts For efficient nuclear fission, it is necessary to slow down the speed of the neutrons. Moderators are materials that have the ability to slow down neutrons quickly with little tendency to absorb them. The neutrons are slowed down most effectively by a head-on collision with a particle of similar mass. Supplementary Information Particles of similar mass such as hydrogen and its isotope deuterium have been found effective as moderators.