A material such as light or heavy water or graphite used in a reactor to slow down fast neutrons by collision with lighter nuclei so as to expedite further fission. Can be used as fuel in heavy water-moderated reactors. An uncharged elementary particle found in the nucleus of every atom except hydrogen. Solitary mobile neutrons travelling at various speeds originate from fission reactions. Sometimes atomic nuclei simply capture neutrons.
All commercial reactors are thermal reactors, using a moderator to slow down the neutrons.
A transuranic element, formed in a nuclear reactor by neutron capture. It has several isotopes, some of which are fissile and some of which undergo spontaneous fission, releasing neutrons. About one third of the energy in a light water reactor comes from the fission of Pu-239, and this is the main isotope of value in reprocessing spent fuel. The spontaneous decay of an unstable atomic nucleus, giving rise to the emission of radiation. The adverse health effect of a radionuclide due to its radioactivity.