Editing Neutron moderator (section)

==Reactor moderators==
{{See also|Nuclear fission}}
In a [[thermal-neutron reactor]], the nucleus of a heavy fuel element such as [[uranium]] absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms ([[Nuclear fission product|fission products]]). The fission process for [[uranium-235|<sup>235</sup>U]] nuclei yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. The free neutrons are emitted with a kinetic energy of ~2 MeV each. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become a self-sustaining [[nuclear chain reaction]] under controlled conditions, thus liberating a tremendous amount of energy.

[[Image:U235 Fission cross section.png|thumb|left|450px|[[Fission cross section]], measured in [[barn (unit)|barns]] (a unit equal to 10<sup>−28</sup>&nbsp;m<sup>2</sup>), is a function of the energy (so-called [[excitation function]]) of the neutron colliding with a <sup>235</sup>U nucleus. Fission probability decreases as neutron energy (and speed) increases. This explains why most reactors fueled with <sup>235</sup>U need a moderator to sustain a chain reaction and why removing a moderator can shut down a reactor.]]

The probability of further fission events is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV-range are much less likely (though not unable) to cause further fission. The newly released fast neutrons, moving at roughly 10% of the [[speed of light]], must be slowed down or "moderated", typically to speeds of a few kilometres per second, if they are to be likely to cause further fission in neighbouring <sup>235</sup>U nuclei and hence continue the chain reaction. This speed occurs at temperatures in the few hundred Celsius range.

In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully ''thermalised'' than others; for example, in a [[CANDU reactor]] nearly all fission reactions are produced by thermal neutrons, while in a [[pressurized water reactor]] (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled [[supercritical water reactor]], the proportion of fast fissions may exceed 50%, making it technically a [[fast-neutron reactor]].

A fast reactor uses no moderator but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction. In some fast reactor designs, up to 20% of fissions can come from direct fast neutron fission of [[uranium-238]], an isotope which is not [[Fissile material|fissile]] at all with thermal neutrons.

Moderators are also used in non-reactor [[neutron source]]s, such as [[plutonium]]-[[beryllium]] (using the {{chem|9|Be}}([[alpha particle|α]],n){{chem|12|C}} reaction) and [[spallation]] sources (using ([[proton|p]],xn) reactions with neutron rich heavy elements as targets).