Editing Fast-neutron reactor (section)

===Moderators in conventional nuclear reactors===

[[Natural uranium]] consists mostly of two [[isotope]]s:

* 99.3% [[uranium-238|{{chem|238|U}}]]
* 0.7% [[uranium-235|{{chem|235|U}}]]

Of these two, {{chem|238|U}} undergoes fission only by fast neutrons.<ref>{{cite web|url=http://www.nuclear-power.net/nuclear-power/reactor-physics/atomic-nuclear-physics/fundamental-particles/neutron/#Neutron_Energy|title=What is Neutron - Neutron Definition|website=www.nuclear-power.net|language=en-US|access-date=2017-09-19}}</ref> 
About 0.7% of natural uranium is {{chem|235|U}}, which will undergo fission by both fast and slow (thermal) neutrons. When the uranium undergoes fission, it releases neutrons with a high energy ("fast").
However, these fast neutrons have a much lower probability of causing another fission than neutrons which are slowed down after they have been generated by the fission process. Slower neutrons have a much higher chance (about 585 times greater) of causing a fission in {{chem|235|U}} than the fast neutrons.

The common solution to this problem is to slow the neutrons down using a [[neutron moderator]], which interacts with the neutrons to slow them. The most common moderator is ordinary water, which acts by [[elastic scattering]] until the neutrons reach [[thermal equilibrium]] with the water (hence the term "thermal neutron"), at which point the neutrons become highly reactive with the {{chem|235|U}}. Other moderators include [[heavy water]], [[beryllium]] and [[Nuclear graphite|graphite]]. The elastic scattering of the neutrons can be likened to the collision of two ping pong balls; when a fast ping pong ball hits one that is stationary or moving slowly, they will both end up having about half of the original kinetic energy of the fast ball. This is in contrast to a fast ping pong ball hitting a bowling ball, where the ping pong ball keeps virtually all of its energy.

Such thermal neutrons are more likely to be absorbed by another heavy element, such as {{chem|238|U}}, {{chem|link=Uranium-232|232|Th}} or {{chem|235|U}}. In this case, only the {{chem|235|U}} has a high probability of fission.

Although {{chem|238|U}} undergoes fission by the fast neutrons released in fission about 11% of the time this can not sustain the chain reaction alone. 
Neutrons produced by fission of {{chem|238|U}} have lower energies than the original neutron, usually below 1 MeV, the fission threshold to cause subsequent fission of {{chem|238|U}}, so fission of {{chem|238|U}} does not sustain a nuclear chain reaction. When hit by thermal neutrons (i.e. neutrons that have been slowed down by a moderator) the neutron can be captured by the {{chem|238|U}} nucleus to transmute the uranium into [[uranium-239|{{chem|239|U}}]] which rapidly decays into [[neptunium-239|{{chem|239|Np}}]] which in turn decays into [[plutonium-239|{{chem|239|Pu}}]]. {{chem|239|Pu}} has a thermal [[neutron cross section]] larger than that of {{chem|235|U}}.

About 73% of the {{chem|239|Pu}} created this way will undergo fission from capturing a thermal neutron while the remaining 27% absorbs a thermal neutron without undergoing fission, {{chem|240|Pu}} is created, which rarely fissions with thermal neutrons. When [[plutonium-240]] in turn absorbs a thermal neutron to become a heavier isotope {{chem|241|Pu}} which is also fissionable with thermal neutrons very close in probability to plutonium-239. In a fast spectrum reactor all three isotopes have a high probability of fission when absorbing a high energy neutron which limits their accumulation in the fuel.

These effects combined have the result of creating, in a moderated reactor, the presence of the [[transuranic]] elements. Such isotopes are themselves unstable, and undergo [[beta decay]] to create ever heavier elements, such as [[americium]] and [[curium]]. Thus, in moderated reactors, plutonium isotopes in many instances do not fission (and so do not release new fast neutrons), but instead just absorb the thermal neutrons. Most moderated reactors use natural uranium or low enriched fuel. As power production continues, around 12–18 months of stable operation in all moderated reactors, the reactor both consumes more fissionable material than it breeds and accumulates neutron absorbing fission products which make it difficult to sustain the fission process. When too much fuel has been consumed the reactor has to be refueled.