Editing Nuclear fission (section)

===Chain reactions===
{{main|Nuclear chain reaction}}
[[File:Fission chain reaction.svg|300px|thumb|A schematic nuclear fission chain reaction. 1.&nbsp;A uranium-235 atom absorbs a neutron and fissions into two new atoms (fission fragments), releasing three new neutrons and some binding energy. 2.&nbsp;One of those neutrons is absorbed by an atom of [[uranium-238]] and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However, the one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. 3.&nbsp;Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases between one and three neutrons, which can then continue the reaction.]]
John Lilley states, "...neutron-induced fission generates extra neutrons which can induce further fissions in the next generation and so on in a chain reaction. The chain reaction is characterized by the ''neutron multiplication factor k'', which is defined as the ratio of the number of neutrons in one generation to the number in the preceding generation. If, in a reactor, ''k'' is less than unity, the reactor is subcritical, the number of neutrons decreases and the chain reaction dies out. If ''k'' > 1, the reactor is supercritical and the chain reaction diverges. This is the situation in a fission bomb where growth is at an explosive rate. If ''k'' is exactly unity, the reactions proceed at a steady rate and the reactor is said to be critical. It is possible to achieve criticality in a reactor using natural uranium as fuel, provided that the neutrons have been efficiently moderated to thermal energies." Moderators include light water, [[heavy water]], and [[graphite]].<ref name=jl/>{{rp|269,274}}

According to John C. Lee, "For all nuclear reactors in operation and those under development, the [[nuclear fuel cycle]] is based on one of three ''fissile'' materials, <sup>235</sup>U, <sup>233</sup>U, and <sup>239</sup>Pu, and the associated isotopic chains. For the current generation of [[LWR]]s, the enriched U contains 2.5~4.5 [[wt%]] of <sup>235</sup>U, which is fabricated into UO<sub>2</sub> [[fuel rod]]s and loaded into fuel assemblies."<ref name=jcl>{{cite book |last1=Lee |first1=John C. |title=Nuclear Reactor Physics and Engineering |date=2020 |publisher=John Wiley & Sons, Inc. |isbn=9781119582328 |pages=324, 327–329}}</ref>

Lee states, "One important comparison for the three major fissile nuclides, <sup>235</sup>U, <sup>233</sup>U, and <sup>239</sup>Pu, is their breeding potential. A ''breeder'' is by definition a reactor that produces more fissile material than it consumes and needs a minimum of two neutrons produced for each neutron absorbed in a fissile nucleus. Thus, in general, the ''conversion ratio (CR) is defined as the ratio of fissile material produced to that destroyed''...when the CR is greater than 1.0, it is called the ''breeding ratio'' (BR)...<sup>233</sup>U offers a superior breeding potential for both thermal and fast reactors, while <sup>239</sup>Pu offers a superior breeding potential for fast reactors."<ref name=jcl/>