Editing Nuclear chain reaction (section)

===Fuel===
{{Mainarticle|Nuclear fuel}}
Nuclear weapons employ high quality, highly enriched fuel exceeding the critical size and geometry ([[critical mass]]) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material (e.g. [[uranium dioxide]], UO<sub>2</sub>). There are two primary isotopes used for fission reactions inside of nuclear reactors. 

The first and most common is [[uranium-235]]. This is the fissile isotope of uranium and it makes up approximately 0.7% of all [[Uranium ore|naturally occurring uranium]].<ref>{{Cite web|url=https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview.aspx|title=Nuclear Fuel Cycle Overview - World Nuclear Association|website=www.world-nuclear.org|access-date=2020-03-18}}</ref> Because of the small amount of <sup>235</sup>U that exists, it is considered a non-renewable energy source despite being found in rock formations around the world.<ref>{{Cite web|url=https://www.eia.gov/energyexplained/nuclear/|title=Nuclear explained - U.S. Energy Information Administration (EIA)|website=www.eia.gov|access-date=2020-03-18}}</ref> Uranium-235 cannot be used as fuel in its base form for energy production; it must undergo a process known as refinement to produce the compound UO<sub>2</sub>. The UO<sub>2</sub> is then pressed and formed into ceramic pellets, which can subsequently be placed into fuel rods. This is when UO<sub>2</sub> can be used for nuclear power production. 

The second most common isotope used in nuclear fission is [[plutonium-239]], because it is able to become fissile with slow neutron interaction. This isotope is formed inside nuclear reactors by exposing <sup>238</sup>U to the neutrons released during fission.<ref>{{Cite web|url=https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx#ECSArticleLink0|title=Plutonium - World Nuclear Association|website=www.world-nuclear.org|access-date=2020-03-18}}</ref> As a result of [[neutron capture]], uranium-239 is produced, which undergoes two [[beta decay]]s to become plutonium-239. Plutonium once occurred as a primordial element in Earth's crust, but only trace amounts remain so it is predominantly synthetic. 

Another proposed fuel for nuclear reactors, which however plays no commercial role as of 2021, is [[uranium-233]], which is "bred" by neutron capture and subsequent beta decays from natural [[thorium]], which is almost 100% composed of the isotope [[thorium-232]]. This is called the [[thorium fuel cycle]].