Editing Nuclear chain reaction (section)

==Process==

Fission chain reactions occur because of interactions between neutrons and [[Fissile material|fissile isotopes]] (such as <sup>235</sup>U). The chain reaction requires both the release of neutrons from fissile isotopes undergoing nuclear fission and the subsequent absorption of some of these neutrons in fissile isotopes. When an atom undergoes nuclear fission, a few neutrons (the exact number depends on uncontrollable and unmeasurable factors; the expected number depends on several factors, usually between 2.5 and 3.0) are ejected from the reaction. These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to produce a reaction that is self-sustaining.

[[Nuclear power plants]] operate by precisely controlling the rate at which nuclear reactions occur. Nuclear weapons, on the other hand, are specifically engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction will lead to an explosive energy release.

===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]].

=== Enrichment process ===
{{Mainarticle|Enriched uranium}}
The fissile isotope uranium-235 in its natural concentration is unfit for the vast majority of nuclear reactors. In order to be prepared for use as fuel in energy production, it must be enriched. The enrichment process does not apply to plutonium. Reactor-grade plutonium is created as a byproduct of neutron interaction between two different isotopes of uranium. 

The first step to enriching uranium begins by converting [[uranium oxide]] (created through the uranium milling process) into a gaseous form. This gas is known as [[uranium hexafluoride]], which is created by combining [[hydrogen fluoride]], [[fluorine]], and uranium oxide. Uranium dioxide is also present in this process and is sent off to be used in reactors not requiring enriched fuel. The remaining uranium hexafluoride compound is drained into metal cylinders where it solidifies. The next step is separating the uranium hexafluoride from the [[Depleted uranium|depleted U-235]] left over. This is typically done with centrifuges that spin fast enough to allow for the 1% mass difference in uranium isotopes to separate themselves. A laser is then used to enrich the hexafluoride compound. The final step involves reconverting the enriched compound back into uranium oxide, leaving the final product: enriched uranium oxide. This form of UO<sub>2</sub> can now be used in fission reactors inside power plants to produce energy.

===Reaction products===
When a fissile atom undergoes nuclear fission, it breaks into two or more fission fragments. Also, several free neutrons, [[gamma ray]]s, and [[neutrino]]s are emitted, and a large amount of energy is released. The sum of the rest masses of the fission fragments and ejected neutrons is less than the sum of the rest masses of the original atom and incident neutron (of course the fission fragments are not at rest). The mass difference is accounted for in the release of energy according to the equation [[Mass–energy equivalence|''E=Δmc<sup>2</sup>'']]:

:'''mass of released energy''' = <math>\frac{E}{c^2} = m_\text{original}-m_\text{final}</math>

Due to the extremely large value of the [[speed of light]], ''c'', a small decrease in mass is associated with a tremendous release of active energy (for example, the kinetic energy of the fission fragments). This energy (in the form of radiation and heat) carries the missing mass when it leaves the reaction system (total mass, like total energy, is always [[conservation of mass|conserved]]). While typical chemical reactions release energies on the order of a few [[electron volt|eVs]] (e.g. the binding energy of the electron to hydrogen is 13.6&nbsp;eV), nuclear fission reactions typically release energies on the order of hundreds of millions of eVs.

Two typical fission reactions are shown below with average values of energy released and number of neutrons ejected:

:<math chem>\begin{align}
\ce{^{235}U + neutron ->} &\ \text{fission fragments} + 2.4\text{ neutrons} + 192.9\text{ MeV} \\
\ce{^{239}Pu + neutron ->} &\ \text{fission fragments} + 2.9\text{ neutrons} + 198.5\text{ MeV}
\end{align}</math><ref name=Duderstadt>{{cite book |last=Duderstadt |first=James |author2=Hamilton, Louis  |title=Nuclear Reactor Analysis |year=1976 |publisher=John Wiley & Sons, Inc |isbn=978-0-471-22363-4 }}</ref>

Note that these equations are for fissions caused by slow-moving (thermal) neutrons. The average energy released and number of neutrons ejected is a function of the incident neutron speed.<ref name=Duderstadt /> Also, note that these equations exclude energy from [[Neutrino|neutrinos]] since these subatomic particles are extremely non-reactive and therefore rarely deposit their energy in the system.