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<onlyinclude>'''Nuclear chain reactions''' are reactions where [[nuclear energy]] is obtained, generally through [[nuclear fission]]. These chain reactions are what provide [[nuclear power plant]]s with the [[energy]] that is then turned into electricity for use by people.</onlyinclude> In these reactions, [[neutron]]s generated by the fission process continue on to initiate fission in other [[atom]]s. These reactions generally occur with heavier [[isotope]]s such as uranium-235 where there is a continuous release and absorption of neutrons. Figure 1 below is a visual representation of what a nuclear fission chain reaction looks like. If at least one neutron from each fission strikes another U-235 nucleus and initiates fission, the chain reaction is sustained and it is said to be '''critical'''.
[[Category:Translated to French]]
[[fr:Réaction nucléaire en chaîne]]
[[Category:Translated to Spanish]]
[[es:Reacción nuclear en cadena]]
[[File:Fission_chain_reaction.svg.png|thumb|Figure 1. A neutron strikes a <sup>235</sup>U nucleus and causes a fission event. This releases more neutrons. Unlike in the figure, on average one new fission event happens as a result of these released neutrons.]]
<onlyinclude>A '''nuclear chain reaction''' occurs when the output of one [[nuclear reaction]] causes more nuclear reactions to occur. These chain reactions are almost always a series of [[nuclear fission|fission]] events, which give off excess [[neutron]]s. It is these excess neutrons that can go on to cause more fission events to occur, hence the name '''chain reaction'''. Nuclear chain reactions are essential to the operation of nuclear power plants.</onlyinclude>


[[File:u235fission.gif|400px|framed|center|Figure 1. A nuclear fission chain reaction of uranium-253 atoms.<ref name=hyp>HyperPhysics. (May 27, 2015). ''Uranium-235 Chain Reaction'' [Online]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html</ref>]]
Chemical reactions involve different chemical species recombining. Nuclear reactions involve different flavours of nuclei (called [[nuclear species]]) interacting. Many [[chemical reaction]]s are also chain reactions, with many similarities to nuclear chain reactions. These similarities include:
* That the reactions are sustained when chemical or nuclear species available to react. The chain reaction stops when the species are removed or are used up.  
* That the chain reactions are controlled (starting, speeding up, slowing down and stopping) by adding or removing chemical or nuclear species in that chain.
* [[Energy]] is often released as the reactions occur.
* Released energy is often output as [[thermal energy]], becoming [[heat]] that can be harnessed by [[heat engine]]s to do useful [[work]] like make [[electricity]].


The mass of [[uranium|uranium-235]] that is required to produce a reaction that is self-sustaining is said to be the "critical mass". This critical chain reaction can be accomplished at relatively low uranium-235 concentrations if the neutrons are [[moderation|moderated]] to lower their speed. A moderator is necessary because the probability for fission is greater with slow neutrons.<ref name=hyp/> The general equation for a nuclear chain reaction of uranium-235 is:<ref name="RE1"/>
While these similarities exist, there are some important differences as well. Nuclear reactions release roughly one '''million''' times as much energy as chemical reactions. This means that chemical chain reactions occur much more easily than nuclear reactions. For example, fire is a chemical chain reaction. Nuclear chain reactions require careful engineering and as far as we know, a natural nuclear chain reaction has only [[Oklo natural nuclear reactor|occurred once.]]<ref> As far as we know! Although it seems unlikely to have occurred more than once.</ref> Nuclear chain reactions require an abundance of careful planning. When they do occur, there is substantially more energy available, leading to nuclear having a much higher [[energy density]] for its fuel.


<center><math>Uranium-235 + ^1_0n \rightarrow \sim 3\;^1_0n + </math> energy </center>
In order to sustain a nuclear chain reaction, every fission event must lead to exactly one more fission event. The most convenient nuclear species to use for nuclear chain reactions is a [[fissile]] [[isotope]] of uranium, <sup>235</sup>U. When <sup>235</sup>U undergoes fission, it gives off, on average, ~2.5 neutrons per fission event. Careful engineering must go into having those neutrons go on to create more fission events. Contrary to what one may expect, difficulties arise in getting enough neutrons to go on and make a sustained nuclear reaction, rather than having too many nuclear reactions. If every fission event leads to ''exactly'' one more fission event, the nuclear chain reaction is said to be '''critical'''. Figure 2 shows a simplification of the fission chain reaction.


If for every one neutron inputted into the equation releases two or three more neutrons, then the number of fission events increases dramatically each generation. However, in reality not all of the released neutrons actually ''cause'' more fissions. Only 1.1 neutron per reaction actually goes on to cause more fissions and continue the chain, however the number of fission events still grows quickly. The process of a nuclear chain reaction releases large amounts of energy, but this energy can be utilized in different ways. On average, there is about 200 [[MeV]] of energy released during fission.<ref name="RE1"/> To put this into context, burning coal provides only a couple eV, while 200 MeV is equal to 200 ''million'' electron volts.<ref>UC Davis' Chem Wiki. (July 9, 2015). ''Fission and Fusion'' [Online]. Available: http://chemwiki.ucdavis.edu/Physical_Chemistry/Nuclear_Chemistry/Fission_and_Fusion</ref> The difference in these energies is enormous. In [[nuclear reactor]]s, the reaction is moderated and progresses at a slow pace to release its energy over a period of time so it can be harnessed and used for peaceful purposes. An atomic bomb utilizes this fission chain reaction as well, however it is designed to release its energy all at once—which is much more damaging. In either case, the release of the energy is controlled, but the time period taken to release the energy differs.<ref name="RE1">Atomic Archive. (June 16, 2015). ''Nuclear Chain Reactions'' [Online]. Available: http://www.atomicarchive.com/Fission/Fission2.shtml</ref>
[[File:u235fission.gif|600px|framed|center|Figure 2. A nuclear fission chain reaction of uranium-235 atoms.<ref name=hyp>HyperPhysics. (May 27, 2015). ''Uranium-235 Chain Reaction'' [Online]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html</ref> In a real nuclear reactor, most of the released neutrons are lost, rather than leading to another fission event.]]


For more information on nuclear chain reactions see HyperPhysics [http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html here].
The video below has a member of the [[About us|Energy Education team]] explaining nuclear chain reactions:
 
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==For Further Reading==
*[[Moderator]]
*[[Isotope]]
*[[Nuclear power plant]]
*[[Nuclear species]]
*[[Strong force]]
*Or explore a [[Special:Random|random page]]


==References==
==References==
{{reflist}}
{{reflist}}
[[Category:Uploaded]]
[[Category:Uploaded]]

Latest revision as of 20:21, 20 December 2021

Figure 1. A neutron strikes a 235U nucleus and causes a fission event. This releases more neutrons. Unlike in the figure, on average one new fission event happens as a result of these released neutrons.

A nuclear chain reaction occurs when the output of one nuclear reaction causes more nuclear reactions to occur. These chain reactions are almost always a series of fission events, which give off excess neutrons. It is these excess neutrons that can go on to cause more fission events to occur, hence the name chain reaction. Nuclear chain reactions are essential to the operation of nuclear power plants.

Chemical reactions involve different chemical species recombining. Nuclear reactions involve different flavours of nuclei (called nuclear species) interacting. Many chemical reactions are also chain reactions, with many similarities to nuclear chain reactions. These similarities include:

  • That the reactions are sustained when chemical or nuclear species available to react. The chain reaction stops when the species are removed or are used up.
  • That the chain reactions are controlled (starting, speeding up, slowing down and stopping) by adding or removing chemical or nuclear species in that chain.
  • Energy is often released as the reactions occur.
  • Released energy is often output as thermal energy, becoming heat that can be harnessed by heat engines to do useful work like make electricity.

While these similarities exist, there are some important differences as well. Nuclear reactions release roughly one million times as much energy as chemical reactions. This means that chemical chain reactions occur much more easily than nuclear reactions. For example, fire is a chemical chain reaction. Nuclear chain reactions require careful engineering and as far as we know, a natural nuclear chain reaction has only occurred once.[1] Nuclear chain reactions require an abundance of careful planning. When they do occur, there is substantially more energy available, leading to nuclear having a much higher energy density for its fuel.

In order to sustain a nuclear chain reaction, every fission event must lead to exactly one more fission event. The most convenient nuclear species to use for nuclear chain reactions is a fissile isotope of uranium, 235U. When 235U undergoes fission, it gives off, on average, ~2.5 neutrons per fission event. Careful engineering must go into having those neutrons go on to create more fission events. Contrary to what one may expect, difficulties arise in getting enough neutrons to go on and make a sustained nuclear reaction, rather than having too many nuclear reactions. If every fission event leads to exactly one more fission event, the nuclear chain reaction is said to be critical. Figure 2 shows a simplification of the fission chain reaction.

Figure 2. A nuclear fission chain reaction of uranium-235 atoms.[2] In a real nuclear reactor, most of the released neutrons are lost, rather than leading to another fission event.

The video below has a member of the Energy Education team explaining nuclear chain reactions:

For Further Reading

References

  1. ↑ As far as we know! Although it seems unlikely to have occurred more than once.
  2. ↑ HyperPhysics. (May 27, 2015). Uranium-235 Chain Reaction [Online]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html
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