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{{short description|Material capable of sustaining a nuclear fission chain reaction}}
{{Redirect|Fissility|the topic in geology|Fissility (geology)}}
[[File:HEUraniumC.jpg|thumb|[[Bar stock|Billet]] of [[enriched uranium]], a fissile material]]
In [[nuclear engineering]], '''fissile material''' is material that can undergo [[nuclear fission]] when struck by a [[neutron]] of low energy.<ref name="fissile">{{cite web |title=NRC: Glossary -- Fissile material |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html |website=www.nrc.gov}}</ref> A self-sustaining thermal [[Nuclear chain reaction#Fission chain reaction|chain reaction]] can only be achieved with fissile material. The predominant [[Neutron temperature|neutron energy]] in a system may be typified by either slow neutrons (i.e.,&nbsp;a thermal system) or fast neutrons. Fissile material can be used to fuel [[thermal-neutron reactor]]s, [[fast-neutron reactor]]s and [[nuclear explosive]]s.

== Fissile vs fissionable ==

{{Ra to Es by HL}}

The term ''fissile'' is distinct from ''fissionable''. A [[nuclide]] that can undergo [[nuclear fission]] (even with a low probability) after capturing a neutron of high or low energy<ref name="fissionable">{{cite web |title=NRC: Glossary -- Fissionable material |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/fissionable-material.html |website=www.nrc.gov}}</ref> is referred to as ''fissionable''. A fissionable nuclide that can undergo fission with a high probability after capturing a low-energy [[thermal neutron]] is referred to as ''fissile''.<ref name="UNENE">{{cite web|title=Slides-Part one: Kinetics|url=https://unene.ca/education/courses/un-0802-nuclear-reactor-analysis|publisher=UNENE University Network of Excellence in Nuclear Engineering|access-date=3 January 2013}}</ref>  Fissionable materials include those (such as [[uranium-238]]) for which fission can be induced only by high-energy neutrons. As a result, fissile materials (such as [[uranium-235]]) are a [[subset]] of fissionable materials.

Uranium-235 fissions with low-energy thermal neutrons because the [[Nuclear binding energy|binding energy]] resulting from the absorption of a neutron is greater than the threshold required for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is fissionable but not fissile.<ref>{{cite book|title=Nuclear Reactor Analysis |author=James J. Duderstadt and Louis J. Hamilton |publisher=John Wiley & Sons, Inc.|year=1976|isbn=0-471-22363-8}}</ref><ref>{{cite book|title=Introduction to Nuclear Engineering |author=John R. Lamarsh and Anthony John Baratta (Third Edition) |publisher=Prentice Hall|year=2001|isbn=0-201-82498-1}}</ref>

An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e.,&nbsp;are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting.  Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a [[nuclear chain reaction]].  As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile.  In the [[arms control]] context, particularly in proposals for a [[Fissile Material Cutoff Treaty]], the term ''fissile'' is often used to describe materials that can be used in the fission primary of a nuclear weapon.<ref>[http://www.fissilematerials.org/ipfm/pages_us_en/fissile/fissile/fissile.php Fissile Materials and Nuclear Weapons] {{Webarchive|url=https://web.archive.org/web/20120206004707/http://www.fissilematerials.org/ipfm/pages_us_en/fissile/fissile/fissile.php |date=2012-02-06 }}, International Panel on Fissile Materials</ref>  These are materials that sustain an explosive [[Neutron temperature#Fast|fast neutron]] [[nuclear fission]] [[nuclear chain reaction|chain reaction]].

Under all definitions above, uranium-238 ({{SimpleNuclide|uranium|238}}) is fissionable, but not fissile. Neutrons produced by fission of {{SimpleNuclide|uranium|238}} have lower [[kinetic energy|energies]] than the original neutron (they behave as in an [[inelastic scattering]]), usually below 1&nbsp;[[MeV]] (i.e.,&nbsp;a speed of about 14,000&nbsp;[[km/s]]), the fission threshold to cause subsequent fission of {{SimpleNuclide|uranium|238}}, so fission of {{SimpleNuclide|uranium|238}} does not sustain a [[nuclear chain reaction]].

Fast fission of {{SimpleNuclide|uranium|238}} in the secondary stage of a thermonuclear weapon, due to the production of high-energy neutrons from [[nuclear fusion]], contributes greatly to the [[nuclear weapon yield|yield]] and to [[nuclear fallout|fallout]] of such weapons. Fast fission of {{SimpleNuclide|uranium|238}} tampers has also been evident in pure fission weapons.<ref>{{cite book |last1=Semkow |first1=Thomas |last2=Parekh |first2=Pravin |last3=Haines |first3=Douglas  |title=Applied Modeling and Computations in Nuclear Science |chapter=Modeling the Effects of the Trinity Test |series=ACS Symposium Series |date=2006 |volume=ACS Symposium Series |pages=142–159 |doi=10.1021/bk-2007-0945.ch011 |isbn=9780841239821 |url=https://pubs.acs.org/doi/abs/10.1021/bk-2007-0945.ch011}}</ref> The fast fission of {{SimpleNuclide|uranium|238}} also makes a significant contribution to the power output of some [[fast-neutron reactor]]s.

== Fissile nuclides ==
{{Actinides vs fission products}}

In general, most [[actinide]] isotopes with an odd [[neutron number]] are fissile. Most nuclear fuels have an odd [[atomic mass number]] ({{nowrap|1=''A'' = ''Z'' + ''N''}} = the total number of [[nucleons]]), and an even [[atomic number]] ''Z''. This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2&nbsp;MeV of energy from absorbing an extra neutron, from the [[semi-empirical mass formula#Pairing term|pairing effect]] which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.

More generally, nuclides with an even number of protons and an even number of neutrons, and located near a [[Beta-decay stable isobars|well-known curve]] in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore"<!--[[scare quotes]]--> the neutron and let it go on its way, or else to [[neutron capture|absorb the neutron]] but without gaining enough energy from the process to deform the nucleus enough for it to fission. These [[Even and odd atomic nuclei|"even-even"]]<!--[[scare quotes]]--> isotopes are also less likely to undergo [[spontaneous fission]], and they also have relatively much longer [[partial half-lives]] for [[alpha decay|alpha]] or [[beta decay|beta]] decay. Examples of these isotopes are uranium-238 and [[thorium-232]]. On the other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons  (odd ''Z'', odd ''N'') are usually short-lived (a notable exception is [[neptunium-236]] with a half-life of 154,000 years) because they readily [[beta decay|decay by beta-particle emission]] to their [[isobar (nuclide)|isobars]] with an even number of protons and an even number of neutrons (even ''Z'', even ''N'') becoming much more stable.  The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.

According to the fissility rule proposed by Yigal Ronen, for a heavy element with ''Z'' between 90 and 100, an isotope is fissile if and only if {{nowrap|1=2 × ''Z'' − ''N'' ∈ {41, 43, 45}}} (where ''N'' = [[Neutron number|number of neutrons]] and ''Z'' = [[Atomic number|number of protons]]), with a few exceptions.<ref>Ronen Y., 2006. A rule for determining fissile isotopes. ''Nucl. Sci. Eng.'', 152:3, pages 334-335. [http://www.ans.org/pubs/journals/nse/a_2588]</ref><ref>{{Cite journal | last1 = Ronen | first1 = Y. | title = Some remarks on the fissile isotopes | doi = 10.1016/j.anucene.2010.07.006 | journal = Annals of Nuclear Energy | volume = 37 | issue = 12 | pages = 1783–1784 | year = 2010 | bibcode = 2010AnNuE..37.1783R }}</ref> This rule holds for all but fourteen nuclides – seven that satisfy the criterion but are nonfissile, and seven that are fissile but do not satisfy the criterion.<ref group="note">The fissile rule thus formulated indicates 33 isotopes as likely fissile: Th-225, 227, 229; Pa-228, 230, 232; U-231, 233, 235; Np-234, 236, 238; Pu-237, 239, 241; Am-240, 242, 244; Cm-243, 245, 247; Bk-246, 248, 250; Cf-249, 251, 253; Es-252, 254, 256; Fm-255, 257, 259. Only fourteen (including a long-lived [[metastable]] [[nuclear isomer]]) have half-lives of at least a year: Th-229, U-233, U-235, Np-236, Pu-239, Pu-241, Am-242m, Cm-243, Cm-245, Cm-247, Bk-248, Cf-249, Cf-251 and Es-252. Of these, only U-235 is [[naturally occurring radioactive material|naturally occurring]]. It is possible to [[breeder reactor|breed]] U-233 and Pu-239 from more common naturally occurring isotopes (Th-232 and U-238 respectively) by single [[neutron capture]]. The others are typically produced in smaller quantities through [[:File:Sasahara.svg|further neutron absorption]].</ref>

== Nuclear fuel ==
{{Mainarticle|Nuclear fuel}}
To be a useful fuel for nuclear fission chain reactions, the material must:

* Be in the region of the [[binding energy]] curve where a fission chain reaction is possible (i.e.,&nbsp;above [[radium]])
* Have a high probability of fission on [[neutron capture]]
* Release more than one neutron on average per neutron capture. (Enough of them on each fission, to compensate for non-fissions and absorptions in non-fuel material)
* Have a reasonably long [[half-life]]
* Be available in suitable quantities.

{| class="wikitable" align="right" id="Capture-fission-ratios"
|+Capture-fission ratios of fissile nuclides
!colspan=3|Thermal neutrons<ref>{{cite web |url=http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |title=Interactive Chart of Nuclides |publisher=Brookhaven National Laboratory |access-date=2013-08-12 |archive-date=2017-01-24 |archive-url=https://web.archive.org/web/20170124175936/http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |url-status=dead }}</ref>!! !!colspan=3|Epithermal neutrons
|-
!σ<sub>F</sub> (b)!!σ<sub>γ</sub> (b)!!%!! !!σ<sub>F</sub> (b)!!σ<sub>γ</sub> (b)!!%
|-
|531||46||8.0%||<sup>233</sup>U||760||140||16%
|-
|585||99||14.5%||<sup>235</sup>U||275||140||34%
|-
|750||271||26.5%||<sup>239</sup>Pu||300||200||40%
|-
|1010||361||26.3%||<sup>241</sup>Pu||570||160||22%
|}

Fissile [[nuclide]]s in nuclear fuels include:

* [[Uranium-233]], bred from [[thorium-232]] by neutron capture with intermediate decays steps omitted.
* [[Uranium-235]], which occurs in [[natural uranium]] and [[enriched uranium]]
* [[Plutonium-239]], bred from uranium-238 by neutron capture with intermediate decays steps omitted.
* [[Plutonium-241]], bred from plutonium-240 directly by neutron capture.
Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture [[Neutron cross-section|cross section]]s for fission (σ<sub>F</sub>), the cross section for neutron capture with emission of a [[gamma ray]] (σ<sub>γ</sub>), and the percentage of non-fissions are in the table at right.

Fertile nuclides in nuclear fuels include:

* [[Thorium-232]], which breeds uranium-233 by neutron capture with intermediate decays steps omitted.
* [[Uranium-238]], which breeds plutonium-239 by neutron capture with intermediate decays steps omitted.
* [[Plutonium-240]], which breeds plutonium-241 directly by neutron capture.

== See also ==
* [[Fertile material]]
* [[Fission product]]
* [[Special nuclear material]]

== Notes ==
{{reflist|group="note"}}

== References ==
{{reflist}}

{{Nuclear technology}}

[[Category:Fissile materials| ]]
[[Category:Nuclear physics]]
[[Category:Nuclear fission]]
[[Category:Nuclear weapon design]]