Editing Fissile material (section)

== 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.