Editing Nuclear weapon (section)

=== Fission weapons ===
[[File:Fission bomb assembly methods.svg|upright=1.4|thumb|The two basic [[Nuclear fission|fission]] weapon designs]]
All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as '''atomic bombs''' or '''atom bombs''' (abbreviated as '''A-bombs'''). This has long been noted as something of a [[misnomer]], as their energy comes from the [[Atomic nucleus|nucleus]] of the atom, just as it does with fusion weapons.

In fission weapons, a mass of [[fissile material]] ([[enriched uranium]] or [[plutonium]]) is forced into [[critical mass|supercriticality]]—allowing an [[exponential growth]] of [[nuclear chain reaction]]s—either by shooting one piece of sub-critical material into another (the "gun" method) or by compression of a sub-critical sphere or cylinder of fissile material using chemically fueled [[explosive lens]]es. The latter approach, the "implosion" method, is more sophisticated and more efficient (smaller, less massive, and requiring less of the expensive fissile fuel) than the former.

A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons (500 [[kiloton]]s) of [[trinitrotoluene|TNT]] ({{convert|1|to|5E5|tTNT|sigfig=2|disp=out}}).<ref name="Hansen">Hansen, Chuck. ''U.S. Nuclear Weapons: The Secret History.'' San Antonio, TX: Aerofax, 1988; and the more-updated Hansen, Chuck, "[http://www.uscoldwar.com/ Swords of Armageddon: U.S. Nuclear Weapons Development since 1945] {{webarchive |url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |date=December 30, 2016}}" (CD-ROM & download available). PDF. 2,600 pages, Sunnyvale, California, Chuklea Publications, 1995, 2007. {{ISBN|978-0-9791915-0-3}} (2nd Ed.)</ref>

All fission reactions generate [[Nuclear fission product|fission products]], the remains of the split atomic nuclei. Many fission products are either highly [[Radioactive decay|radioactive]] (but short-lived) or moderately radioactive (but long-lived), and as such, they are a serious form of [[radioactive contamination]]. Fission products are the principal radioactive component of [[nuclear fallout]]. Another source of radioactivity is the burst of free neutrons produced by the weapon. When they collide with other nuclei in the surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive.

The most commonly used fissile materials for nuclear weapons applications have been [[uranium-235]] and [[plutonium-239]]. Less commonly used has been [[uranium-233]]. [[Neptunium-237]] and some isotopes of [[americium]] may be usable for nuclear explosives as well, but it is not clear that this has ever been implemented, and their plausible use in nuclear weapons is a matter of dispute.<ref>{{cite web |last1=Albright |first1=David |author-link=David Albright |last2=Kramer |first2=Kimberly |date=August 22, 2005 |title=Neptunium 237 and Americium: World Inventories and Proliferation Concerns |url=http://isis-online.org/uploads/isis-reports/documents/np_237_and_americium.pdf |publisher=[[Institute for Science and International Security]] |access-date=October 13, 2011 |url-status=live |archive-url=https://web.archive.org/web/20120103234833/http://isis-online.org/uploads/isis-reports/documents/np_237_and_americium.pdf |archive-date=January 3, 2012}}</ref>