Editing Nuclear weapon design (section)

==Specific designs==
While every nuclear weapon design falls into one of these categories, specific designs have occasionally become the subject of news accounts and public discussion, often with incorrect descriptions about how they work and what they do. Examples:

==={{Anchor|Alarm Clock}}Alarm Clock/Sloika===
[[File:Castle Union.gif|thumb|Castle-''Union'', 6.9&nbsp;megatons.]]
The first effort to exploit the symbiotic relationship between fission and fusion was a 1940s design that mixed fission and fusion fuel in alternating thin layers. As a single-stage device, it would have been a cumbersome application of boosted fission. It first became practical when incorporated into the secondary of a two-stage thermonuclear weapon.<ref>"The 'Alarm Clock' ... became practical only by the inclusion of Li6 (in 1950) and its combination with the radiation implosion." Hans A. Bethe, [https://fas.org/nuke/guide/usa/nuclear/bethe-52.htm Memorandum on the History of Thermonuclear Program] {{webarchive |url=https://web.archive.org/web/20160304030002/https://fas.org/nuke/guide/usa/nuclear/bethe-52.htm |date=March 4, 2016}}, May 28, 1952.</ref>

The U.S. name, Alarm Clock, came from Teller: he called it that because it might "wake up the world" to the possibility of the potential of the Super.{{sfn|Rhodes|1995|p=256}} The Russian name for the same design was more descriptive: Sloika ({{langx|ru|Слойка}}), a layered pastry cake. A single-stage Soviet Sloika was tested as [[RDS-6s]] on August 12, 1953. No single-stage U.S. version was tested, but the code named [[Castle Union]] shot of [[Operation Castle]], April 26, 1954, was a two-stage thermonuclear device code-named Alarm Clock. Its yield, at [[Bikini Atoll|Bikini]], was 6.9 megatons.{{Citation needed|date=June 2021}}

Because the Soviet Sloika test used dry lithium-6 deuteride eight months before the first U.S. test to use it (Castle Bravo, March 1, 1954), it was sometimes claimed that the USSR won the H-bomb race, even though the United States tested and developed the first hydrogen bomb: the Ivy Mike H-bomb test. The 1952 U.S. Ivy Mike test used cryogenically cooled liquid deuterium as the fusion fuel in the secondary, and employed the D-D fusion reaction. However, the first Soviet test to use a radiation-imploded secondary, the essential feature of a true H-bomb, was on November 23, 1955, three years after Ivy Mike. In fact, real work on the implosion scheme in the Soviet Union only commenced in the very early part of 1953, several months after the successful testing of Sloika.{{Citation needed|date=June 2021}}

===Clean bombs {{Anchor|Clean bombs}}===

==== Designs with lead tampers ====
[[File:Bassoon Prime.jpg|right|thumb|upright|Bassoon, the prototype for a 9.3-megaton clean bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956 test. The two attachments on the left are ''[[#Light pipes|light pipes]]''; see below for elaboration.]]
On March 1, 1954, the largest-ever U.S. nuclear test explosion, the 15-megaton [[Castle Bravo]] shot of [[Operation Castle]] at Bikini Atoll, delivered a promptly lethal dose of fission-product fallout to more than {{convert|6000|sqmi|km2}} of Pacific Ocean surface.<ref>See [[:File:Bravo fallout2.png|map]].</ref> Radiation injuries to [[Castle Bravo#Inhabited islands affected|Marshall Islanders]] and [[Daigo Fukuryū Maru|Japanese fishermen]] made that fact public and revealed the role of fission in hydrogen bombs.

In response to the public alarm over fallout, an effort was made to design a clean multi-megaton weapon, relying almost entirely on fusion. The energy produced by the fissioning of [[uranium-238|unenriched natural uranium]], when used as the tamper material in the secondary and subsequent stages in the Teller-Ulam design, can far exceed the energy released by fusion, as was the case in the Castle Bravo test. Replacing the [[fissionable]] material in the tamper with another [[Atomic number|high-Z]] material ([[lead]]) is essential to producing a "clean" bomb. In such a device, the tamper no longer contributes energy, so for any given weight, a clean bomb will have less yield. This was called the "materials substitution method".<ref name="q320" /> The earliest known incidence of a three-stage device being tested, with the third stage, called the tertiary, being ignited by the secondary, was May 27, 1956, in the Bassoon device. This device was tested in the Zuni shot of [[Operation Redwing]]. This shot used non-fissionable tampers; an inert substitute material such as tungsten or lead was used. Its yield was 3.5 megatons, 85% fusion and only 15% fission.{{Citation needed|date=June 2021}}

On July 19, 1956, AEC Chairman Lewis Strauss said that the [[Operation Redwing|Redwing Zuni]] shot clean bomb test "produced much of importance ... from a humanitarian aspect." However, less than two days after this announcement, the dirty version of Bassoon, called Bassoon Prime, with a [[uranium-238]] tamper in place, was tested on a barge off the coast of Bikini Atoll as the [[Operation Redwing|Redwing Tewa]] shot. The Bassoon Prime produced a 5-megaton yield, of which 87% came from fission. Data obtained from this test, and others, culminated in the eventual deployment of the highest-yielding US nuclear weapon known, and the highest [[nuclear weapon yield|yield-to-weight weapon]] ever mass produced, a three-stage thermonuclear weapon with a maximum "dirty" yield of 25 megatons, designated as the [[B41 nuclear bomb]], which was to be carried by U.S. Air Force bombers until it was decommissioned; this weapon was never fully tested.{{Citation needed|date=June 2021|reason=also relevancy}}

In the Soviet [[peaceful nuclear explosion]] program "Nuclear Explosions for the National Economy", "clean" bombs were used for a 1971 triple salvo test related to the [[Pechora–Kama Canal]] project. It was reported that about 250 nuclear devices might be used to get the final goal. The ''Taiga'' test was to demonstrate the feasibility of the project. Three of these devices of 15&nbsp;kiloton yield each were placed in separate boreholes, simultaneously detonated, catapulting a radioactive plume into the air that was carried eastward by wind. The resulting trench was around {{convert|700|m|ft}} long and {{convert|340|m|ft}} wide, with an unimpressive depth of just {{convert|10|to|15|m|ft|sigfig=1}}.<ref>{{cite journal |last1=Ramzaev |first1=V. |last2=Repin |first2=V. |last3=Medvedev |first3=A. |last4=Khramtsov |first4=E. |last5=Timofeeva |first5=M. |last6=Yakovlev |first6=V. |date=July 2011 |title=Radiological investigations at the "Taiga" nuclear explosion site: Site description and in situ measurements |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X11000750 |journal=Journal of Environmental Radioactivity |language=en |volume=102 |issue=7 |pages=672–680 |bibcode=2011JEnvR.102..672R |doi=10.1016/j.jenvrad.2011.04.003 |pmid=21524834}}</ref> Despite their "clean" nature, the area still exhibits a noticeably higher (albeit mostly harmless) concentration of [[fission products]], the intense [[Neutron irradiation|neutron bombardment]] of the soil, the device itself and the support structures also activated their stable elements to create a significant amount of man-made radioactive elements like [[60Co|<sup>60</sup>Co]]. A larger scale project as was envisioned, however, would have had significant consequences both from the fallout of radioactive plume and the radioactive elements created by the neutron bombardment.<ref>{{cite journal |last1=Ramzaev |first1=V. |last2=Repin |first2=V. |last3=Medvedev |first3=A. |last4=Khramtsov |first4=E. |last5=Timofeeva |first5=M. |last6=Yakovlev |first6=V. |date=July 2012 |title=Radiological investigations at the "Taiga" nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X11003043 |journal=Journal of Environmental Radioactivity |language=en |volume=109 |pages=1–12 |bibcode=2012JEnvR.109....1R |doi=10.1016/j.jenvrad.2011.12.009 |pmid=22541991}}</ref>

Other high fusion yield fraction tests include the 50-megaton [[Tsar Bomba]] at 97% fusion,<ref>[https://nuclearweaponarchive.org/Nwfaq/Nfaq4-5.html 4.5 Thermonuclear Weapon Designs and Later Subsections] {{webarchive|url=https://web.archive.org/web/20160303170957/https://nuclearweaponarchive.org/Nwfaq/Nfaq4-5.html|date=March 3, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref> the 9.3-megaton [[Operation Hardtack I|Hardtack Poplar]] test at 95%,<ref>[https://nuclearweaponarchive.org/Usa/Tests/Hardtack1.html Operation Hardtack I] {{webarchive|url=https://web.archive.org/web/20160910232153/https://nuclearweaponarchive.org/Usa/Tests/Hardtack1.html|date=September 10, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref> and the 4.5-megaton [[Operation Redwing|Redwing Navajo]] test at 95% fusion.<ref>[https://nuclearweaponarchive.org/Usa/Tests/Redwing.html Operation Redwing] {{webarchive|url=https://web.archive.org/web/20160910232205/https://nuclearweaponarchive.org/Usa/Tests/Redwing.html|date=September 10, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref>

==== Designs with no tampers ====
[[File:DominicHousatonic.gif|thumb|[[Operation Dominic]] shot Housatonic, the cleanest and highest yield-to-weight ratio test ever, testing the Ripple design.]]
The Ripple concept, which used ablation to achieve fusion using very little fission, was and still is by far the cleanest design. Unlike previous clean bombs, which were clean simply by replacing the uranium-238 tamper with lead, Ripple was inherently clean. The fission sparkplug was replaced by a large deuterium-tritium gas core, surrounded by a tamper-like lithium deuteride shell. It is assumed that thin concentric shells of a high-Z material like lead, driven by the small [[Kinglet (nuclear primary)|Kinglet primary]] allowed propagated sustained shockwaves to the core, sustaining the thermonuclear burn and giving the device its name. The design was influenced by the nascent field of [[inertial confinement fusion]]. Ripple was also extremely efficient; plans for a 15 kt/kg were made during [[Operation Dominic]]. Shot Androscoggin featured a proof-of-concept Ripple design, resulting in a 63-kiloton fizzle (significantly lower than the predicted 15 megatons). It was repeated in shot Housatonic, which featured a 9.96 megaton explosion that was reportedly >99.9% fusion.<ref name="q320">{{cite journal |last=Grams |first=Jon |date=2021-06-06 |title=Ripple: An Investigation of the World's Most Advanced High-Yield Thermonuclear Weapon Design |url=https://muse.jhu.edu/article/794729/pdf |journal=Journal of Cold War Studies |publisher=The MIT Press |volume=23 |issue=2 |pages=133–161 |issn=1531-3298 |access-date=2025-04-07}}</ref>

===Third generation===
First and second generation nuclear weapons release energy as omnidirectional blasts. Third generation<ref>{{cite book |title=The Role and Control of Weapons in the 1990s |last1=Barnaby |first1=Frank |date=2012 |publisher=Routledge |isbn=978-1134901913 |url=https://books.google.com/books?id=H8wwRGrD6V4C&q=third+generation+nuclear+weapons+project+excalibur+prometheus&pg=PT148 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=H8wwRGrD6V4C&q=third+generation+nuclear+weapons+project+excalibur+prometheus&pg=PT148 |archive-date=2021-09-04}}</ref><ref>{{cite web |title=Bulletin of the Atomic Scientists |publisher=Educational Foundation for Nuclear Science, Inc |date=March 1991 |url=https://books.google.com/books?id=rwwAAAAAMBAJ&q=shaped+nuclear+charge+third+generation+nuclear+weapons&pg=PA31 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=rwwAAAAAMBAJ&q=shaped+nuclear+charge+third+generation+nuclear+weapons&pg=PA31 |archive-date=2021-09-04}}</ref><ref>{{cite book |title=SDI: Technology, survivability, and software |publisher=DIANE |isbn=978-1428922679 |url=https://books.google.com/books?id=XDTo_35uQcUC&q=sdi+nuclear+shotgun&pg=PA122 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=XDTo_35uQcUC&q=sdi+nuclear+shotgun&pg=PA122 |archive-date=2021-09-04}}</ref> nuclear weapons are experimental special effect warheads and devices that can release energy in a directed manner, some of which were tested during the [[Cold War]] but were never deployed. These include:
* Project Prometheus, also known as "Nuclear Shotgun", which would have used a nuclear explosion to accelerate kinetic penetrators against ICBMs.<ref>{{cite book |title=The Role and Control of Weapons in the 1990s |isbn=978-1134901913 |last1=Barnaby |first1=Frank |date=2012 |publisher=Routledge |url=https://books.google.com/books?id=H8wwRGrD6V4C&q=prometheus+nuclear+shotgun&pg=PT148 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154854/https://books.google.com/books?id=H8wwRGrD6V4C&q=prometheus+nuclear+shotgun&pg=PT148 |archive-date=2021-09-04}}</ref>
* [[Project Excalibur]], a nuclear-pumped X-ray laser to [[ballistic missile defense|destroy ballistic missiles]].
* [[Nuclear shaped charge]]s that focus their energy in particular directions.
* [[Project Orion (nuclear propulsion)|Project Orion]] explored the use of nuclear explosives for rocket propulsion.

===Fourth generation===
The idea of "4th-generation" nuclear weapons has been proposed as a possible successor to the examples of weapons designs listed above. These methods tend to revolve around using non-nuclear primaries to set off further fission or fusion reactions. For example, if [[antimatter]] were usable and controllable in macroscopic quantities, a reaction between a small amount of antimatter and an equivalent amount of matter could release energy comparable to a small fission weapon, and could in turn be used as the first stage of a very compact thermonuclear weapon. Extremely-powerful  lasers could also potentially be used this way, if they could be made powerful-enough, and compact-enough, to be viable as a weapon. Most of these ideas are versions of [[pure fusion weapon]]s, and share the common property that they involve hitherto unrealized technologies as their "primary" stages.<ref>{{cite arXiv |eprint=physics/0510071 |last1=Gsponer |first1=Andre |title=Fourth Generation Nuclear Weapons: Military effectiveness and collateral effects |year=2005}}</ref>

While many nations have invested significantly in [[inertial confinement fusion]] research programs, since the 1970s it has not been considered promising for direct weapons use, but rather as a tool for weapons- and energy-related research that can be used in the absence of full-scale nuclear testing. Whether any nations are aggressively pursuing "4th-generation" weapons is not clear. In many case (as with antimatter) the underlying technology is presently thought to be very far from being viable, and if it was viable would be a powerful weapon in and of itself, outside of a nuclear weapons context, and without providing any significant advantages above existing nuclear weapons designs<ref>[http://whyfiles.org/167new_nukes/4.html Never say "never"] {{webarchive |url=https://web.archive.org/web/20160418234450/http://whyfiles.org/167new_nukes/4.html |date=April 18, 2016}}. Whyfiles.org. Retrieved on 2011-05-01.</ref>

===Pure fusion weapons===
{{Main|Pure fusion weapon}}

Since the 1950s, the United States and Soviet Union investigated the possibility of releasing significant amounts of nuclear fusion energy without the use of a fission primary. Such "pure fusion weapons" were primarily imagined as low-yield, tactical nuclear weapons whose advantage would be their ability to be used without producing fallout on the scale of weapons that release fission products. In 1998, the [[United States Department of Energy]] declassified the following:
{{blockquote|
(1) Fact that the DOE made a substantial investment in the past to develop a pure fusion weapon

(2)	That the U.S. does not have and is not developing a pure fusion weapon; and
(3)	That no credible design for a pure fusion weapon resulted from the DOE investment.<ref>{{cite web|title=Restricted Data Declassification Decisions, 1946 to the Present (RDD-7)|url=https://sgp.fas.org/othergov/doe/rdd-7.html|date=1 January 2001}}</ref>}}

[[Red mercury]], a likely hoax substance, has been hyped as a catalyst for a pure fusion weapon.{{Citation needed|date=April 2024}}

===Cobalt bombs===
{{Main|Cobalt bomb}} {{See also|Salted bomb}}
A doomsday bomb, made popular by [[Nevil Shute]]'s 1957 [[On the Beach (novel)|novel]], and subsequent 1959 movie, ''[[On the Beach (1959 film)|On the Beach]]'', the cobalt bomb is a hydrogen bomb with a jacket of cobalt. The neutron-activated cobalt would have maximized the environmental damage from radioactive fallout. These bombs were popularized in the 1964 film ''[[Dr. Strangelove|Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb]]''; the material added to the bombs is referred to in the film as 'cobalt-thorium G'.{{Citation needed|date=June 2021}}

Such "salted" weapons were investigated by U.S. Department of Defense.<ref>{{cite book |first1=Samuel |last1=Glasstone |title=The Effects of Nuclear Weapons |year=1962 |publisher=U.S. Department of Defense, U.S. Atomic Energy Commission |pages=464–466 |url=https://books.google.com/books?id=Ovu108BraNUC}}</ref> Fission products are as deadly as neutron-activated cobalt.

Initially, gamma radiation from the fission products of an equivalent size fission-fusion-fission bomb are much more intense than [[Cobalt-60]] ({{SimpleNuclide|cobalt|60}}): 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that {{SimpleNuclide|cobalt|60}} fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long-lived isotopes produced by fission would overtake the {{SimpleNuclide|cobalt|60}} again after about 75 years.<ref name="Nuclear Weapons FAQ: 1.6"/>

The triple "taiga" nuclear [[salvo]] test, as part of the preliminary March 1971 [[Pechora–Kama Canal]] project, produced a small amount of fission products and therefore a comparatively large amount of case material activated products are responsible for most of the residual activity at the site today, namely {{SimpleNuclide|cobalt|60}}. {{As of|2011|post=,}} [[neutron activation|fusion generated neutron activation]] was responsible for about half of the gamma dose at the test site. That dose is too small to cause deleterious effects, and normal green vegetation exists all around the lake that was formed.<ref>{{cite journal |title=Radiological investigations at the 'Taiga' nuclear explosion site: Site description and in situ measurements |pmid=21524834 |volume=102 |issue=7 |journal=Journal of Environmental Radioactivity |pages=672–680 |year=2011 |last1=Ramzaev |first1=V |last2=Repin |first2=V |last3=Medvedev |first3=A |last4=Khramtsov |first4=E |last5=Timofeeva |first5=M |last6=Yakovlev |first6=V |doi=10.1016/j.jenvrad.2011.04.003|bibcode=2011JEnvR.102..672R }}</ref><ref>{{cite journal |title=Radiological investigations at the 'Taiga' nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air |pmid=22541991 |volume=109 |journal=Journal of Environmental Radioactivity |pages=1–12 |year=2012 |last1=Ramzaev |first1=V |last2=Repin |first2=V |last3=Medvedev |first3=A |last4=Khramtsov |first4=E |last5=Timofeeva |first5=M |last6=Yakovlev |first6=V |doi=10.1016/j.jenvrad.2011.12.009|bibcode=2012JEnvR.109....1R }}</ref>

===Arbitrarily large multi-staged devices===
The idea of a device which has an arbitrarily large number of Teller-Ulam stages, with each driving a larger radiation-driven implosion than the preceding stage, is frequently suggested,<ref>{{cite book
|last1=Winterberg
|first1=Friedwardt
|title=The Release of Thermonuclear Energy by Inertial Confinement: Ways Towards Ignition
|publisher=World Scientific
|date=2010
|pages=192–193
|isbn=978-9814295918
|url=https://books.google.com/books?id=B7RV_vASz0EC&q=arbitrarily+large+gains%22staged+Teller-Ulam&pg=PA192
|access-date=2020-11-02
|url-status=live
|archive-url=https://web.archive.org/web/20210805053441/https://books.google.com/books?id=B7RV_vASz0EC&q=arbitrarily+large+gains%22staged+Teller-Ulam&pg=PA192
|archive-date=2021-08-05
}}</ref><ref>{{cite book
|last1=Croddy
|first1=Eric A.
|last2=Wirtz
|first2=James J.
|last3=Larsen
|first3=Jeffrey, Eds.
|title=Weapons of Mass Destruction: An Encyclopedia of Worldwide Policy, Technology, and History
|publisher=ABC-CLIO, Inc.
|date=2005
|page=376
|isbn=978-1-85109-490-5
|url=https://books.google.com/books?id=ZzlNgS70OHAC&q=almost+unlimited+yield&pg=RA1-PA376
|access-date=2020-11-02
|url-status=live
|archive-url=https://web.archive.org/web/20210904154854/https://books.google.com/books?id=ZzlNgS70OHAC&q=almost+unlimited+yield&pg=RA1-PA376
|archive-date=2021-09-04
}}</ref> but technically disputed.<ref name=ieri>{{cite web |title=Fission, Fusion and Staging |website=[[IERI]] |url=https://www.ieri.be/fr/publications/ierinews/2011/juillet/fission-fusion-and-staging |access-date=2013-05-22 |url-status=live |archive-url=https://web.archive.org/web/20160305053224/http://ieri.be/fr/publications/ierinews/2011/juillet/fission-fusion-and-staging |archive-date=2016-03-05}}.</ref> There are "well-known sketches and some reasonable-looking calculations in the open literature about two-stage weapons, but no similarly accurate descriptions of true three stage concepts."<ref name=ieri/>

During the mid-1950s through early 1960s, scientists working in the weapons laboratories of the United States investigated weapons concepts as large as 1,000&nbsp;megatons,<ref>[https://nsarchive2.gwu.edu/nukevault/ebb249/doc09.pdf The Air Force and Strategic Deterrence 1951–1960. USAF historical division Liaison Office by George F. Lemmer 1967, p. 13. Formerly restricted data] {{webarchive |url=https://web.archive.org/web/20140617080527/http://www2.gwu.edu/~nsarchiv/nukevault/ebb249/doc09.pdf |date=June 17, 2014}}.</ref> and [[Edward Teller]] announced the design of a 10,000-megaton weapon code-named [[Sundial (weapon)|SUNDIAL]] at a meeting of the General Advisory Committee of the Atomic Energy Commission.<ref>{{cite web|title=In Search of a Bigger Boom|url=https://blog.nuclearsecrecy.com/2012/09/12/in-search-of-a-bigger-boom/|last=Wellerstein|first=Alex|date=12 September 2012}}</ref> Much of the information about these efforts remains classified,<ref>{{cite web |title=2013 FOIA Log |url=https://documents.theblackvault.com/documents/foia/FOIA%2014-00108-H.pdf |access-date=2014-10-06 |url-status=live |archive-url=https://web.archive.org/web/20160304063659/http://documents.theblackvault.com/documents/foia/FOIA%2014-00108-H.pdf |archive-date=2016-03-04}}</ref><ref>{{cite web |title=Case No. FIC-15-0005 |url=https://www.energy.gov/sites/prod/files/2016/04/f30/FIC-15-0005.pdf |access-date=2016-10-25 |url-status=live |archive-url=https://web.archive.org/web/20161025114419/http://energy.gov/sites/prod/files/2016/04/f30/FIC-15-0005.pdf |archive-date=2016-10-25}}</ref> but such "gigaton" range weapons do not appear to have made it beyond theoretical investigations.

While both the US and Soviet Union investigated (and in the case of the Soviets, tested) "very high yield" (e.g. 50 to 100-megaton) weapons designs in the 1950s and early 1960s,<ref>{{cite web|url=https://thebulletin.org/2021/11/the-untold-story-of-the-worlds-biggest-nuclear-bomb/|title=An Unearthly Spectacle: The Untold Story of the World's Biggest Bomb|last=Wellerstein|first=Alex|publisher=Bulletin of the Atomic Scientists|date=29 October 2021}}</ref> these appear to represent the upper-limit of Cold War weapon yields pursued seriously, and were so physically heavy and massive that they could not be carried entirely within the bomb bays of the largest bombers. Cold War warhead development trends from the mid-1960s onward, and especially after the [[Limited Test Ban Treaty]], instead resulted in highly-compact warheads with yields in the range from hundreds of kilotons to the low megatons that gave greater options for deliverability.

Following the concern caused by the estimated gigaton scale of the 1994 [[Comet Shoemaker-Levy 9]] impacts on the planet [[Jupiter]], in a 1995 meeting at [[Lawrence Livermore National Laboratory]] (LLNL), [[Edward Teller]] proposed to a collective of U.S. and Russian ex-[[Cold War]] weapons designers that they collaborate on designing a 1,000-megaton [[asteroid impact avoidance#Nuclear explosive device|nuclear explosive device for diverting extinction-class asteroids]] (10+ km in diameter), which would be employed in the event that one of these asteroids were on an impact trajectory with Earth.<ref>{{cite web |title=A new use for nuclear weapons: hunting rogue asteroids A persistent campaign by weapons designers to develop a nuclear defense against extraterrestrial rocks slowly wins government support 2013 |website=Center for Public Integrity |date=2013-10-16 |url=https://publicintegrity.org/national-security/a-new-use-for-nuclear-weapons-hunting-rogue-asteroids/ |access-date=7 October 2014 |url-status=live |archive-url=https://web.archive.org/web/20160320055111/http://www.publicintegrity.org/2013/10/16/13547/new-use-nuclear-weapons-hunting-rogue-asteroids |archive-date=2016-03-20}}</ref><ref>{{cite web |title=The mother of all bombs would sit in wait in an orbitary platform |author=Jason Mick |date=October 17, 2013 |url=http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm |url-status=dead |archive-url=https://web.archive.org/web/20141009190305/http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm#sthash.rQvVzS6m.dpuf |archive-date=October 9, 2014}}</ref><ref>[https://web.archive.org/web/20150909023233/https://e-reports-ext.llnl.gov/pdf/232015.pdf planetary defense workshop LLNL 1995]</ref>

===Neutron bombs===
{{Main|Neutron bomb}}

<nowiki>A neutron bomb, technically referred to as an enhanced radiation weapon (ERW), is a type of tactical nuclear weapon designed specifically to release a large portion of its energy as energetic neutron radiation. This contrasts with standard thermonuclear weapons, which are designed to capture this intense neutron radiation to increase its overall explosive yield. In terms of yield, ERWs typically produce about one-tenth that of a fission-type atomic weapon. Even with their significantly lower explosive power, ERWs are still capable of much greater destruction than any conventional bomb. Meanwhile, relative to other nuclear weapons, damage is more focused on biological material than on material infrastructure (though extreme blast and heat effects are not eliminated).</nowiki>{{Citation needed|date=June 2021}}

ERWs are more accurately described as suppressed yield weapons. When the yield of a nuclear weapon is less than one kiloton, its lethal radius from blast, {{convert|700|m|ft|abbr=on}}, is less than that from its neutron radiation. However, the blast is more than potent enough to destroy most structures, which are less resistant to blast effects than even unprotected human beings. Blast pressures of upwards of {{cvt|20|psi|kPa}} are survivable, whereas most buildings will collapse with a pressure of only {{cvt|5|psi|kPa|sigfig=1}}.{{Citation needed|date=June 2021}}

Commonly misconceived as a weapon designed to kill populations and leave infrastructure intact, these bombs (as mentioned above) are still very capable of leveling buildings over a large radius. The intent of their design was to kill tank crews – tanks giving excellent protection against blast and heat, surviving (relatively) very close to a detonation. Given the Soviets' vast tank forces during the Cold War, this was the perfect weapon to counter them. The neutron radiation could instantly incapacitate a tank crew out to roughly the same distance that the heat and blast would incapacitate an unprotected human (depending on design). The tank chassis would also be rendered highly radioactive, temporarily preventing its re-use by a fresh crew.{{Citation needed|date=June 2021}}

Neutron weapons were also intended for use in other applications, however. For example, they are effective in anti-nuclear defenses – the neutron flux being capable of neutralising an incoming warhead at a greater range than heat or blast. Nuclear warheads are very resistant to physical damage, but are very difficult to harden against extreme neutron flux.{{Citation needed|date=June 2021}}

{| class="wikitable" style="float:right; text-align:center;"
|+ Energy distribution of weapon
|-
! !! Standard !! Enhanced
|-
| Blast || 50% || 40%
|-
| Thermal energy || 35% || 25%
|-
| Instant radiation || 5% || 30%
|-
| Residual radiation || 10% || 5%
|}

ERWs were two-stage thermonuclears with all non-essential uranium removed to minimize fission yield. Fusion provided the neutrons. Developed in the 1950s, they were first deployed in the 1970s, by U.S. forces in Europe. The last ones were retired in the 1990s.{{Citation needed|date=June 2021}}

A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1–10 kilotons, with fission proportion varying from 50% at 1&nbsp;kiloton to 25% at 10&nbsp;kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10 to 15 times greater than for a pure fission implosion weapon or for a strategic warhead like a [[W87]] or [[W88]].<ref name="Neutron bomb: Why 'clean' is deadly"/>

=== Minor actinide fission weapons ===
{{multiple image|perrow = 2|total_width=350
| image1 = Np_sphere.jpg
| image2 = Americium_microscope.jpg
| caption1 = Macroscopic shell of neptunium-237
| caption2 = Microscopic quantity of americium
| footer = [[Minor actinides]] of concern to hypothetical fission weapons
From top, left to right
}}

Some isotopes of [[protactinium]], [[neptunium]], [[americium]], [[curium]], [[californium]], [[berkelium]], and [[einsteinium]] have calculated [[critical mass]] values, ranging in the kilograms to tens of kilograms. Few possess an adequate combination of high [[fission cross section]] (for detonation), low [[spontaneous fission]] rate (to limit [[predetonation]]), low alpha or gamma decay rate (to allow handling).{{Citation needed|date=April 2025}}

All suffer from a far higher cost of production compared to standard [[fissile material]]. This is due to both production of the quantity required, often in [[High flux reactor|high flux reactors]], and complex chemical separation procedures. For elements curium and, total global production has never exceeded a single critical mass of separated material.{{Citation needed|date=April 2025}}

[[Neptunium-237]] is considered the most immediately concerning minor actinide isotope for weaponization. Comprising ~0.05% of [[spent nuclear fuel]], ~5 tons are produced annually worldwide. The [[International Atomic Energy Agency]] has established monitoring for facilities capable of separation of the isotope, but is yet to classify it as a "special fissionable material", alongside [[plutonium-239]], and high enrichments of [[uranium-235]] and [[uranium-233]].<ref name="c496">{{cite web |last=An |first=J S |last2=Shin |first2=J S |last3=Kim |first3=J S |last4=Kwack |first4=E H |last5=Kim |first5=B K |date=2000-05-01 |title=The proliferation potential of neptunium and americium |url=https://www.osti.gov/etdeweb/biblio/20101205 |access-date=2025-04-30 |website=The proliferation potential of neptunium and americium (Technical Report)}}</ref> In September 2002, researchers at the [[Los Alamos National Laboratory]] briefly produced the first known nuclear [[critical mass]] involving a significant quantity of neptunium, in combination with shells of [[enriched uranium]] ([[uranium-235]]), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties,"<ref name="criticality">{{cite web |last1=Sanchez |first1=Rene G. |last2=Loaiza |first2=David J. |last3=Kimpland |first3=Robert H. |last4=Hayes |first4=David K. |last5=Cappiello |first5=Charlene C. |last6=Myers |first6=William L. |last7=Jaegers |first7=Peter J. |last8=Clement |first8=Steven D. |last9=Butterfield |first9=Kenneth B. |title=Criticality of a <sup>237</sup>Np Sphere |url=http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/2.14_107.pdf |url-status=dead |archive-url=https://web.archive.org/web/20140512214219/http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/2.14_107.pdf |archive-date=2014-05-12 |access-date=2014-08-06 |publisher=Japanese Atomic Energy Agency}}</ref> showing that it "is about as good a bomb material as [uranium-235]."<ref name="Weiss">{{cite journal |last=Weiss |first=Peter |date=2 July 2009 |title=Neptunium nukes?: Little-studied metal goes critical |journal=Science News |volume=162 |issue=17 |pages=259 |doi=10.2307/4014034 |jstor=4014034}}</ref> The United States federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in [[Nevada]].<ref name="Nevada">{{cite web |last=Yarris |first=Lynn |date=2005-11-29 |title=Getting the Neptunium out of Nuclear Waste |url=http://newscenter.lbl.gov/feature-stories/2005/11/29/getting-the-neptunium-out-of-nuclear-waste/ |access-date=2014-07-26 |publisher=Berkeley laboratory, U.S. Department of Energy}}</ref>

Certain isotopes of [[americium]] are also considered weaponizable, despite considerable challenge, based on the testimony of nuclear weapons physicists.<ref name="s816">{{cite journal |last=Pellaud |first=Bruno |date=2002-07-12 |title=Proliferation aspects of plutonium recycling |url=https://comptes-rendus.academie-sciences.fr/physique/item/10.1016/S1631-0705(02)01364-6.pdf |journal=Comptes Rendus. Physique |publisher=Cellule MathDoc/Centre Mersenne |volume=3 |issue=7-8 |pages=1067–1079 |doi=10.1016/s1631-0705(02)01364-6 |issn=1878-1535 |access-date=2025-04-30 |doi-access=free}}</ref>