Editing Neutron moderator (section)

== Nuclear weapon design ==
{{Main|Uranium hydride bomb}}
Early speculation about [[nuclear weapon]]s assumed that an "atom bomb" would be a large amount of fissile material moderated by a neutron moderator, similar in structure to a [[nuclear reactor]] or "pile".<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq8.html#nfaq8.2.1 Nuclear Weapons Frequently Asked Questions - 8.2.1 Early Research on Fusion Weapons]</ref> Only the [[Manhattan Project]] embraced the idea of a chain reaction of fast neutrons in pure metallic uranium or plutonium. Other moderated designs were also considered by the Americans; proposals included [[Uranium hydride bomb|using uranium deuteride]] as the fissile material.<ref name="upshot">[http://www.nuclearweaponarchive.org/Usa/Tests/Upshotk.html Operation Upshot–Knothole]</ref><ref name="globalsecurity">[http://www.globalsecurity.org/wmd/systems/w48.htm W48] - globalsecurity.org</ref> 

In 1943 [[Robert Oppenheimer]] and [[Niels Bohr]] considered the possibility of using a "pile" as a weapon.<ref>{{Cite web |url=http://www.ask.ne.jp/~hankaku/english/np5y.html |title=Atomic Bomb Chronology: 1942-1944 |access-date=2008-12-16 |archive-url=https://web.archive.org/web/20080528074940/http://www.ask.ne.jp/~hankaku/english/np5y.html |archive-date=2008-05-28 |url-status=dead }}</ref> The motivation was that with a graphite moderator it would be possible to achieve the chain reaction without the use of any isotope separation. However, plutonium can be produced ("[[Breeder reactor|bred]]") sufficiently isotopically pure as to be usable in a bomb and then has to be "only" separated chemically, a much easier processes than isotope separation, albeit still a challenging one. In August 1945, when information of the [[Atomic bombings of Hiroshima and Nagasaki|atomic bombing of Hiroshima]] was relayed to the scientists of the German nuclear program who were interred at Farm Hall in England, chief scientist [[Werner Heisenberg]] hypothesized that the device must have been "something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator".<ref>[[Hans Bethe]] in ''[[Physics Today]]'' Vol 53 (2001) [http://www.nd.edu/~nsl/Lectures/phys205/pdf/Nuclear_warfare_3.pdf]</ref> The German program, which had been much less advanced, had never even considered the plutonium option and did not discover a feasible method of large scale isotope separation in uranium.

After the success of the Manhattan Project, all major [[:Category:Nuclear weapons programs|nuclear weapons programs]] have relied on fast neutrons in their weapons designs. The notable exception is the ''Ruth'' and ''Ray'' test explosions of [[Operation Upshot–Knothole]]. The aim of the [[University of California Radiation Laboratory]] (UCRL) designs was the exploration of deuterated polyethylene charge containing uranium<ref name="herk">{{cite book |author-link=Gregg Herken |first=Gregg |last=Herken |title=Brotherhood of the Bomb |url=https://archive.org/details/brotherhoodofbom0000herk |url-access=registration |date=2003}}</ref>{{refpage|chapter 15}} as a candidate thermonuclear fuel,<ref name="swordsoarIII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=III |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|203}} hoping that [[deuterium]] would fuse (becoming an active medium) if compressed appropriately. If successful, the devices could also lead to a compact primary containing minimal amount of fissile material, and powerful enough to ignite RAMROD<ref name="swordsoarIII" />{{refpage|149}} a [[thermonuclear weapon]] designed by UCRL at the time. For a "hydride" primary, the degree of compression would not make deuterium to fuse, but the design could be subjected to boosting, raising the yield considerably.<ref name="swordsoarI">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=I |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|258}} The [[Pit (nuclear weapon)|cores]] consisted of a mix of [[uranium deuteride]] (UD<sub>3</sub>),<ref name="swordsoarIII" />{{refpage|202}} and deuterated polyethylene. The core tested in ''Ray'' used uranium low enriched in U<sup>235</sup>, and in both shots deuterium acted as the neutron moderator.<ref name="swordsoarI" />{{refpage|260}} The predicted [[Nuclear weapon yield|yield]] was 1.5 to 3 kt for ''Ruth'' (with a maximum potential yield of 20 kt<ref name="swordsoarVII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=VII |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|96}}) and 0.5-1 kt for ''Ray''. The tests produced yields of 200 [[tons of TNT]] each; both tests were considered to be [[Fizzle (nuclear explosion)|fizzles]].<ref name="upshot" /><ref name="globalsecurity" />

A side effect of using a moderator in a nuclear explosive is that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons. Another effect of moderation is that the time between subsequent neutron generations is increased, slowing down the reaction. This makes the containment of the explosion a problem; the [[inertia]] that is used to confine [[Nuclear weapon design#Implosion-type weapon|implosion type]] bombs will not be able to confine the reaction. The result may be a fizzle. The explosive power of a fully moderated explosion is thus limited; at worst it may be equal to a chemical explosive of similar mass. According to Heisenberg: "One can never make an explosive with slow neutrons, not even with the heavy water machine, as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete."<ref name="Rose1998">{{cite book|author=Paul Lawrence Rose|author-link=Paul Lawrence Rose|title=Heisenberg and the Nazi Atomic Bomb Project: A Study in German Culture|url=https://archive.org/details/isbn_9780520229266|url-access=registration|access-date=6 May 2017|year=1998|publisher=[[University of California Press]]|isbn=978-0-520-21077-6|page=[https://archive.org/details/isbn_9780520229266/page/211 211]}}</ref> While a nuclear bomb working on thermal neutrons may be impractical, modern weapons designs may still benefit from some level of moderation. A beryllium tamper used as a [[neutron reflector]] will act as a moderator.<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.7.3 Nuclear Weapons Frequently Asked Questions - 4.1.7.3.2 Reflectors]</ref><ref name="killus">[http://unintentional-irony.blogspot.com/2007/07/n-moderation.html N Moderation]</ref>