Editing Manhattan Project (section)

== Feasibility ==
=== Proposals ===
[[File:Lawrence Compton Bush Conant Compton Loomis 83d40m March 1940 meeting UCB.JPG|thumb|March 1940 meeting at Berkeley, California: [[Ernest O. Lawrence]], [[Arthur H. Compton]], [[Vannevar Bush]], [[James B. Conant]], [[Karl T. Compton]], and [[Alfred Lee Loomis|Alfred L. Loomis]]|alt=Six men in suits sitting on chairs, smiling and laughing]]
The S-1 Committee meeting on 18 December 1941 was "pervaded by an atmosphere of enthusiasm and urgency"<ref>{{harvnb|Jones|1985|p=35}}.</ref> in the wake of the [[attack on Pearl Harbor]] and the [[United States declaration of war on Japan]] and [[United States declaration of war on Germany (1941)|on Germany]].<ref>{{harvnb|Williams|1960|pp=3–4}}.</ref> Work was proceeding on three techniques for [[isotope separation]]: Lawrence and his team at the [[University of California]] investigated [[electromagnetic separation]], [[Eger Murphree]] and [[Jesse Wakefield Beams]]'s team looked into [[gaseous diffusion]] at [[Columbia University]], and [[Philip Abelson]] directed research into [[Thermophoresis|thermal diffusion]] at the [[Carnegie Institution of Washington]] and later the [[Naval Research Laboratory]].<ref name="Jones, pp. 37-39" /> Murphree also headed an unsuccessful separation project using [[gas centrifuge]]s.<ref>{{harvnb|Nichols|1987|pp=32}}.</ref>

Meanwhile, there were two lines of investigation into [[nuclear reactor technology]]: [[Harold Urey]] researched [[heavy water]] at Columbia, while Arthur Compton organized the [[Metallurgical Laboratory]] in early 1942 to study plutonium and reactors using [[nuclear graphite|graphite]] as a [[neutron moderator]].<ref>{{harvnb|Jones|1985|pp=35–36}}.</ref> The S-1 Committee recommended pursuing all five technologies. This was approved by Bush, Conant, and [[Brigadier General (United States)|Brigadier General]] [[Wilhelm D. Styer]], who had been designated the Army's representative on nuclear matters.<ref name="Jones, pp. 37-39" />

Bush and Conant then took the recommendation to the Top Policy Group with a budget proposal for $54&nbsp;million for construction by the [[United States Army Corps of Engineers]], $31&nbsp;million for research and development by OSRD and $5&nbsp;million for contingencies in fiscal year 1943. They sent it on 17 June 1942, to the President, who approved it by writing "OK FDR" on the document.<ref name="Jones, pp. 37-39">{{harvnb|Jones|1985|pp=37–39}}.</ref>

=== Bomb design concepts ===
[[File:Los Alamos Primer assembly methods.png|thumb|upright|Different fission bomb assembly methods explored during the July 1942 conference|alt=A series of doodles]]
Arthur Compton asked theoretical physicist [[J. Robert Oppenheimer]] of the University of California to take over research into [[fast neutron calculations]]—key to calculations of critical mass and weapon detonation—from [[Gregory Breit]], who had quit on 18 May 1942 because of concerns over lax operational security.<ref>{{harvnb|Rhodes|1986|p=416}}.</ref> [[John H. Manley]], a physicist at the Metallurgical Laboratory, was assigned to assist Oppenheimer by coordinating experimental physics groups scattered across the country.<ref>{{harvnb|Hewlett|Anderson|1962|p=103}}.</ref> Oppenheimer and [[Robert Serber]] of the [[University of Illinois at Urbana–Champaign|University of Illinois]] examined the problems of [[neutron]] diffusion—how neutrons moved in a nuclear chain reaction—and [[hydrodynamics]]—how the explosion produced by a chain reaction might behave.<ref name="auto">{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=42–44}}</ref>

To review this work and the general theory of fission reactions, Oppenheimer and Fermi convened meetings at the University of Chicago in June and at the University of California in July 1942 with theoretical physicists [[Hans Bethe]], [[John Van Vleck]], Edward Teller, [[Emil Konopinski]], Robert Serber, [[Stan Frankel]], and Eldred C. (Carlyle) Nelson, and [[experimental physicist]]s [[Emilio Segrè]], [[Felix Bloch]], [[Franco Rasetti]], Manley, and [[Edwin McMillan]]. They tentatively confirmed that a fission bomb was theoretically possible.<ref name="auto"/>

The properties of pure uranium-235 were relatively unknown, as were those of plutonium, which had only been isolated by [[Glenn Seaborg]] and his team in February 1941. The scientists at the July 1942 conference envisioned creating plutonium in nuclear reactors where uranium-238 atoms absorbed neutrons that had been emitted from fissioning uranium-235. At this point no reactor had been built, and only tiny quantities of plutonium were available from [[cyclotron]]s.{{sfn|Hewlett|Anderson|1962|pp=33–35, 183}} Even by December 1943, only two milligrams had been produced.<ref>{{harvnb|Groves|1962|p=41}}.</ref> There were many ways of arranging the fissile material into a critical mass. The simplest was shooting a "cylindrical plug" into a sphere of "active material" with a "tamper"—dense material to focus neutrons inward and keep the reacting mass together to increase its efficiency.<ref>{{harvnb|Serber|Rhodes|1992|p=21}}.</ref> They also explored designs involving [[spheroid]]s, a primitive form of "[[implosion nuclear weapon|implosion]]" suggested by [[Richard C. Tolman]], and the possibility of [[autocatalysis|autocatalytic methods]] to increase the efficiency of the bomb as it exploded.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=54–56}}</ref>

As the idea of the fission bomb was theoretically settled—at least until more experimental data was available—Edward Teller pushed for discussion of a more powerful bomb: the "super", now usually referred to as a "[[hydrogen bomb]]", which would use the force of a detonating fission bomb to ignite a [[nuclear fusion]] reaction in [[deuterium]] and [[tritium]].<ref>{{harvnb|Rhodes|1986|p=417}}.</ref> Teller proposed scheme after scheme, but Bethe refused each one. The fusion idea was put aside to concentrate on producing fission bombs.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=44–45}}</ref> Teller raised the speculative possibility that an atomic bomb might "ignite" the atmosphere because of a hypothetical fusion reaction of nitrogen nuclei.{{efn|The reaction Teller was most concerned with was: {{nuclide|nitrogen|14}} + {{nuclide|nitrogen|14}} → {{nuclide|magnesium|24}} + {{nuclide|helium|4}} (alpha particle) + 17.7 MeV.<ref>{{harvnb|Bethe|1991|p=30}}.</ref>}} Bethe calculated that it was "extremely unlikely".<ref>{{harvnb|Rhodes|1986|p=419}}.</ref> A postwar report co-authored by Teller concluded that "whatever the temperature to which a section of the atmosphere may be heated, no self-propagating chain of nuclear reactions is likely to be started."<ref>{{cite web |author-link=Emil Konopinski |last1=Konopinski |first1=E. J |first2=C. |last2=Marvin |first3=Edward |last3=Teller |author-link3=Edward Teller |title=Ignition of the Atmosphere with Nuclear Bombs |issue=LA–602 |publisher=Los Alamos National Laboratory |url=https://fas.org/sgp/othergov/doe/lanl/docs1/00329010.pdf |year=1946 |access-date=23 November 2008}}</ref> In Serber's account, Oppenheimer mentioned the possibility of this scenario to [[Arthur Compton]], who "didn't have enough sense to shut up about it. It somehow got into a document that went to Washington" and was "never laid to rest".{{efn|In Bethe's account, the possibility of this ultimate catastrophe came up again in 1975 when it appeared in a magazine article by H.C. Dudley, who got the idea from a report by [[Pearl Buck]] of an interview she had with [[Arthur Compton]] in 1959. The worry was not entirely extinguished in some people's minds until the [[Trinity test]].<ref>{{harvnb|Bethe|1991|pp=xi, 30}}.</ref>}}