Editing Nuclear fission (section)

==History==

===Discovery of nuclear fission===

{{main|Discovery of nuclear fission}}
[[File:Hahn and Meitner in 1912.jpg|thumb|373x373px|[[Otto Hahn]] and [[Lise Meitner]] in 1912]]
The discovery of nuclear fission occurred in 1938 in the buildings of the [[Kaiser Wilhelm Society]] for Chemistry, today part of the [[Free University of Berlin]], following over four decades of work on the science of [[radioactivity]] and the elaboration of new nuclear physics that described the components of atoms. In 1911, [[Ernest Rutherford]] proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the [[Rutherford model]]).<ref>{{cite journal |author=E. Rutherford|year=1911|title=The scattering of α and β particles by matter and the structure of the atom|journal=Philosophical Magazine|volume= 21|pages=669–688|url=http://web.ihep.su/dbserv/compas/src/rutherford11/eng.pdf|bibcode=2012PMag...92..379R|doi=10.1080/14786435.2011.617037|issue=4|s2cid=126189920}}</ref> [[Niels Bohr]] improved upon this in 1913 by reconciling the quantum behavior of electrons (the [[Bohr model]]). In 1928, [[George Gamow]] proposed the [[Liquid drop model]], which became essential to understanding the physics 
of fission.<ref name=rr/>{{rp|49–51,70–77,228}}<ref name=ww/>{{rp|6–7}}

In 1896, [[Henri Becquerel]] had found, and [[Marie Curie]] named, radioactivity. In 1900, Rutherford and [[Frederick Soddy]], investigating the radioactive gas emanating from [[thorium]], "conveyed the tremendous and  inevitable conclusion that the element thorium was slowly and spontaneously [[nuclear transmutation|transmuting]] itself into argon gas!"<ref name=rr/>{{rp|41–43}}

In 1919, following up on an earlier anomaly [[Ernest Marsden]] noted in 1915, Rutherford attempted to "break up the atom." Rutherford was able to accomplish the first artificial transmutation of [[nitrogen]] into [[oxygen]], using alpha particles directed at nitrogen <sup>14</sup>N + α → <sup>17</sup>O + p.&nbsp; Rutherford stated, "...we must conclude that the nitrogen atom is disintegrated," while the newspapers stated he had ''split the atom''. This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. It also offered a new way to study the nucleus. Rutherford and [[James Chadwick]] then used alpha particles to "disintegrate" boron, fluorine, sodium, aluminum, and phosphorus before reaching a limitation associated with the energy of his alpha particle source.<ref name="rr">{{cite book |last1=Rhodes |first1=Richard |title=The Making of the Atomic Bomb |date=1986 |publisher=Simon & Schuster Paperbacks |location=New York |isbn=9781451677614 |pages=135–138}}</ref> Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues [[Ernest Walton]] and [[John Cockcroft]], who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", and would win them the 1951 Nobel Prize in Physics for ''"Transmutation of atomic nuclei by artificially accelerated atomic particles"'', although it was not the nuclear fission reaction later discovered in heavy elements.<ref>{{cite web |url=http://www-outreach.phy.cam.ac.uk/camphy/cockcroftwalton/cockcroftwalton9_1.htm |title=Cockcroft and Walton split lithium with high energy protons April 1932 |publisher=Outreach.phy.cam.ac.uk |date=1932-04-14 |access-date=2013-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20120902195556/http://www-outreach.phy.cam.ac.uk/camphy/cockcroftwalton/cockcroftwalton9_1.htm |archive-date=2012-09-02 }}</ref><ref>{{Cite web |title=Trump claims Manchester atom split as US achievement |url=https://www.bbc.com/news/articles/cg451wx2n63o |access-date=2025-01-21 |website=www.bbc.com |date=21 January 2025 |language=en-GB}}</ref><ref>{{Cite magazine |last=AP |first=Charlotte Graham-McLay / |date=2025-01-21 |title=New Zealanders Balk at Trump Claim That Americans Split Atom |url=https://time.com/7208628/trump-america-split-atom-new-zealand-fact-check/ |access-date=2025-01-21 |magazine=TIME |language=en}}</ref>

English physicist [[James Chadwick]] discovered the neutron in 1932.<ref>{{cite journal |author=J. Chadwick|doi=10.1038/129312a0|title=Possible Existence of a Neutron|year=1932|page=312|issue=3252|volume=129|journal=Nature|url=http://web.mit.edu/22.54/resources/Chadwick.pdf|bibcode= 1932Natur.129Q.312C|s2cid=4076465|doi-access=free}}</ref> Chadwick used an [[ionization chamber]] to observe protons knocked out of several elements 
by beryllium radiation, following up on earlier observations made by [[Irène Joliot-Curie|Joliot-Curies]]. In Chadwick's words, "...In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge..." The existence of the neutron was first postulated by Rutherford in 1920, and in the words of Chadwick, "...how on earth were you going to build up a big nucleus with a large positive charge? And the answer was a neutral particle."<ref name=rr/>{{rp|153–165}} Subsequently, he communicated his findings in more detail.<ref>{{cite journal |doi=10.1098/rspa.1932.0112|author=Chadwick, J.|year=1932|title=The existence of a neutron|journal=Proceedings of the Royal Society A|volume=136|issue=830|pages=692–708|url=http://www.chemteam.info/Chem-History/Chadwick-1932/Chadwick-neutron.html|bibcode= 1932RSPSA.136..692C|doi-access=free}} and {{cite journal |doi=10.1098/rspa.1933.0152|author=Chadwick, J.|year=1933|title=The Bakerian Lecture: The neutron|journal=Proceedings of the Royal Society A|volume=142|issue=846 |pages=1–25|bibcode= 1933RSPSA.142....1C|doi-access=free}}</ref>

In the words of [[Richard Rhodes]], referring to the neutron, "It would therefore serve as a new nuclear probe of surpassing power of penetration." [[Philip Morrison]] stated, "A beam of [[thermal neutron]]s moving at about the speed of sound...produces nuclear reactions in many materials much more easily than a beam of protons...traveling thousands of times faster."  
According to Rhodes, "Slowing down a neutron gave it more time in the vicinity of the nucleus, and that gave it more time to be captured." Fermi's team, studying radiative capture which is the emission of gamma radiation after the nucleus captures a neutron, studied sixty elements, inducing radioactivity in forty. In the process, they discovered the ability of hydrogen to slow down the neutrons.<ref name=rr/>{{rp|165,216–220}}

[[Enrico Fermi]] and his colleagues in [[Rome]] studied the results of bombarding uranium with neutrons in 1934.<ref>E. Fermi, E. Amaldi, O. D'Agostino, F. Rasetti, and E. Segrè (1934) "Radioattività provocata da bombardamento di neutroni III", ''La Ricerca Scientifica'', vol.&nbsp;5, no.&nbsp;1, pages&nbsp;452–453.</ref> Fermi concluded that his experiments had created new elements with 93 and 94 protons, which the group dubbed [[ausenium and hesperium]]. However, not all were convinced by Fermi's analysis of his results, though he would win the 1938  [[Nobel Prize in Physics]] for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". The German chemist [[Ida Noddack]] notably suggested in 1934 that instead of creating a new, heavier element 93, that "it is conceivable that the nucleus breaks up into several large fragments."<ref>{{cite journal |author=Ida Noddack|year=1934|page=653|issue=37|title=Über das Element 93|volume=47|journal=Zeitschrift für Angewandte Chemie|url=http://www.chemteam.info/Chem-History/Noddack-1934.html|doi=10.1002/ange.19340473707|bibcode=1934AngCh..47..653N}}</ref> However, the quoted objection comes some distance down, and was but one of several gaps she noted in Fermi's claim. Although Noddack was a renowned analytical chemist, she lacked the background in physics to appreciate the enormity of what she was proposing.<ref>{{cite book |last=Hook |first=Ernest B. |editor-last=Hook |editor-first=Ernest B. |title=Prematurity in Scientific Discovery: On Resistance and Neglect |contribution=Interdisciplinary Dissonance and Prematurity: Ida Noddack’s Suggestion of Nuclear Fission |pages=124–148 |publisher=University of California Press |location=Berkeley and Los Angeles |date=2002 |isbn=978-0-520-23106-1 |oclc=883986381 }}</ref>

[[File:Nuclear Fission Experimental Apparatus 1938 - Deutsches Museum - Munich.jpg|thumb|left|The nuclear fission display at the [[Deutsches Museum]] in [[Munich]]. The table and instruments are originals,<ref>{{cite web | url=https://digital.deutsches-museum.de/de/digital-catalogue/collection-object/71930/ | title=Originalgeräte zur Entdeckung der Kernspaltung, "Hahn-Meitner-Straßmann-Tisch" }}</ref><ref>{{cite web | url=https://www.youtube.com/watch?v=ww8rqqVCBxo | title=Entdeckung der Kernspaltung 1938, Versuchsaufbau, Deutsches Museum München &#124; Faszination Museum | website=[[YouTube]] | date=7 July 2015 }}</ref> but would not have been together in the same room.]]

After the Fermi publication, [[Otto Hahn]], [[Lise Meitner]], and [[Fritz Strassmann]] began performing similar experiments in [[Berlin]]. Meitner, an Austrian Jew, lost her Austrian citizenship with the ''[[Anschluss]]'', the union of Austria with Germany in March 1938, but she fled in July 1938 to Sweden and started a correspondence by mail with Hahn in Berlin. By coincidence, her nephew [[Otto Robert Frisch]], also a refugee, was also in Sweden when Meitner received a letter from Hahn dated 19 December describing his chemical proof that some of the product of the bombardment of uranium with neutrons was [[barium]]. Hahn suggested a ''bursting'' of the nucleus, but he was unsure of what the physical basis for the results were. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and the techniques were well known. Meitner and Frisch then correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. Frisch suggested the process be named "nuclear fission", by analogy to the process of living cell division into two cells, which was then called [[fission (biology)|binary fission]]. Just as the term nuclear "chain reaction" would later be borrowed from chemistry, so the term "fission" was borrowed from biology.<ref>{{cite book |last=Frisch |first=Otto Robert |title=What Little I Remember |year=1980 |publisher=Cambridge University Press |isbn=0-52-128010-9 |pages=114–117 |quote=The paper was composed by several long-distance telephone calls, Lise Meitner having returned to Stockholm in the meantime. I asked an American biologist who was working with Hevesy what they call the process by which single cells divide in two; 'fission', he said, so I used the term 'nuclear fission' in that paper. Placzek was sceptical; couldn’t I do some experiments to show the existence of those fast-moving fragments of the uranium nucleus? Oddly enough that thought hadn’t occurred to me, but now I quickly set to work, and the experiment (which was really very easy) was done in two days, and a short note about it was sent off to Nature together with the other note I had composed over the telephone with Lise Meitner.}}</ref>

News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Meitner's and Frisch's interpretation of the discovery of Hahn and Strassmann crossed the Atlantic Ocean with Niels Bohr, who was to lecture at [[Princeton University]]. [[Isidor Isaac Rabi|I.I. Rabi]] and [[Willis Lamb]], two [[Columbia University]] physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found [[Herbert L. Anderson]]. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics."<ref>Richard Rhodes. (1986) ''The Making of the Atomic Bomb'', Simon and Schuster, p. 268, {{ISBN|0-671-44133-7}}.</ref>

It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States,<ref>{{cite journal |author1=H. L. Anderson |author2=E. T. Booth |author3=J. R. Dunning |author4=E. Fermi |author5=G. N. Glasoe |author6=F. G. Slack  |name-list-style=amp |title=The Fission of Uranium|journal=Physical Review|volume=55|issue=5|page=511|year=1939|doi=10.1103/PhysRev.55.511.2|bibcode= 1939PhRv...55..511A}}</ref> which was done in the basement of [[Pupin Hall]]. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The results confirmed that fission was occurring and hinted strongly that it was the isotope [[uranium 235]] in particular that was fissioning. The next day, the fifth [[Washington Conference on Theoretical Physics]] began in [[Washington, D.C.]] under the joint auspices of the George Washington University and the [[Carnegie Institution of Washington]]. There, the news on nuclear fission was spread even further, which fostered many more experimental demonstrations.<ref>Richard Rhodes (1986). ''The Making of the Atomic Bomb'', Simon and Schuster, pp. 267–270, {{ISBN|0-671-44133-7}}.</ref>
The 6 January 1939 Hahn and Strassman paper announced the discover of fission. In their second publication on nuclear fission in February 1939, Hahn and Strassmann used the term ''Uranspaltung'' (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.<ref>{{cite journal |last1=Hahn |first1=O. |last2=Strassmann |first2=F. |author-link2=Fritz Strassmann |title=Nachweis der Entstehung aktiver Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung; Nachweis weiterer aktiver Bruchstücke bei der Uranspaltung |journal=Naturwissenschaften |volume=27 |issue=6 |pages=89–95 |date=February 1939 |doi=10.1007/BF01488988 |bibcode=1939NW.....27...89H |s2cid=33512939 }}</ref> The 11 February 1939 paper by Meitner and Frisch compared the process to the division of a liquid drop and estimated the energy released at 200 MeV.<ref>{{cite journal |last1=Meitner |first1=Lisa |last2=Frisch |first2=O.R. |title=Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction |url=https://www.nature.com/articles/143239a0 |journal=Nature |access-date=20 September 2023 |date=1939|volume=143 |issue=3615 |pages=239–240 |doi=10.1038/143239a0 |bibcode=1939Natur.143..239M |s2cid=4113262 }}</ref>  The 1 September 1939 paper by Bohr and Wheeler used this liquid drop model to quantify fission details, including the energy released, estimated the cross section for neutron-induced fission, and deduced {{chem|235|U}} was the major contributor to that cross section and slow-neutron fission.<ref>{{cite journal |last1=Bohr |first1=Niels |last2=Wheeler |first2=John |title=The Mechanism of Nuclear Fission |journal=Physical Review |date=1939|volume=56 |issue=5 |pages=426–450 |doi=10.1103/PhysRev.56.426 |bibcode=1939PhRv...56..426B |doi-access=free }}</ref><ref name=rr/>{{rp|262,311}}<ref name=ww/>{{rp|9–13}}

===Fission chain reaction realized===
During this period the Hungarian physicist [[Leó Szilárd]] realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Rutherford's disparaging remarks about generating power from neutron collisions. However, Szilárd had not been able to achieve a neutron-driven chain reaction using beryllium. Szilard stated, "...if we could find an element which is split by neutrons and which would emit ''two'' neutrons when it absorbs ''one'' neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction."  On 25 January 1939, after learning of Hahn's discovery from [[Eugene Wigner]], Szilard noted, "...if enough neutrons are emitted...then it should be, of course, possible to sustain a chain reaction. All of the things which [[H. G. Wells]] predicted appeared suddenly real to me." After the Hahn-Strassman paper was published, Szilard noted in a letter to [[Lewis Strauss]], that during the fission of uranium, "the energy released in this new reaction must be very much higher than all previously known cases...," which might lead to "large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs."<ref name="tz">{{cite book |last1=Zoellner |first1=Tom |title=Uranium |date=2009 |publisher=Viking Penguin |isbn=978-0-670-02064-5 |pages=28–30}}</ref><ref name=rr/>{{rp|26–28,203–204,213–214,223–225,267–268}}

Szilard now urged Fermi (in New York) and [[Frédéric Joliot-Curie]] (in Paris) to refrain from publishing on the possibility of a chain reaction, lest the Nazi government become aware of the possibilities on the eve of what would later be known as [[World War II]]. With some hesitation Fermi agreed to self-censor. But Joliot-Curie did not, and in April 1939 his team in Paris, including [[Hans von Halban]] and [[Lew Kowarski]], reported in the journal ''Nature'' that the number of neutrons emitted with nuclear fission of uranium was then reported at 3.5 per fission.<ref>{{cite journal |author1=H. Von Halban |author2=F. Joliot |author3=L. Kowarski  |name-list-style=amp |title=Number of Neutrons Liberated in the Nuclear Fission of Uranium|journal=Nature|volume=143|issue=3625|page=680|year=1939|doi=10.1038/143680a0|bibcode= 1939Natur.143..680V|s2cid=4089039 |doi-access=free}}</ref> Szilard and [[Walter Zinn]] found "...the number of neutrons emitted by fission to be about two." Fermi and Anderson estimated "a yield of about two neutrons per each neutron captured."<ref name=rr/>{{rp|290–291,295–296}}

[[File:Stagg Field reactor.jpg|thumb|Drawing of the first artificial reactor, [[Chicago Pile-1]]]]

With the news of fission neutrons from uranium fission, Szilárd immediately understood the possibility of a nuclear chain reaction using uranium. In the summer, Fermi and Szilard proposed the idea of a nuclear reactor (pile) to mediate this process. The pile would use natural uranium as fuel. Fermi had shown much earlier that neutrons were far more effectively captured by atoms if they were of low energy (so-called "slow" or "thermal" neutrons), because for quantum reasons it made the atoms look like much larger targets to the neutrons. Thus to slow down the secondary neutrons released by the fissioning uranium nuclei, Fermi and Szilard proposed a graphite "moderator", against which the fast, high-energy secondary neutrons would collide, effectively slowing them down. With enough uranium, and with sufficiently pure graphite, their "pile" could theoretically sustain a slow-neutron chain reaction. This would result in the production of heat, as well as the creation of radioactive fission products.<ref name=rr/>{{rp|291,298–302}}

In August 1939, Szilard, [[Edward Teller|Teller]] and [[Eugene Wigner|Wigner]] thought that the [[German nuclear weapons program|Germans might make use of the fission chain reaction]] and were spurred to attempt to attract the attention of the United States government to the issue. Towards this, they persuaded [[Albert Einstein]] to lend his name to a letter directed to President [[Franklin Roosevelt]]. On 11 October, the [[Einstein–Szilárd letter]] was delivered via [[Alexander Sachs]]. Roosevelt quickly understood the implications, stating, "Alex, what you are after is to see that the Nazis don't blow us up." Roosevelt ordered the formation of the [[Advisory Committee on Uranium]].<ref name=rr/>{{rp|303–309,312–317}}

In February 1940, encouraged by Fermi and [[John R. Dunning]], [[Alfred O. C. Nier]] was able to separate U-235 and U-238 from [[uranium tetrachloride]] in a [[mass spectrometry|glass mass spectrometer]]. Subsequently, Dunning, bombarding the U-235 sample with neutrons generated by the Columbia University [[cyclotron]], confirmed "U-235 was responsible for the slow neutron fission of uranium."<ref name=rr/>{{rp|297–298,332}}

At the [[University of Birmingham]], Frisch teamed up with [[Rudolf Peierls|Peierls]], who had been working on a critical mass formula. assuming isotope separation was possible, they considered <sup>235</sup>U, which had a [[nuclear cross section|cross section]] not yet determined, but which was assumed to be much larger than that of natural uranium. They calculated only a pound or two in a volume less than a golf ball, would result in a chain reaction faster than vaporization, and the resultant explosion would generate temperature greater than the interior of the sun, and pressures greater than the center of the earth. Additionally, the costs of isotope separation "would be insignificant compared to the cost of the war." By March 1940, encouraged by [[Mark Oliphant]], they wrote the [[Frisch–Peierls memorandum]] in two parts, "On the construction of a 'super-bomb; based on a nuclear chain reaction in uranium," and "Memorandum on the properties of a radioactive 'super-bomb.' ". On 10 April 1940, the first meeting of the [[MAUD Committee]] was held.<ref name=rr/>{{rp|321–325,330–331,340–341}}

In December 1940, [[Franz Simon]] at Oxford wrote his Estimate of the size of an actual separation plant." Simon proposed [[gaseous diffusion]] as the best method for uranium isotope separation.<ref name=rr/>{{rp|339,343}}

On 28 March 1941, [[Emilio Segré]] and [[Glen Seaborg]] reported on the "strong indications that <sup>239</sup>Pu undergoes fission with slow neutrons." This meant chemical separation was an alternative to uranium isotope separation. Instead, a nuclear reactor fueled with ordinary uranium could produce a plutonium isotope as a nuclear explosive substitute for <sup>235</sup>U. In May, they demonstrated the cross section of plutonium was 1.7 times that of U235. When plutonium's cross section for fast fission was measured to be ten times that of U238, plutonium became a viable option for a bomb.<ref name=rr/>{{rp|346–355,366–368}}

In October 1941, MAUD released its final report to the U.S. Government. The report stated, "We have now reached the conclusion that it will be possible to make an effective uranium bomb...The material for the first bomb could be ready by the end of 1943..."<ref name=rr/>{{rp|368–369}}

In November 1941, John Dunning and [[Eugene T. Booth]] were able to demonstrate the enrichment of uranium through gaseous barrier diffusion. On 27 November, Bush delivered to third [[National Academy of Sciences]] report to Roosevelt. The report, amongst other things, called for parallel development of all isotope-separation systems. On 6 December, Bush and Conant reorganized the Uranium Committee's tasks, with [[Harold Urey]] developing gaseous diffusion, Lawrence developing electromagnetic separation, [[Eger V. Murphree]] developing centrifuges, and [[Arthur Compton]] responsible for theoretical studies and design.<ref name=rr/>{{rp|381,387–388}}

On 23 April 1942, [[Metallurgical Laboratory|Met Lab]] scientists discussed seven possible ways to extract plutonium from irradiated uranium, and decided to pursue investigation of all seven. On 17 June, the first batch of uranium nitrate hexahydrate (UNH) was undergoing neutron bombardment in the [[Washington University in St. Louis]] cyclotron. On 27 July, the irradiated UNH was ready for [[Glenn T. Seaborg]]'s team. On 20 August, using ultramicrochemistry techniques, they successfully extracted plutonium.<ref name=rr/>{{rp|408–415}}

In April 1939, creating a chain reaction in natural uranium became the goal of Fermi and Szilard, as opposed to isotope separation. Their first efforts involved five hundred pounds of uranium oxide from the Eldorado Radium Corporation. Packed into fifty-two cans two inches in diameter and two feet long in a tank of manganese solution, they were able to confirm more neutrons were emitted than absorbed. However, the hydrogen within the water absorbed the slow neutrons necessary for fission. Carbon in the form of graphite, was then considered, because of its smaller capture cross section. In April 1940, Fermi was able to confirm carbon's potential for a slow-neutron chain reaction, after receiving [[National Carbon Company]]'s graphite bricks at their [[Pupin Laboratories]]. In August and September, the Columbia team enlarged upon the cross section measurements by making a series of exponential "piles".  The first piles consisted of a uranium-graphite lattice, consisting of 288 cans, each containing 60 pounds of uranium oxide, surrounded by graphite bricks. Fermi's goal was to determine critical mass necessary to sustain neutron generation. Fermi defined the [[reproduction factor]] k for assessing the chain reaction, with a value of 1.0 denoting a sustained chain reaction.  In September 1941, Fermi's team was only able to achieve a k value of 0.87.  In April 1942, before the project was centralized in Chicago, they had achieved 0.918 by removing moisture from the oxide. In May 1942, Fermi planned a full-scale chain reacting pile, Chicago Pile-1, after one of the exponential piles at [[Stagg Field]] reached a k of 0.995. Between 15 September and 15 November, [[Herbert L. Anderson]] and [[Walter Zinn]] built sixteen exponential piles. Acquisition of purer forms of graphite, without traces of boron and its large cross section, became paramount. Also important was the acquisition of highly purified forms of oxide from [[Mallinckrodt]] Chemical Works. Finally, acquiring pure uranium metal from the [[Ames process]], meant the replacement of oxide pseudospheres with [[Frank Spedding]]'s "eggs". Starting on 16 November 1942, Fermi had Anderson and Zinn working in two twelve-hours shifts, constructing a pile that eventually reached 57 layers by 1 Dec. The final pile consisted of 771,000 pounds of graphite, 80,590 pounds of uranium oxide, and 12,400 pounds of uranium metal, with ten cadmium [[control rod]]s. Neutron intensity was measured with a [[boron trifluoride]] counter, with the control rods removed, after the end of each shift. On 2 Dec. 1942, with k approaching 1.0, Fermi had all but one of the control rod removed, and gradually removed the last one. The neutron counter clicks increased, as did the pen recorder, when Fermi announced "The pile has gone critical." They had achieved a k of 1.0006, which meant neutron intensity doubled every two minutes, in addition to breeding plutonium.<ref name=rr/>{{rp|298–301,333–334,394–397,400–401,428–442}}

===Manhattan Project and beyond===
{{see also|Manhattan Project}}
In the United States, an all-out effort for making atomic weapons was begun in late 1942. This work was taken over by the [[U.S. Army Corps of Engineers]] in 1943, and known as the Manhattan Engineer District. The top-secret [[Manhattan Project]], as it was colloquially known, was led by General [[Leslie R. Groves]]. Among the project's dozens of sites were: [[Hanford Site]] in Washington, which had the first industrial-scale nuclear reactors and produced [[plutonium]]; [[Oak Ridge, Tennessee]], which was primarily concerned with [[uranium enrichment]]; and [[Los Alamos National Laboratory|Los Alamos]], in New Mexico, which was the scientific hub for research on bomb development and design. Other sites, notably the [[Berkeley Radiation Laboratory]] and the [[Metallurgical Laboratory]] at the University of Chicago, played important contributing roles. Overall scientific direction of the project was managed by the physicist [[J. Robert Oppenheimer]].

In July 1945, the first atomic explosive device, dubbed "The Gadget", was detonated in the New Mexico desert in the [[Trinity test|Trinity]] test. It was fueled by plutonium created at Hanford. In August 1945, two more atomic devices – "[[Little Boy]]", a uranium-235 bomb, and "[[Fat Man]]", a plutonium bomb – were [[atomic bombings of Hiroshima and Nagasaki|used against the Japanese cities of Hiroshima and Nagasaki]].

===Natural fission chain-reactors on Earth===
[[Natural nuclear fission reactor|Criticality in nature]] is uncommon. At three ore deposits at [[Oklo]] in [[Gabon]], sixteen sites (the so-called [[Oklo Fossil Reactors]]) have been discovered at which self-sustaining nuclear fission took place approximately 2&nbsp;billion years ago. French physicist [[Francis Perrin (physicist)|Francis Perrin]] discovered the Oklo Fossil Reactors in 1972, but it was postulated by [[Paul Kuroda]] in 1956.<ref>{{cite journal |author=P. K. Kuroda|doi=10.1063/1.1743058|title=On the Nuclear Physical Stability of the Uranium Minerals|year=1956|page=781|issue=4|volume=25|journal=The Journal of Chemical Physics|url=http://nuclearplanet.com/Kuroda%201956.pdf|bibcode= 1956JChPh..25..781K}}</ref> Large-scale natural uranium fission chain reactions, moderated by normal water, had occurred far in the past and would not be possible now. This ancient process was able to use normal water as a moderator only because 2&nbsp;billion years before the present, natural uranium was richer in the shorter-lived fissile isotope <sup>235</sup>U (about 3%), than natural uranium available today (which is only 0.7%, and must be enriched to 3% to be usable in light-water reactors).