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The now S-1 Committee held an urgent meeting on the 18th of December of 1941 provoked by the Attack on Pearl Harbour and the subsequent declaration of war on Japan and Germany by the United States. Work was proceeding on three different techniques for isotope separation to separate uranium-235 from the more abundant uranium-238. The physicists Eger Murphree and Jesse Wakefield Beam's team investigated gaseous diffusion at the Columbia University, while Lawrence with his team looked into electromagnetic separation and Philip Abelson's team researched thermal fusion. 

At the same time, two lines if research into nuclear reactor technology were taking place, with Harold Urey continuing research into Heavy Water (which is needed for certain types of nuclear reactors). On the 23rd of May, 1942, Briggs, Compton, Lawrence, Murphree and Urey met to finalize the S-1 Committee recommendations, which called for all those technologies to be pursued. This was approved by Bush and Conant, as well as Brigadier General Wilhelm D. Styler, who then took the recommendation to the Top Policy Group, proposing a budget of around $54 million for the construction by the United Sates Army Corps of Engineers, $31 million for research and development by OSRD and additional $5 million for contingencies that may arise. The Top Policy Group then proceeded to send this recommendation to Roosevelt, who approved it on the 17th of June, 1942.

Compton needed someone to take over research into fast neutron calculations (the key to calculations of critical mass and weapon detonation) as the previous researcher, Gregory Breit, had quit on the 18th of May of 1942. The man he found to replace his was J. Robert Oppenheimer of the University of California. Oppenheim would come to be one of, if not the, most important people in the Manhattan Project. John H. Manley, a physicist at Metallurgical Laboratory, was assigned to assist Oppenheimer by contacting and coordinating experimental physic groups all across the country. Oppenheim and the physicist Robert Serber of the University of Illinois researched the problems of how neutrons moved in a nuclear chain reaction as well as how the explosion produced by a chain reaction may theoretically behave. After a lot of work in this area, it was uncertainly confirmed that a fission bomb is theoretically possible to create.

Despite some success in the research, there were still many unknown factors, such as the properties of pure Uranium-235 as well as plutonium. In July of 1942, the scientists at the Berkeley Conference suggested creating plutonium in nuclear reactors where uranium-238 atoms absorbed neutrons that had been emitted from the fission of uranium-235 atoms. While the suggestion was interesting, no reactor had been actually built at this point, and only small amounts of plutonium were available. Even by the end of 1943, only two milligrams had been produced. Many ways of arranging the fissile materials into a critical mass were an option. The simplest one was shooting a 'cylindrical plug' into a sphere of active material with a dense material that would focus neutrons inward and keep the reacting mass together to increase its efficiency, which would be perfect for a bomb. Scientists also explored designs involving spheroids, a primitive form of 'implosion' which was suggested by the physicist Richard C. Tolman as well the possibility of autocatalytic methods (if one of the reaction products is also a catalyst for the same or a coupled reaction), which would significantly increase the efficiency of the bomb when it would explode. 



With the idea of a fission bomb theoretically settled, the 1942 conference turned in a different direction. Edward Teller started pushing for a discussion for a theoretically even more powerful bomb, at the time referred to as the super bomb (today more commonly referred to as a hydrogen bomb), which would use the explosive force of a detonating fission bomb to ignite a a nuclear fusion reaction in deuterium and tritium. He proposed scheme after scheme, but each one was refused, and so the idea was put aside to concentrate on producing fission bombs. One concern that was raised by Teller is the possibility that an atomic bomb might quite literally ignite the atmosphere in an unstoppable reaction, which would have devastating effects for Earth. 

With that, the next step was to organize the building of the bomb.



Attack on Pearl Harbour (December 7th, 1941)
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J. Robert Oppenheimer
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Different fission bomb assembly options that were explored during the July 1942 conference
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