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Nuclear graphite is any grade of graphite, usually electro-graphite, specifically manufactured for use as a moderator or reflector within a nuclear reactor. Graphite is an important material for the construction of both historical and modern nuclear reactors, due to its extreme purity and its ability to withstand extremely high temperatures. ==History== The potential for creating a chain reaction in uranium became apparent in 1939 following the nuclear fission experiments of Otto Hahn and Fritz Strassman, and the interpretation of these results by Lise Meitner and Otto Frisch. The exciting possibilities that this presented rapidly spread throughout the world physics community. In order for the fission process to chain react, the neutrons created by uranium fission must be slowed down by interacting with a neutron moderator (an element with a low atomic weight, that will "bounce", when hit by a neutron) before they will be captured by other uranium atoms. It became well known by late 1939 that the two most promising moderators were heavy water and graphite. In February 1940, using funds that were allocated partly as a result of the Einstein-Szilard letter to President Roosevelt, Leo Szilard purchased several tons of graphite from the Speer Carbon Company and from the National Carbon Company (the National Carbon Division of the Union Carbide and Carbon Corporation in Cleveland Ohio) for use in Enrico Fermi's first fission experiments, the so-called exponential pile. Fermi writes that "The results of this experiment was () somewhat discouraging" presumably due to the absorption of neutrons by some unknown impurity. So, in December 1940 Fermi and Szilard met with Herbert G. MacPherson and V. C. Hamister at National Carbon to discuss the possible existence of impurities in graphite. During this conversation it became clear that minute quantities of boron impurities were the source of the problem.〔 As a result of this meeting, over the next two years, MacPherson and Hamister developed thermal and gas extraction purification techniques at National Carbon for the production of boron-free graphite.〔 The resulting product was designated AGOT Graphite ("Atcheson Graphite Ordinary Temperature") by National Carbon, and it was "the first true nuclear grade graphite". During this period, Fermi and Szilard purchased graphite from several manufacturers with various degrees of neutron absorption cross section: AGX graphite from National Carbon Company with 6.68 mb (millibarns) cross section, US graphite from United States Graphite Company with 6.38 mb cross section, Speer graphite from the Speer Carbon Company with 5.51 mb cross section, and when it became available, AGOT graphite from National Carbon, with 4.97 mb cross section.〔 (See also Haag ().) By November 1942 National Carbon had shipped 250 tons of AGOT graphite to the University of Chicago〔 where it became the primary source of graphite to be used in the construction of Fermi's Chicago Pile-1, the first nuclear reactor to generate a sustained chain reaction (December 2, 1942).〔 AGOT graphite was used to build the X-10 graphite reactor in Oak Ridge TN (early 1943) and the first reactors at the Hanford Site in Washington (mid 1943),〔 for the production of plutonium during and after World War II.〔〔 The AGOT process and its later refinements became standard techniques in the manufacture of nuclear graphite.〔 The cross section of graphite was also investigated during the second world war in Germany by Walter Bothe, P. Jensen, and Werner Heisenberg. The purest graphite available to them was a product from the Siemens Plania company, which exhibited a neutron absorption cross section of about 6.4 mb〔 to 7.5 mb (Haag 2005). Heisenberg therefore decided that graphite would be unsuitable as a moderator in a reactor design using natural uranium, due to this apparently high rate of neutron absorption.〔 Consequently, the German effort to create a chain reaction involved attempts to use heavy water, an expensive and scarce alternative, made all the more difficult to acquire as a consequence of the Norwegian heavy water sabotage by Norwegian and Allied forces. Writing as late as 1947, Heisenberg still did not understand that the only problem with graphite was the boron impurities.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nuclear graphite」の詳細全文を読む スポンサード リンク
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