X-10 Graphite Reactor
The X-10 Graphite Reactor is a decommissioned nuclear reactor at Oak Ridge National Laboratory in Oak Ridge, Tennessee. Formerly known as the Clinton Pile and X-10 Pile, it was the world's second artificial nuclear reactor and the first intended for continuous operation. It was built during World War II as part of the Manhattan Project.
While Chicago Pile-1 demonstrated the feasibility of nuclear reactors, the Manhattan Project's goal of producing enough plutonium for atomic bombs required reactors a thousand times as powerful, along with facilities to chemically separate the plutonium bred in the reactors from uranium and fission products. An intermediate step was considered prudent. The next step for the plutonium project, codenamed X-10, was the construction of a semiworks where techniques and procedures could be developed and training conducted. The centerpiece of this was the X-10 Graphite Reactor. It was air-cooled, used nuclear graphite as a neutron moderator, and pure natural uranium in metal form for fuel.
Using designs by the Metallurgical Laboratory, DuPont commenced construction of the plutonium semiworks at the Clinton Engineer Works in Oak Ridge on February 2, 1943. The reactor went critical on November 4, 1943, and produced its first plutonium in early 1944. The reactor and chemical separation plant provided invaluable experience for engineers, technicians, reactor operators, and safety officials who then moved on to the Hanford Site. It supplied the Los Alamos Laboratory with its first significant amounts of plutonium and its first reactor-bred product. Studies of these samples in comparison to those from cyclotrons revealed a higher content of plutonium-240, making the gun-type Thin Man design impossible, leading to the Gadget and Fat Man bombs of the now-ubiquitous implosion-type.
X-10 operated as a plutonium production plant until January 1945, when it was turned over to research activities and the production of radioactive isotopes for scientific, medical, industrial and agricultural uses. In August 1948, it became the first nuclear reactor to produce electricity, lighting a single bulb. It was shut down in 1963 and was designated a National Historic Landmark in 1965.
Origins
The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, followed by its theoretical explanation by Lise Meitner and Otto Frisch, opened up the possibility of a controlled nuclear chain reaction with uranium. At Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be done. Szilard drafted a confidential letter to the President of the United States, Franklin D. Roosevelt, explaining the possibility of atomic bombs, and warning of the danger of a German nuclear weapon project. He convinced his old friend and collaborator Albert Einstein to co-sign it, lending his fame to the proposal. This resulted in support by the U.S. government for research into nuclear fission, which became the Manhattan Project.In April 1941, the National Defense Research Committee asked Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program. His report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium. In October he wrote another report on the practicality of an atomic bomb. Niels Bohr and John Wheeler had theorized that heavy isotopes with even atomic numbers and odd number of neutrons were fissile. If so, then plutonium-239 was likely to be fissile.
Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch cyclotron there in May 1941 and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time plutonium-239 had been produced in minute quantities using cyclotrons, but it was not possible to produce large quantities that way. Compton discussed with Eugene Wigner from Princeton University how plutonium might be produced in a nuclear reactor, and with Robert Serber how the plutonium produced in a reactor might be separated from uranium.
The final draft of Compton's November 1941 report made no mention of using plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible. In December, Compton was placed in charge of the plutonium project, which was codenamed X-10. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built. He felt that having teams at Columbia, Princeton, the University of Chicago and the University of California was creating too much duplication and not enough collaboration, and he concentrated the work at the Metallurgical Laboratory at the University of Chicago.
Site selection
By June 1942, the Manhattan Project had reached the stage where the construction of production facilities could be contemplated. On June 25, 1942, the Office of Scientific Research and Development S-1 Executive Committee deliberated on where they should be located. Moving directly to a megawatt production plant looked like a big step, given that many industrial processes do not easily scale from the laboratory to production size. An intermediate step of building a pilot plant was considered prudent. For the pilot plutonium separation plant, a site was wanted close to the Metallurgical Laboratory, where the research was being carried out, but for reasons of safety and security, it was not desirable to locate the facilities in a densely populated area like Chicago.Compton selected a site in the Argonne Forest, part of the Forest Preserve District of Cook County, about southwest of Chicago. The full-scale production facilities would be co-located with other Manhattan Project facilities at a still more remote location in Tennessee. Some of land was leased from Cook County for the pilot facilities, while an site for the production facilities was selected at Oak Ridge, Tennessee. By the S-1 Executive Committee meeting on September 13 and 14, it had become apparent that the pilot facilities would be too extensive for the Argonne site, so instead a research reactor would be built at Argonne, while the plutonium pilot facilities would be built at the Clinton Engineer Works in Tennessee.
The Oak Ridge site was selected on the basis of several criteria. The plutonium pilot facilities needed to be from the site boundary and any other installation, in case radioactive fission products escaped. While security and safety concerns suggested a remote site, it still needed to be near sources of labor, and accessible by road and rail transportation. A mild climate that allowed construction to proceed throughout the year was desirable. Terrain separated by ridges would reduce the impact of accidental explosions, but they could not be so steep as to complicate construction. The substratum needed to be firm enough to provide good foundations but not so rocky that it would hinder excavation work. It needed large amounts of electrical power and cooling water. Finally, a War Department policy held that, as a rule, munitions facilities should not be located west of the Sierra or Cascade Ranges, east of the Appalachian Mountains, or within of the Canadian or Mexican borders.
In December, it was decided that the plutonium production facilities would not be built at Oak Ridge after all, but at the even more remote Hanford Site in Washington state. Compton and the staff at the Metallurgical Laboratory then reopened the question of building the plutonium semiworks at Argonne, but the engineers and management of DuPont, particularly Roger Williams, did not support this proposal. They felt that there would be insufficient space at Argonne and that there were disadvantages in having a site that was so accessible, as they were afraid that it would permit the research staff from the Metallurgical Laboratory to interfere unduly with the design and construction, which they considered their prerogative. A better location, they felt, would be with the remote production facilities at Hanford. In the end a compromise was reached. On January 12, 1943, Compton, Williams, and Brigadier General Leslie R. Groves, Jr., the director of the Manhattan Project, agreed that the semiworks would be built at the Clinton Engineer Works.
Both Compton and Groves proposed that DuPont operate the semiworks. Williams counter-proposed that the semiworks be operated by the Metallurgical Laboratory. He reasoned that it would primarily be a research and educational facility, and that expertise was to be found at the Metallurgical Laboratory. Compton was shocked; the Metallurgical Laboratory was part of the University of Chicago, and therefore the university would be operating an industrial facility from its main campus. James B. Conant told him that Harvard University "wouldn't touch it with a ten-foot pole", but the University of Chicago's Vice President, Emery T. Filbey, took a different view and instructed Compton to accept. When University President Robert Hutchins returned, he greeted Compton with "I see, Arthur, that while I was gone you doubled the size of my university".
Design
The fundamental design decisions in building a reactor are the choice of fuel, coolant and neutron moderator. The choice of fuel was straightforward; only natural uranium was available. The decision that the reactor would use graphite as a neutron moderator caused little debate. Although with heavy water as moderator the number of neutrons produced for every one absorbed was 10 percent more than in the purest graphite, heavy water would be unavailable in sufficient quantities for at least a year. This left the choice of coolant, over which there was much discussion. A limiting factor was that the fuel slugs would be clad in aluminum, so the operating temperature of the reactor could not exceed . The theoretical physicists in Wigner's group at the Metallurgical Laboratory developed several designs. In November 1942, the DuPont engineers chose helium gas as the coolant for the production plant, mainly on the basis that it did not absorb neutrons but also because it was inert, which removed the issue of corrosion.Not everyone agreed with the decision to use helium. Szilard, in particular, was an early proponent of using liquid bismuth; but the major opponent was Wigner, who argued forcefully in favor of a water-cooled reactor design. He realized that since water absorbed neutrons, k would be reduced by about 3 percent, but he had sufficient confidence in his calculations that the water-cooled reactor would still be able to achieve criticality. From an engineering perspective, a water-cooled design was straightforward to design and build, while helium posed technological problems. Wigner's team produced a preliminary report on water cooling, designated CE-140 in April 1942, followed by a more detailed one, CE-197, titled "On a Plant with Water Cooling", in July 1942.
Fermi's Chicago Pile-1 reactor, constructed under the west viewing stands of the original Stagg Field at the University of Chicago, "went critical" on December 2, 1942. This graphite-moderated reactor only generated up to 200 W, but it demonstrated that k was higher than anticipated. This removed most of the objections to air-cooled and water-cooled reactor designs, and it greatly simplified other aspects of the design. Wigner's team submitted blueprints of a water-cooled reactor to DuPont in January 1943. By this time, the concerns of DuPont's engineers about the corrosiveness of water had been overcome by the mounting difficulties of using helium, and all work on helium was terminated in February. At the same time, air cooling was chosen for the reactor at the pilot plant. Since it would be of a quite different design from the production reactors, the X-10 Graphite Reactor lost its value as a prototype, but its value as a working pilot facility remained, providing plutonium needed for research. It was hoped that problems would be found in time to deal with them in the production plants. The semiworks would also be used for training and for developing procedures.