Science
EXPERIMENTAL REACTORS
Science > Particle Accelerators and Other Technologies
A reactor is an assembly of nuclear fuel capable of sustaining a controlled nuclear chain reaction. It can also be referred to as a "pile," because early reactors such as CP-1 resembled piles of fissionable material. Experimental nuclear reactors including CP-1, CP-2, CP-3, X-10, and the water boilers at Los Alamos played a central role in the science of the Manhattan Project. Reactors at Hanford were geared towards the production of plutonium, but experimental reactors at Chicago, Argonne, Oak Ridge and Los Alamos generated scientific data on the physics of fission reactions, the operation of reactors, and produced material for use in various experiments. Experimental reactors, for example, allowed scientists to conduct critical mass calculations, and measure cross sections for fission reactions, for neutron capture and for scattering in a variety of materials, particularly those under consideration as moderators and tampers. 
During the war, the first experimental reactors took root around the Chicago Met Lab. CP-1 (for "Chicago Pile-1"), a construction of uranium and graphite bricks assembled under a playing field at the University of Chicago, became the world's first self-sustaining chain reaction by 1942. However, the effort leading up to CP-1 appeared, at times, uncertain. Shortages of graphite and uranium delayed construction by mid-1942, but by the end of the year, on Wednesday, December 2, 1942, CP-1 went critical. Experiments continued over the months that followed with the aim of acquiring data on the properties of fission. The reactor was generally operated at the relatively low power level of only half a watt, reaching up as high as 200 watts for brief intervals. In the spring of 1943, scientists at Chicago dismantled CP-1 and relocated and reconstructed it outside Chicago at what would become Argonne National Laboratory. This new and improved reactor was re-named CP-2. The second-generation reactor was built with radiation shielding, improved controls to permit more powerful routine operation (capable of operating at a power level about 10 times larger than CP-1), and improved experimental facilities. CP-2 was dismantled in 1954. Scientists completed construction on the third generation experimental reactor CP-3, in 1944. Like CP-2, CP-3 was used to gather experimental data in support of the Manhattan Project, and to further scientific knowledge of reactor physics. As originally built, CP-3 was fueled with natural uranium and operated at a power level of 300 kilowatts. In 1950 CP-3 started using enriched uranium for fuel; it was retired in 1955. 
At Oak Ridge, DuPont engineers sought to construct a smaller, air-cooled experimental reactor before they constructed the full-scale plutonium production piles (reactors) at the Hanford Engineer Works. This smaller experimental reactor could offer useful practice at the operation of industrial-sized piles, as well as potentially produce early samples of plutonium for experimentation at the bomb laboratory at Los Alamos. The result was the X-10 Graphite Reactor. Los Alamos scientists dubbed their experimental reactors "water boilers" to hide the true purpose of the machines. Enrico Fermi, who had already developed a special expertise at working on reactors through his work at Chicago, oversaw the Los Alamos reactors. Two Los Alamos reactors went critical during the war; their operation influenced design decisions and aided the development of the bomb. The first reactor, constructed in 1943 and critical the following year, ran on liquid enriched uranium fuel, and due to its very low power output took the name LOPO (low-power). LOPO involved many important physicists in its design and operation, including Fermi and Richard Feynman. The LOPO reactor successfully contributed to the determination of critical mass and allowed for the testing of a new reactor concept. The second reactor at Los Alamos became operational in 1944 and, as it could be operated at power levels up to 5.5 kilowatts (about fifty times more powerful than CP-1), took the name HYPO (high-power). HYPO generated neutrons the Laboratory needed for cross-section measurements and other studies. After the war, a third-generation water boiler at Los Alamos, SUPO (super-power) operated at a much higher power and produced many more neutrons for experimental purposes.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. A very useful discussion of experimental reactors of the Manhattan Project is, "Early Reactors: From Fermi's Water Boiler to Novel Power Prototypes," Los Alamos Science Winter/Spring 1983, by Merle E. Bunker. On reactor design and construction at Oak Ridge see Swords to Plowshares: A Short History of Oak Ridge National Laboratory (osti.gov). See also, Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972); and F. G. Gosling, The Manhattan Project: Making the Atomic Bomb (DOE/MA-0001; Washington: History Division, Department of Energy, January 1999). The drawing of CP-1 is courtesy the National Archives. The photograph of CP-2 is courtesy the Argonne National Laboratory. The photograph of X-10 is courtesy the Oak Ridge National Laboratory.