text: Enrico Fermi and his team from Columbia University arrived at the University of Chicago in January 1942 as part of an effort to concentrate the Manhattan Project's reactor work at the new Metallurgical Laboratory. Fermi arranged with Wigner for Christy to join his group, which was building a nuclear reactor, which Fermi called a "pile", in the squash court under Stagg Field at the University of Chicago. Construction began on November 6, 1942, and Christy was present when Chicago Pile-1 went critical on December 2.
  In early 1943, Christy joined Oppenheimer's Los Alamos Laboratory in New Mexico, where he became an American citizen in 1943 or 1944. Hans Bethe, the head of T (Theoretical) Division, detailed his physicists to assist with the projects at the laboratory. With his knowledge of reactors, Christy's assignment was to help Donald W. Kerst's Water Boiler  group. The Water Boiler was an aqueous homogeneous reactor intended as a laboratory instrument to test critical mass calculations and the effect of various tamper materials. It was the first reactor to use enriched uranium as a fuel, and the first to use liquid fuel in the form of soluble uranium sulfate dissolved in water. Christy estimated that it would require  of pure uranium-235, a figure he subsequently revised to . When the reactor went critical on May 9, 1944, with , the accuracy of Christy's figures raised the laboratory's confidence in T Division's calculations.