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.