 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 600 grams ( 21 oz ) of pure uranium @-@ 235 , a figure he subsequently revised to 575 grams ( 20 @.@ 3 oz ) . When the reactor went critical on May 9 , 1944 with 565 grams ( 19 @.@ 9 oz ) , the accuracy of Christy 's figures raised the laboratory 's confidence in T Division 's calculations . 
