 In 1942 he joined the Manhattan Project at the University of Chicago , where he was recruited by Enrico Fermi to join the effort to build the first nuclear reactor , having been recommended as a theory resource by Oppenheimer . When Oppenheimer formed the Manhattan Project 's Los Alamos Laboratory in 1943 , Christy was one of the early recruits to join the Theory Group . Christy is generally credited with the insight that a solid sub @-@ critical mass of plutonium could be explosively compressed into supercriticality , a great simplification of earlier concepts of implosion requiring hollow shells . For this insight the solid @-@ core plutonium model is often referred to as the " Christy pit " . 
