 As the reactors at Hanford came online , the Metallurgical Laboratory turned its attention back to theoretical designs . The discovery of spontaneous fission in reactor @-@ bred plutonium due to contamination by plutonium @-@ 240 led Wigner to propose switching to breeding uranium @-@ 233 from thorium , but the challenge was met by the Los Alamos Laboratory developing an implosion @-@ type nuclear weapon design . Wigner was also intrigued by the possibility of doing away with much of the complexities of a reactor by having the uranium in solution or a slurry in heavy water . The Metallurgical Laboratory attempted to find a way of doing this . 
