 On April 19 , 1942 , Seaborg reached Chicago and joined the chemistry group at the Metallurgical Laboratory of the Manhattan Project at the University of Chicago , where Enrico Fermi and his group would later convert uranium @-@ 238 to plutonium @-@ 239 in a controlled nuclear chain reaction . Seaborg 's role was to figure out how to extract the tiny bit of plutonium from the mass of uranium . Plutonium @-@ 239 was isolated in visible amounts using a transmutation reaction on August 20 , 1942 , and weighed on September 10 , 1942 , in Seaborg 's Chicago laboratory . He was responsible for the multi @-@ stage chemical process that separated , concentrated and isolated plutonium . This process was further developed at the Clinton Engineering Works in Oak Ridge , Tennessee , and then entered full @-@ scale production at the Hanford Engineer Works , in Richland , Washington . 
