 Wigner led the Theoretical Group that included Creutz , Leo <unk> , Alvin M. Weinberg , <unk> Way and Gale Young . The group 's task was to design the reactors that would convert uranium into plutonium . At the time , reactors existed only on paper , and no reactor had yet gone critical . In July 1942 , Wigner chose a conservative 100 MW design , with a graphite neutron <unk> and water cooling . The choice of water as a coolant was controversial at the time because water was known to absorb neutrons , thereby reducing the efficiency of the reactor ; but Wigner was confident that his group 's calculations were correct and that water would work , while the technical difficulties involved in using helium or liquid metal as a coolant would delay the project . Working seven days a week , the group designed the reactors between September 1942 and January 1943 . Creutz studied the corrosion of metals in a water @-@ cooled system , and designed the cooling system . In 1959 a patent for the reactor design would be issued in the name of Creutz , <unk> , Weinberg , Wigner , and Young . 
