 Wigner led the Theoretical Group at the Metallurgical Laboratory that included Alvin Weinberg , Katharine Way , Gale Young and Edward Creutz . The group 's task was to design the production nuclear 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 moderator and water cooling . The choice of water as a coolant was controversial at the time . 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 coolants would delay the project . 
