Walter Zinn (1907 – 2000) a Canadian physicist who, on December 2, 1942, pulled out the emergency control rod from the reactor at the University of Chicago that released the world’s first self-sustaining nuclear reaction. Zinn reinserted it to terminate the chain reaction after 28 minutes of operation. Recruited by Enrico Fermi to work on the Manhattan Project, Zinn supervised all phases of the construction of the first experimental nuclear reactor, or "atomic pile" as it was then called. He also served as the first director of Argonne National Laboratory from 1946-1956. Zinn designed the Experimental Breeder Reactor-I, the first nuclear reactor to produce electric power (Dec. 20, 1951) in Idaho at the National Reactor Testing Site. EBR-I is now a National Historic Landmark. It also was the first nuclear reactor to demonstrate the breeding principle (that reactors can generate more nuclear fuel than they consume). In 1963, the EBR-I was the first nuclear reactor to achieve a chain reaction with plutonium as well as the first to demonstrate the feasibility of using liquid metals at high temperatures as a reactor coolant.

Further Readings
Walter Zinn 1907-2000 (Canadian Nuclear Society)
Walter Henry Zinn (National Academy of Sciences)