William Stewart was an engineer in the Special Engineering Detachment at Los Alamos, New Mexico. Specifically, Stewart worked in “Project Trinity” under the leadership of George Kistiakowsky and Kenneth Bainbridge. Stewart helped set up the Trinity test of the plutonium implosion weapon and witnessed the first atomic explosion in history on July 16, 1945.
He was discharged in March of 1946 and hired as a civilian (via the University of California, Berekley) to go to the Bikini Atoll for work with Operation Crossroads, the first series of postwar atomic tests.