During
World War II, Bachelder enlisted in the
Women's Army Corps (WAC) in November 1942, at the
Springfield, Massachusetts, headquarters. After spending time in training at military bases in several U.S. states, she received orders assigning her to the Company 'D' WAC Detachment of the
Manhattan District,
United States Army Corps of Engineers. Her secret assignment was to lead a group of 15 to 20 women from the WAC, stationed in
Des Moines, Iowa, to
Fort Sill, Oklahoma, and from there to
Santa Fe, New Mexico. She and the women under her command arrived at
Los Alamos, New Mexico, on October 21, 1943. Bachelder however reported to one more woman named Helen E. Mulvihill. Bachelder and the other women lived in a dorm. "Manhattan" was the code name for the special military division dedicated to developing an atomic weapon. In
the clandestine laboratory at the remote Los Alamos desert site, Bachelder was responsible for the analysis of the
spectroscopy of
uranium isotopes and discovering techniques for x-radiation. Since the
uranium-235 isotope is
fissile, whereas the
uranium-238 isotope is not, her role in the project was a crucial task: to ensure the purity of the sub-critical material, and therefore the
nuclear explosion, of the world's first atomic bombs. These methods were used during the preparation of
plutonium-239, the fissile material used in the construction of the atomic bomb for the
Trinity nuclear test, on July 16, 1945. Analogous methods were used for the uranium weapon, code-named "
Little Boy", which destroyed
Hiroshima, Japan on August 6, 1945, and for the plutonium bomb which
destroyed Nagasaki, Japan, on August 9, 1945, leading to the
Japanese surrender. The secret program was under the general direction of
J. Robert Oppenheimer, whom Bachelder described as: ==Contribution to post-war developments in nuclear energy==