Little Boy

Little boy

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a reworking of their unsuccessful Thin Man nuclear bomb. 

...However, by mid-1946, the Hanford Site reactors began suffering badly from the Wigner effect, the dislocation of atoms in a solid caused by neutron radiation, and plutonium became scarce, so six Little Boy assemblies were produced at Sandia Base.