Little Boy - Development of The Bomb

Development of The Bomb

The "Little Boy" bomb was constructed through the Manhattan Project during World War II. Because enriched uranium was known to be fissionable, it was the first approach to bomb development pursued. The vast majority of the work in constructing "Little Boy" came in the form of the isotope enrichment of the uranium necessary for the weapon. Enrichment at Oak Ridge, Tennessee began in February 1943, after many years of research.

The development of the first prototypes and the experimental work started in early 1943, at the time when the Los Alamos Design Laboratory became operational in the framework of the Manhattan Project. Originally gun-type designs were pursued for both a uranium and plutonium weapon (the "Thin Man" design), but in April 1944 it was discovered that the spontaneous fission rate for plutonium was too great to use in a gun-type weapon. In July 1944, almost all research at Los Alamos was redirected to the implosion plutonium weapon. In contrast, the uranium bomb was straightforward if not trivial to design.

With plutonium found unsuitable for the gun-type design, the team working on the gun weapon (led by A. Francis Birch), faced another problem: the bomb was simple, but they lacked the quantity of uranium-235 necessary for its production. Enough fissile material was not going to be available before mid-1945. Despite this, Birch managed to convince others that this concept was worth pursuing, so that in case of a failure of the plutonium bomb, it would still be possible to use the gun principle. In February 1945, the specifications were completed (model 1850). The bomb, except for the uranium payload, was ready at the beginning of May 1945.

Most of the uranium necessary for the production of the bomb came from the Shinkolobwe mine and was made available thanks to the foresight of the CEO of the High Katanga Mining Union, Edgar Sengier, who had 1000 tons of uranium ore transported to a New York warehouse in 1939. A small amount may have come from a captured German submarine, U-234, after the German surrender in May 1945. Other sources state that at least part of the 1100 tons of uranium ore and uranium oxide captured by US troops in the second half of April 1945 in Stassfurt, Germany, became 235U for the bomb. The majority of the uranium for Little Boy was enriched in Oak Ridge, Tennessee, primarily by means of electromagnetic separation in calutrons and through gaseous diffusion plants, with a small amount contributed by the cyclotrons at Ernest O. Lawrence's Radiation Laboratory. The core of Little Boy contained 64 kg of uranium, of which 50 kg was enriched to 88%, and the remaining 14 kg at 50%, resulting in an average enrichment of 79.68%.