Since June 1944 Los Alamos Laboratory had begun receiving highly enriched uranium from the Y-12 laboratory located in Oak Ridge, Tennessee. The large quantities of uranium necessary to extract the fissile material was obtained mainly from the Shinkolobwe mine, located in the Congo Republic, although part of it was also obtained through the “Alsos Mission” carried out in Europe and through the capture of the German U-234 submarine carrying 559 kg of uranium oxide bound for Japan. Although the first operational bomb was intended to be made of plutonium-239, it was discovered that enriching this material created impurities in the form of plutonium-240 isotopes that were extremely unstable and capable of detonating apart before creating the chain reaction, thereby that bomb would not work properly.After problems with the use of plutonium-239 as fissile material, it was then decided to use the same “gun” trigger system to start the chain reaction but with uranium-235 fissile material. The trigger system consisted of firing a sub-critical mass of enriched uranium-235 in hollow cylinder shape, (the “bullet”), against a kind of enriched uranium-235 firing pin, (the “target spike”), through a 170mm caliber artillery barrel to achieve supercritical mass inside the bomb. In the case of the Little Boy bomb, the bullet weighed 38.5 kg and the target spike weighed 25.6 kg. The bullet was propelled by a cordite charge at a speed of about 300 m/s by the barrel of 1.80 meters long, and in the absence of about 25 cm for contact, both masses became critical causing neutrons to begin the chain reaction even before joining both masses (bullet and target spike).In February 1945, the design of the bomb had been completed and the manufacture of the different components began in three different places, so that none of them would have all blueprints with which to manufacture a complete bomb. The plants commissioned were the Naval Gun Factory in Washington D.C., the Naval Ordnance Plant in Center Line, Michigan and the Expert Tool and Die Company in Detroit, Michigan. The bombs (without the uranium payload) were finished in May, but the fissile material was not finished until June (bullet) and July (target spike). From July 30, all components were ready to be assembled and form a fully operational bomb.Eight bomb casings were built, of which 4 were launched from B-29 bombers to study the dynamic behavior of the bomb and to test the components. Safety and emergency ground tests and loading and unloading procedures were performed, however, the most important system of the bomb, the chain reaction initiation system (gun type), was not. The main reason is that there was not enough enriched uranium for such a test and it was also considered that the start-up system was simple enough to be certain that it would work after performing only a laboratory test. Due to this lack of live testing, there was some concern that bomb could explode if the aircraft bomber were to crash or be shot down. While the odds of a nuclear explosion were quite remote, it was likely to cause nuclear contamination if something went wrong.The Little Boy bomb weighed 4.4 tons and contained 64.1 kg of 80% enriched uranium, and only one kg of this material would end up carrying out the nuclear fission. Using only 1.7% of the fissile material, the bomb was considered very inefficient, in military terms. The uranium-235 “bullet” consisted of nine 159mm diameter, 100mm bore rings compressed into a 413mm long projectile partially filled with a carbide tungsten disc. When the “bullet” was fired by the cordite charge and hit the “target spike”, the chain reaction occurred completely surrounded by a tamper and neutron reflector of tungsten carbide and steel. After the impact of both parts, the neutron initiators formed of polonium-beryllium were activated helping the chain reaction.The bomb had several fuzes that activated the trigger system that generated the explosion. It was calculated that the bomb would obtain the best destructive result if it exploded at about 580 meters above the target. There was a time fuze to ensure that the bomb did not explode within 15 seconds from the launch. It also carried a barometric type fuze that would activate a radar altimeter firing command circuit at an altitude of 2,000 meters. Once these radar altimeters were activated, the ignition of the cordite charge, (composed by four bags of 900 grams each), was initiated, which launched the uranium bullet against the target spike. After 10 milliseconds, the chain reaction occurred, lasting no more than one microsecond (0.000001 s), and the full destructive power of the bomb was released.Finally, on August 6, 1945, the trigger system and the uranium charge were mounted inside the L-11 casing and Little Boy bomb was installed in the B-29 bomber named Enola Gay. The 4 cordite bags were installed inside the bomb during the flight to the target, as an accidental explosion was feared if the aircraft crashed during takeoff. After inserting the cordite and activating several connectors and wires, the bomb was armed and ready for launch. This occurred over the Japanese city of Hiroshima at 08:15.17, and 44.4 seconds after leaving the bomber at about 600 meters above the ground the bomb exploded with a power equivalent to 15,000 tonnes of TNT explosive (15 kilotons). The result, 66,000 people killed by a single bomb.
