Little Boy


August 19, 2022

Little Boy was the code name of the Mk.1 bomb, the second atomic bomb built as part of the Manhattan Project and the first nuclear weapon in history to have been used in a conflict through the bombing of Hiroshima. during the last days of World War II. To make this bomb, uranium oxides of different quality and origin were used. Most of it was enriched at the Oak Ridge National Laboratory facilities in Tennessee, primarily by the gas diffusion method of uranium hexafluoride, marginally by other techniques.

Construction features

The Mk.1 was equipped with a conventional steel casing, 3 meters long and 0.71 meters in diameter, and weighed 4 037 kilograms. It was a "gun type" atomic bomb with fissile material consisting of highly enriched uranium. In the case of "Little Boy", the enriched uranium "bullet" weighed 38.53 kilograms and the target, likewise of enriched uranium, weighed 25.6 kilograms. The bomb therefore contained a total of 64.13 kilograms of uranium enriched at 80%, equal to 2.4 critical masses. The bullet consisted of a hollow cylinder made up of nine washers while the target was a cylinder made up of six washers with an internal hole diameter of 25.4 millimeters, i.e. the one necessary to contain a steel bar of the same diameter that would have constituted the "backbone" of super-critical mass. The target was completed on July 24, 1945 where the washers that would make up the bullet were cast between June 15, 1945 and July 3 of the same year. The barrel was an ordinary modified anti-aircraft gun barrel, 183 centimeters long, with an outer diameter of 165 millimeters and a caliber of 76 millimeters and capable of withstanding a pressure of 2 700 bar. The barrels were tested by firing two or three shots each with 90 kilogram bullets and at a speed of 300 meters per second. It had been calculated that the fissile activity of the critical mass of "Little Boy" lasted a total of 1.35 milliseconds and that the first part of which (lasting 0.5 milliseconds) occurred during the approach of the bullet to the target ( and therefore even before the union of the two sub-critical masses). If the chain reaction failed to start, some beryllium-polonium alloy "ABNER" initiators would have acted as a neutron source capable of externally initiating the reaction. The decision to add "ABNER" to the trigger mechanism was made by Robert Oppenheimer only on March 15, 1945. As a result, sixteen of these initiators were sent directly to the US military base on the island of Tinian (Mariana archipelago) in the 'Pacific Ocean, where the final assembly of the bomb was in progress, and four of them were first mounted in it.


The components of "Little Boy" were sent to the island of Tinian starting from May 1945, that is, even before the "Trinity test" had been carried out. The cruiser Indianapolis, departing on July 16, 1945 from San Francisco and arriving on the island ten days later, transported the parts to make up the body of the bomb and the bullet while three C-54 planes carried the target on July 28, 1945. The bomb (called "L11") was set up on July 31, 1945. Its use was originally planned first for August 1, 1945 and later for August 3, 1945 but bad weather conditions prevented the bomber from taking off on those days. On August 4, 1945, it was then scheduled to take off two days later and so the next day the bomb was loaded into the cargo hold of the United States Army Air Forces Boeing B-29-45-MO Superfortress strategic heavy bomber (serial number 44-86292). renamed with the pseudon