Science Fair Project Encyclopedia
"Little Boy" was the codename given to the nuclear weapon dropped on Hiroshima, Japan on Monday, August 6, 1945. Little Boy was dropped from a B-29 Superfortress, the Enola Gay piloted by Lt. Col. Paul Tibbets, from about 31,000 feet (9450 m). The device exploded at approximately 8:15 a.m. (JST) when it reached an altitude of 1,800 ft (550 m).
It was the first of the only two nuclear weapons that have ever been used in warfare to date.
The Mk I "Little Boy" was 10 feet (3 m) in length, 28 inches (71 cm) wide and weighed 8,900 lb (4000 kg). The design used a gun arrangement to explosively force a sub-critical mass of uranium-235 and three U-235 target rings together into a super-critical mass, initiating a nuclear chain reaction. It contained 60 kg U-235, of which 1 kg underwent fission.
Approximately 70,000 people were killed as a direct result of the blast, and a similar number were injured. A great number more would later die as a result of nuclear fallout and cancer. Pregnant mothers lost their children, or they were born with deformations. Clothing was burned into the skin.
At the time there had never been a test explosion with this type of weapon. The only test explosion of a nuclear weapon was with the plutonium-type, on July 16, 1945 at the Trinity site. This was because tests of controlled nuclear reactions with U-235 (as opposed to the uncontrolled reaction that occurs in a bomb) had already been done, and the principles involved were so simple that it was taken to be unnecessary to test the weapon in advance. Furthermore, there was relatively little of the necessary uranium-235. The military was also anxious to drop the bomb, and testing the device would have delayed its use until more uranium was ready.
Although used occasionally in later experimental devices, the design was used only once as a weapon because of the extreme danger of a misfire. A simple crash could drive the "bullet" into the "target" and release lethal radiation doses or even a full nuclear detonation. The danger of misfire was even greater over water. Even if the force of a crash did not set the bomb off, if water entered the fail safe system, it would be shorted out, possibly leading to a detonation of the bomb. The British Red Beard nuclear weapon also suffered from this flaw.
It has been conjectured that German produced uranium may have been used in the production of the Little Boy or Fat Man bombs. The uranium was reportedly secured by Manhattan Project scientific director Robert Oppenheimer from the surrendering German submarine U-234. The German U-boat had been en route to deliver the uranium and other top secret German warfare technology to the then Empire of Japan (meaning the uranium had been intended for Japanese atomic bombs to get dropped on the US). However, U-234 surrendered when ordered to do so by German military authorities following the end of hostilities in the European war theatre after Germany's unconditional surrender. (Two Japanese military officials on board of the German vessel committed suicide and were buried at sea.) There are conflicting assessments of the importance of the German material to the Manhattan Project. The German uranium was likely only to have been non-enriched uranium oxide which would have yielded a comparatively small amount of bomb-grade material. Whatever the importance however, reports suggest that it does indeed seem relatively likely that some German nuclear bomb making material found its way into either of the said two U.S. atomic bombs (recent literature has claimed that uranium oxide recovered from Germany had certainly been used for the Soviet nuclear program).
- Description at Nuclear Weapons Archive
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