Little Boy

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 (9,450 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 two nuclear weapons that were ever used in warfare.

The Mk I "Little Boy" was 10 feet (3 m) in length, 28 inches (71 cm) wide and weighed 8,900 lb (4,035 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. The yield of "Little Boy" was about 13 kilotons of TNT equivalent in explosive force, i.e. 5.5 x 10¹³ joule = 55 TJ (terajoule). Around 75,000 people were killed as a direct result of the blast, though more died later as a result of fallout and cancer.

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.

See: Fat Man, Manhattan Project