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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 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 (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. The yield of "Little Boy" was about 13 kilotons of TNT equivalent
in explosive force, i.e. 5.5×1013 joule = 55 TJ (terajoule). Approximately
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. The military were also anxious to drop the bomb, and
testing the device would have delayed its use until more uranium was ready.
Although used occaionally 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.
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