Nuclear — fission · United States · 1945
Little Boy
Little Boy (Mk I)
The first nuclear weapon used in war. A fission bomb using highly enriched uranium, dropped by a B-29 bomber.
See Little Boy in the 3D simulator- Yield
- 15 ktPublic estimate
- Energy
- 62.8 TJ
- Mass
- 4.4 tDocumented
- Dimensions
- 3 m · Ø 0.71 mDocumented
- Type
- Fission atomic bomb (uranium)
- Period
- World War II
- Status
- Historical
Key points
- The design was considered reliable enough to be used without a full prior test.
- Only a very small fraction of the fissile material actually fissioned (around 1 to 2% according to public estimates).
- Roughly 3,000 to 4,000 times less powerful than the Tsar Bomba.
History
Developed by the Manhattan Project, Little Boy was dropped on Hiroshima on 6 August 1945. Three days later, Fat Man was dropped on Nagasaki. These two bombings remain the only uses of nuclear weapons in armed conflict.
- 1942 The Manhattan Project begins.
- 1944–1945 Design and assembly at Los Alamos.
- 6 August 1945 Dropped on Hiroshima by the B-29 “Enola Gay”.
- 1945–1950 A few units built, then quickly retired in favor of more efficient designs.
How does it work?
A fission weapon releases energy by splitting heavy nuclei (uranium-235 or plutonium-239). Each fission releases about 200 MeV and a few neutrons.
If these neutrons cause further fissions, a chain reaction runs away in less than a millionth of a second.
The energy heats matter to tens of millions of degrees: fireball, thermal radiation and shock wave.
Little Boy used a so-called “gun-type” assembly: one piece of uranium was fired into another to briefly exceed the critical mass.
Deliberately general explanations: the application provides no design or manufacturing information.
Frequently asked questions
How powerful is Little Boy?
Little Boy has an estimated yield of 15 kt of TNT equivalent, about 62.8 TJ of energy.
What is the history of Little Boy?
Used once, on 6 August 1945 over Hiroshima. Developed by the Manhattan Project, Little Boy was dropped on Hiroshima on 6 August 1945. Three days later, Fat Man was dropped on Nagasaki. These two bombings remain the only uses of nuclear weapons in armed conflict.
What does the Little Boy explosion look like?
The Bomb Simulator shows the fireball, shock wave and cloud of Little Boy at real scale, over Paris, London or New York, with the sources and reliability of each value.
Sources
- Little Boy and Fat Man — Atomic Heritage Foundation / National Museum of Nuclear Science & History
- The Manhattan Project — The Atomic Bombing of Hiroshima — U.S. Department of Energy (OSTI)
- The Effects of Nuclear Weapons, 3rd ed. (Glasstone & Dolan, 1977) — U.S. DoD / DOE (OSTI)
Other weapons
- Trinity — 25 kt
- Fat Man — 21 kt
- RDS-1 — 22 kt
- Hurricane — 25 kt
- RDS-6s — 400 kt
- Gerboise Bleue — 70 kt
- 596 — 22 kt
- Davy Crockett — 20 t
- Ivy Mike — 10.4 Mt
- Castle Bravo — 15 Mt
- RDS-37 — 1.6 Mt
- Mk 17 — 15 Mt
This application is an educational tool. The animations and effect zones are simplified representations meant to illustrate orders of magnitude. They are neither a prediction of the effects of a real weapon nor a military planning tool.