Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > Types of Nuclear Weapons > Pure Fission Weapons

These are weapons that only use fission reactions as a source of energy. Fission bombs operate by rapidly assembling a subcritical configuration of fissile material (plutonium or enriched uranium) into one that is highly supercritical. The original atomic bombs tested in 16 July 1945 (device name: Gadget, test name: Trinity) and dropped on Japan in 6 August 1945 (Little Boy, over Hiroshima) and 9 August 1945 (Fat Man, over Nagasaki) were pure fission weapons.

These are the easiest nuclear weapons to design and manufacture, and the capability to do so is a prerequisite for developing any of the other weapon types. In addition to the five declared nuclear powers (the U.S., the USSR/Russia, Britain, France, and China) which have all acquired and tested these weapons, they have also been acquired by Israel, India, South Africa, and Pakistan. India has tested a fission bomb, while Israel and South Africa are suspected of having tested one.

There are practical limits to the size of pure fission bombs. Larger bombs require more fissionable material, which:

  1. becomes increasingly difficult to maintain as a subcritical mass before detonation and
  2. makes it harder to assemble into a high efficiency supercritical mass before stray neutrons cause predetonation.

Due to secrecy, and the boosting issue described below, it is somewhat difficult to identify the largest pure fission bomb ever tested for certain. It appears to have been the 500 kt Ivy King test by the U.S. (15 November 1952). The device exploded in this test was the Mk-18 Super Oralloy Bomb ("SOB") designed by a team led by Ted Taylor.


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