The Manhattan Project Heritage Preservation Association, Inc.

"Preserving, Exhibiting, Interpreting and Teaching the History of the Manhattan Project"


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"Fat Man" 

Above photos courtesy Bradbury Science Museum - "Click" on photo for enlargement

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"Fat Man" was the second plutonium, implosion-type bomb.  The first was the "gadget" detonated at the Trinity site on July 16, 1945.

Keep in mind that an explosive nuclear chain reaction occurs when a sufficient quantity of nuclear fuel, such as uranium or plutonium, is brought together to form a critical mass.  This is the minimum amount of fissionable material needed to start a chain reaction.  The chain reaction starts when neutrons strike the heavy uranium or plutonium nucleus which splits releasing a tremendous amount of energy along with two or more neutrons which, in turn split more nuclei, and so on.  In the implosion-type device, a core of sub-critical plutonium is surrounded by several thousand pounds of high-explosive designed in such a way that the explosive force of the HE is directed inwards thereby crushing the plutonium core into a super-critical state.

Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war.  Following are some approximate statistics for Fat Man.  If you require more extensive information on this weapon, please contact us via feedback:

  • Weight: 10,800 lbs
  • Length: 10 ft 8 in.; Diameter: 60 in.
  • Fuel:  Highly enriched plutonium 239
  • Plutonium Fuel: approx. 13.6 lbs; approx. size of a softball
  • Plutonium core surrounded by 5,300 lbs of high explosives; plutonium core reduced to size of tennis ball
  • Bomb Initiator:  Beryllium - Polonium
  • All components of Fat Man ferried to Tinian Island aboard B-29's of the 509th CG
  • Efficiency of weapon: 10 times that of Little Boy
  • Approx 1.176 Kilograms of plutonium converted to energy
  • Explosive force: 21,000 tons of TNT equivalent
  • Use:  Dropped on Japanese city of Nagasaki; August 9, 1945
  • Nuclear Weaponeer: Cdr. Frederick Ashworth
  • Delivery:  B-29 Bockscar piloted by Maj. Charles Sweeney

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