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 this gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical.  The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins.

Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war.  Following are some approximate statistics for Little Boy.  If you require more extensive information on this weapon, please contact us via feedback:

• Weight: 9,700 lbs
• Length: 10 ft.; Diameter: 28 in.
• Fuel:  Highly enriched uranium; "Oralloy"
• Uranium Fuel: approx. 140 lbs; target - 85 lbs and projectile - 55 lbs
• Target case, barrel, uranium projectile, and other main parts ferried to Tinian Island via USS Indianapolis
• Uranium target component ferried to Tinian via C-54 aircraft of the 509th Composite Group
• Efficiency of weapon: poor
• Approx. 1.38% of the uranium fuel actually fissioned
• Explosive force: 15,000 tons of TNT equivalent
• Use:  Dropped on Japanese city of Hiroshima; August 6, 1945
• Delivery:  B-29 Enola Gay piloted by Col. Paul Tibbets

"Click" here to view "Fat Man" page