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. The first was the 'Gadget' detonated at the Trinity site on July 16, 1945. 'Fat Man' was the second plutonium, implosion-type bomb. Delivery: B-29 Enola Gay piloted by Col.Use: Dropped on Japanese city of Hiroshima August 6, 1945.Explosive force: 15,000 tons of TNT equivalent.1.38% of the uranium fuel actually fissioned Uranium target component ferried to Tinian via C-54 aircraft of the 509th Composite Group.Target case, barrel, uranium projectile, and other main parts ferried to Tinian Island via USS Indianapolis.140 lbs target - 85 lbs and projectile - 55 lbs Fuel: Highly enriched uranium 'Oralloy'.Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins. 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. For more information, see The Science Behind the Atom Bomb. 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. This is the minimum amount of fissionable material needed to start a chain reaction.

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.

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