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. For more information, see The Science Behind the Atom Bomb.

Little Boy

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.

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

Fat Man

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

Caption: Fat Man being lowered and checked on transport dolly for airfield trip

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 Composite Group

Efficiency of weapon: 10 times that of Little Boy

Approximately 1 kilogram of plutonium fissioned

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

Photos courtesy the National Archives and An illustrated guide to the Atomic Bombs by Ryan Crierie.