Little Boy and Fat Man



The United States detonated three nuclear devices during the Second World War II: the first was a test blast which took place on July 16, 1945; the other two devices were dropped on Japan.

The race for the atomic bomb is a remarkable story. Not only did the Americans design, produce and successfully deploy these weapons, they also built two entirely different types of bombs. The first to be dropped on Japan was the ‘Little Boy.’ It was a uranium device that was 10 ½ feet long with a 29 inch diameter. It weighed 9,700 lb and its explosive yield was the equivalent of 12,500 tons of TNT. The power of the ‘Little Boy’ was derived from the rare isotope – uranium 235. This illusive isotope had to be physically separated from the more plentiful uranium 238 isotope and took a vast amount of energy and technical ingenuity. The result being that when the bomb was dropped on Hiroshima it used all of the U-235 in existence. The ‘Little Boy’ was also a gun-type fission weapon and was so certain to work no test was carried out before its use – there was also not enough U-235 for such a test anyway. In order to work a mass of U-235 was simply shot down a conventional gun barrel into another mass of U-235. The critical mass created resulted in the second largest nuclear explosion created during World War II. Remarkably, the bomb’s design was known to be inefficient and prone to accidental discharge.

The other nuclear device tested (The Trinity Test blast on July 16, 1945, New Mexico) and dropped on Nagasaki was a plutonium bomb. The ‘Fat Man’ bomb was 10 feet 8 in long and 5 feet in diameter, weighed 10,000 lb with an explosive yield that was the equivalent of 22,000 tons of TNT. Originally designed to use the same gun type method used in the ‘Little Boy’, it was soon realized that the amount of energy released using plutonium-239 would in fact reduce the explosive yield of the weapon. In order to maximize the yield the idea of ‘implosion’ was adapted. By creating a perfectly spherical explosive wave and compressing a sphere of plutonium located in the middle of that wave, critical mass would occur followed by an explosion.