Little Boy - Basic Weapon Design

Basic Weapon Design

The Mk I "Little Boy" was 120 inches (300 cm) in length, 28 inches (71 cm) in diameter and weighed approximately 9,700 pounds (4,400 kg). The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target cylinder together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of chemical explosives. It contained 64 kg (140 lb) of uranium, of which less than a kilogram underwent nuclear fission, and of this mass only 0.6 g (0.021 oz) was transformed into a different type of energy (initially kinetic energy, then heat and light).

No full test of a gun-type nuclear weapon had occurred before the "Little Boy" device was dropped over Hiroshima. The only test explosion of a nuclear weapon concept had been of an implosion-type device employing plutonium as its fissionable material, and took place on July 16, 1945 at the Trinity test. There were several reasons for not testing a "Little Boy" type of device. Primarily, there was little uranium-235 as compared with the relatively large amount of plutonium which, it was expected, could be produced by the Hanford reactors. Additionally, the weapon design was simple enough that it was only deemed necessary to do laboratory tests with the gun-type assembly. Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work.

Although occasionally used in later experimental devices, the gun design was only used once as a weapon because of the danger of accidental detonation. Little Boy's design is considered unsafe when compared to modern nuclear weapons, which incorporate safety features to endure various accident scenarios. The objective of Little Boy was to create a weapon that was absolutely guaranteed to work. Consequently, Little Boy incorporated only basic safety mechanisms, thus an accidental detonation could easily occur during one or more of the following scenarios:

• A crash could drive the hollow "bullet" onto the "target" cylinder resulting in a massive release of radiation, or possibly nuclear detonation.
• An electrical short circuit of some sort.
• The danger of a misfire was greater over water. If the force of a crash did not trigger the bomb, water leakage into the system could short it out, possibly leading to detonation. The British Red Beard nuclear weapon also suffered from this design flaw.
• Furthermore if immersed in water, the uranium halves were subject to a moderator-effect of the liquid.
• Fire.
• Lightning strike.