gun-type fission weapon

gun-type fission weapon. A gun-type fission weapon is a nuclear weapon design that uses conventional artillery principles to initiate a nuclear chain reaction. It involves firing one sub-critical mass of fissile material into another to form a super-critical mass, leading to an explosive release of energy. This relatively simple design was the first type of atomic bomb ever used in warfare, exemplified by the Little Boy bomb dropped on Hiroshima.

Design and mechanism

The core mechanism involves a smooth-bore gun barrel assembly inside the weapon's casing. One sub-critical piece of highly enriched uranium, shaped as a cylindrical target, is fixed at one end of the barrel. Another sub-critical piece, the bullet, is positioned at the opposite end. Upon triggering, a conventional explosive propellant, such as cordite, fires the bullet down the barrel at high velocity. The two masses collide and combine, almost instantaneously forming a super-critical mass. A neutron initiator at the moment of assembly provides a burst of neutrons to start the chain reaction. The entire assembly process must occur rapidly to prevent predetonation from spontaneous fission.

History and development

Early theoretical work on the concept was conducted by the MAUD Committee in the United Kingdom. The design was pursued aggressively by the Manhattan Project at the Los Alamos Laboratory under the direction of J. Robert Oppenheimer. Because it was considered a reliably simple design using uranium-235, significant research was not required, unlike the more complex implosion-type nuclear weapon. The Little Boy gun-type device was developed by the team at Project Alberta and used in combat without prior full-scale testing. Following World War II, further designs like the Mark 8 and Mark 11 were developed for tactical use by the United States Armed Forces.

Nuclear physics and criticality

The design relies exclusively on uranium-235 as its fissile material, as plutonium-239 has a high rate of spontaneous fission that makes it unsuitable. The critical mass for a bare sphere of highly enriched uranium is approximately 52 kilograms. In the gun assembly, the combined mass of the target and bullet exceeds this threshold once merged. The speed of assembly is critical; a slow assembly allows premature neutrons from spontaneous fission to start a chain reaction in a still-subcritical configuration, causing a fizzle. The efficiency and yield are lower than implosion designs because the super-critical state is not maintained as long before explosive disassembly.

Known examples and deployments

The only nuclear weapon of this type ever used in warfare is the Little Boy bomb, detonated over Hiroshima on August 6, 1945, by the B-29 Superfortress Enola Gay. Subsequent United States models included the Mark 8, designed for use against fortified targets, and the Mark 11, an aerodynamic weapon. These were deployed during the early Cold War by units such as the Strategic Air Command. No other nation is known to have deployed a pure gun-type weapon in its arsenal, as subsequent nuclear powers like the Soviet Union and the United Kingdom initially developed implosion designs.

Advantages and limitations

The primary advantage is mechanical and conceptual simplicity, offering high reliability without the need for sophisticated explosive lenses or perfect simultaneity in detonation. This made it an attractive first-generation weapon. However, significant limitations include inherent inefficiency, requiring large quantities of scarce uranium-235, and a susceptibility to predetonation if used with plutonium. The design is also generally longer, heavier, and less suitable for modern delivery systems like ICBMs or SLBMs compared to more compact implosion weapons. These factors led to its obsolescence in strategic arsenals.

Category:Nuclear weapon design Category:Nuclear weapons of the United States Category:Manhattan Project