What is the Gun-type Fission Bomb?

The gun-type fission bomb is one of the simplest and earliest designs of nuclear weapons, marked by significant historical importance and technological development. This article delves into the engineering and operational details of the Little Boy, the most notable example of this design, providing a comprehensive understanding of its construction, function, and historical context.

Introduction to the Gun-type Fission Bomb

The gun-type fission bomb is a straightforward yet crucial nuclear weapon system, both in terms of its design and historical significance. It was utilized in only a few warheads, with the Little Boy dropping on Hiroshima being the most famous example. Other notable examples include the W9 artillery shell, the W33 testing in Nougat/Aardvark, and the XW33 artillery shell tested at Plumbbob/Laplace. The development of this weapon was primarily driven by the simpler mechanics required to achieve a chain reaction, making it a significant step in nuclear weaponry history.

The Design and Function of the Little Boy

The core of the Little Boy design is a barrel-like setup, reminiscent of a firearm's barrel but with critical differences. At the 'breech end,' the 'bullet' (in this case, a cylindrical assembly) is composed of stacked rings of highly enriched uranium, with about 80% of the fissile isotope U-235. This design ensures that the uranium as a whole remains subcritical, unable to sustain a chain reaction, by reducing the contact between fissile materials.

The uranium 'bullet' is propelled down the 165mm wide, 1800mm long steel tube by a cordite propellant charge, aiming to meet an enriched uranium target spike of mass 26kg. This target is surrounded by tungsten carbide blocks, serving as a neutron reflector to minimize neutron loss and reduce the critical mass of U-235.

The fuze system is designed to delay detonation to ensure the bomb is clear of the aircraft. Upon release, the weapon switches over to internal power and starts a fifteen-second timer. The barometric switch then activates altitude-determining radar altimeters to signal the optimal detonation height, usually around 580m above the ground. The altimeter closes a switch, activating the primers that fire the cordite charge, propelling the 'bullet' towards the target at a velocity of 980 m/s.

Operational Details and Fission Process

Once the two pieces of uranium come within nine inches of each other, the neutron flux causes fission to commence. Initiators in the target well, made of polonium and beryllium, inject additional neutrons to ensure the precise detonation. Within one microsecond, one kilogram of the critical mass undergoes uncontrolled nuclear fission, releasing approximately 0.6 grams in the form of energy. This amounts to a significant energy release, equivalent to 12,000 tonnes of conventional explosives from just half a gram of matter.

Later analysis estimated the yield of Little Boy at 15 kilotons, which became the official figure. The design faces limitations in efficiency and safety; the process requires a substantial amount of fissile material and is more prone to accidental criticality than implosion-type weapons. Over time, the implosion-type weapon became more favored for its greater efficiency and lower risk of uncommanded detonation.

Conclusion

The gun-type fission bomb, particularly the Little Boy, played a pivotal role in the historical development of nuclear weaponry. Understanding its design, function, and limitations offers valuable insights into the evolution of nuclear technology and its impact on global security.

Note: This discussion is for educational purposes only and in line with ethical considerations.