The first atomic bomb, dropped on Hiroshima, Japan on August 6th 1945, was of a unique uranium-based ‘gun principle’ design never subsequently employed in a military weapon because of its inherent risks and the scarcity of the fissionable material. The second bomb, dropped on Nagasaki three days later, was of completely different design. Together, the two bombs it brought about a quick end to the conflict with Japan in World War II.
‘Little Boy’: An Unusual and Dangerous Bomb
The Manhattan Project, working on developing an ‘atomic’ bomb based on Einstein’s equations, had designed two different types of bomb, one code-named ‘Little Boy’ based on the rare isotope Uranium 235 and the second, code-named ‘Fat Man’ based on more readily obtainable plutonium.
Little Boy was an unhealthy boy of 9700 pounds, 120 inches long and 28 inches in diameter (4400 kg and 300 x 71cm Ø). The bomb contained a 64 kg (141 lbs) of enriched, fissionable Uranium 235 but only a little over 1% of this underwent nuclear fission and only around 0.1% was physically converted into energy. Nevertheless, the explosion is officially estimated to have been equivalent to around 16 kilotons of TNT.
With Little Boy, the Manhattan project wished to create a simple weapon that was absolutely guaranteed to work. It was constructed on the ‘gun principle’, in which one of the two shaped uranium components is fired into the other one and, for simplicity, incorporated only a minimum of basic safety mechanisms that could break down and prevent detonation. Because of the high risk of premature detonation once the bomb was primed, the bomb was only filled with chemical explosive and then electrically armed during its flight to Hiroshima.
The ‘Gun Principle’
The bomb was constructed on the ‘gun principle’ in that a sub-critical mass of uranium was fired at another sub-critical mass to create an accelerated critical mass explosion. This principle was much simpler than the ‘implosion’ design required for the plutonium-based bomb and used a much rarer material so only the plutonium-based bomb had been previously tested in the Trinity test at White Sands, Nevada. It was felt that Little Boy’s simple design would absolutely ensure detonation but the bomb was much more inherently unstable and some complicated measures were required to guard against premature explosion. The ‘gun principle’ has never been used since in any military weapon.
A Counter-Intuitive Design
Critical mass was to be achieved by firing one mass of uranium 235 into another similar sub-critical mass. The principle was of inserting a ‘bullet’ or plug into a shell of the same material to create a super-critical mass. The high-speed impact would speed up the nuclear chain reaction leading to an explosion.
Because of critical mass considerations, instead of the ‘bullet’ of uranium being fired into the uranium shell, the process was reversed , in other words, the uranium shell had to be fired at the ‘bullet”. Conventional chemical explosive was used to fire the projectile (the shell) down what was essentially a smooth-bore gun barrel 72” (180cm) long and 6.5” (17cm) in diameter to encapsulate and impact with the target (the ‘bullet’) 7” (18cm) long and 4” (10cm) in diameter.
Construction Details
The bomb’s precise specifications remain classified but an idea of its internal dimensions has been garnered from the limited photographic evidence available, interviews with former Manhattan Project personnel, and piecing together information from declassified sources.
According to the website Nuclear Weapon Archive, inside the weapon, the uranium-235 material was divided into two parts. The projectile shell was a hollow cylinder with 60% of the total mass (38.5 kg). It consisted of a stack of 9 uranium rings, each 6.25 “ (159mm) in diameter with a 4”-diameter (100mm) hole in the center and a total length of 7” (180mm), pressed together into the front end of a thin-walled projectile 16.25” (413mm) long. Filling in the remainder of the space behind these rings in the projectile was a tungsten carbide disc and a steel back. At detonation, the projectile was pushed 42” (1100mm) down a 72” (1800mm) long, 6.5” (170mm) smooth-bore gun barrel. The target "insert", comprising 40% of the total mass (25.6 kg), was a 4”-diameter cylinder, 7” long with a 1” (25mm) diameter hole in the center. This insert was composed of a stack of 6 uranium discs or washers a little thicker than the projectile rings. These discs were slid over a 1”-diameter (25mm) rod. This rod then extended forward through the tungsten carbide tamper plug, the impact-absorbing anvil, and the nose plug backstop, eventually protruding out the front of the bomb casing. This entire target assembly was secured at both ends with locknuts.
When the hollow-front projectile reached the target and slid over the target insert, the assembled super-critical mass of uranium would be completely surrounded by a tamper and neutron reflector of tungsten carbide and steel, both materials having a combined mass of 2,300 kg. Neutron generators at the base of the projectile would be activated by the impact.
The bomb was detonated by 4 bags of cordite powder loaded into the gun breech.
For a sketch of the Little Boy’s estimated internal construction, see “Atom Bombs: The Top Secret Story of Little Boy and Fat Man” 2003, P112 (John Coster Drawing)
The Inherent Risks
With Little Boy, the Manhattan project wished to create a simple weapon that was absolutely guaranteed to work. It therefore incorporated only a minimum of basic safety mechanisms that could break down and prevent detonation. Accidental detonation could result from an impact causing the ‘bullet’ and the shell to collide, resulting in a massive release of radiation and possible nuclear detonation. Once the bomb was primed and armed, the risks increased several-fold and included an electrical short circuit, for instance from water leaking into the circuit, fire and lightning strike. As a safety measure, the bomb was only primed with chemical explosive and armed electrically at the very last moment, just before reaching the drop site.
Sources
Los Alamos National Laboratory report LA-8819, The yields of the Hiroshima and Nagasaki nuclear explosions by John Malik, September 1985
The majority of the details are taken from the description of the "Little Boy" by Carey Sublette in Section 8 of his " Nuclear Weapons Frequently Asked Questions ". The most recent updates come from John Coster-Mullen's Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man, 2003 (first printed in 1996), a self-published account based largely on oral histories but which contains, in its extensive appendix, a declassified U.S. government document detailing the exact mass and configuration of the U-235 rings.
Jeremy Bernstein (October 15, 2007). Nuclear Weapons: What You Need to Know. Cambridge University Press. p. 133. ISBN 052188408X. "a 2,300 kilogram mixture of tungsten carbide and steel was used
Martin Cross - A translator, former chef and marketeer, currently disabled. I write articles on food,, travel, politics, religion and technology.
