This is a comparison of the two types of atomic bombs used against Japan, one based on uranium-235 and the other based on plutonium.

Fat Man and Little Boy, are the nicknames given to the first and only two Atomic bombs ever used in combat. Little boy was dropped on Hiroshima, Japan on August 6, 1945. Fat Man was detonated three days later over the city of Nagasaki. These two weapons were pivotal in saving the lives of thousands of Americans, and bringing the Second World War to a victorious end. The designs of these weapon systems were experimental, and technologically amazing. It was these first two designs which paved the way for future scientist to build weapons that yielded 200 to 300 times the destructive force of these two pioneers.

The design of the Fat man device was basically the same as an experimental test version named Gadget. A test of the implosion bomb was considered essential due to the newness of the explosive wave shaping technology, and the complexity of the system. The first Soviet atomic bomb, code named RDS-1, is identical to the design of Gadget. This is because detailed descriptions of the design were given to Soviet intelligence by spies who worked for the United States at Los Alamos.

The basic structure of the Fat Man was based on a series of 6 concentric nested spheres. The outermost was the explosive lens system, followed by the absorber shell, the Uranium reflector shell, the plutonium pit, and lastly the innermost shell, the neutron initiator.

The shell system basically worked as an implosion device. The outer shell was made of high-powered explosive that, when detonated, compressed the inner spheres and charged the uranium. The design was suggested by Robert Christy in order to minimize asymmetry and instability problems during implosion.

The innermost shell, the plutonium pit, contained 6.2 kg of plutonium alloy contained in a 9.0 cm shell. It was solid except for an approximately 2.5 cm cavity in the center where the neutron initiator was placed. The pit was formed in two hemispheres. Since plutonium is a chemically very reactive metal, as well as a significant health hazard, each half-sphere was electroplated with nickel to avoid deteriorating reactions. The pit had a 2.5 cm hole capped with a plutonium plug to allow the insertion of the initiator after assembly.

The neutron initiator used was called the "Urchin" or "screwball" design. It was a sphere consisting of a hollow beryllium shell, with a solid beryllium pellet inside. The whole initiator weighed about 7 grams. The outer shell was 2 cm wide and 6 mm thick while the solid inner pellet was 8 mm wide. 15 parallel wedge-shaped grooves, each 2 mm deep, were cut into the inner surface of the shell. Like the pit, the shell was formed in two halves. The Urchin was activated by the arrival of the implosion shock wave caused by the outer lens shell. When the shock wave reached the beryllium walls of the cavity, they vaporized and formed a separate shock wave that struck the initiator. This collapsed the wedge shaped grooves in the outer initiator shell and created turbulence that rapidly mixed the plutonium and beryllium of the inner and outer spheres together. The alpha particles emitted by the new isotope then struck beryllium atoms, periodically knocking loose neutrons, perhaps one every 5-10 nanoseconds. It was these neutrons which progressed to the uranium tamper surrounding the pit, and provided the backbone for the explosion.

The pit was surrounded by a natural uranium tamper weighing 120 kg, with a diameter of 23 cm. The tamper formed a 7 cm thick layer around the pit. About 20% of the bomb yield was from fast fission of this layer. The pit and the tamper together comprised an essential component of the bomb. The nuclear implosion was so powerful that it compressed the pit and inner assemblies to 2.5 times their original density.

Surrounding the tamper was an 11.5 cm thick aluminum sphere also weighing 120 kg. The primary purpose of this sphere, called the "pusher" or "absorber shell", was to reduce the effect of the Taylor wave. The Taylor wave is the rapid drop in pressure that occurs behind a shock wave. In other words, the scientists wanted as much pressure behind the shock wave as possible in order to increase the destructive power. By reducing the Taylor wave, the pressure of the transmitted shock wave increases, enhancing the pressure reached at the center of core, and thereby increasing the size of the explosion.

The outermost shell was the high explosive lens system. This made a layer some 47 cm thick weighing at least 2500 kg. This assembly consisted of 32 explosive lenses; 20 of them hexagonal, and 12 pentagonal. The lenses fitted together in the same pattern as a soccer ball, forming a complete spherical explosive assembly that was 140 cm wide. Each lens had three pieces: two made of high velocity explosive, and one of low velocity explosive. The outermost piece of high velocity explosive had a conical cavity in its inner surface into which fitted an appropriately shaped piece of slow explosive. The purpose of these paired pieces was to form a lens that shaped a convex, expanding shock wave into a convex converging one. The lenses were made by precision casting, so meltable explosives were utilized.

The lens system had to be made to very precise tolerances. The composition and densities of the explosives had to be accurately controlled and extremely uniform. The pieces had to fit together with an accuracy of less than 1 mm to prevent irregularities in the shock wave which could alter the fission of the uranium. Accurate alignment of the lens surfaces was even more important than a close fit. A great deal of tissue paper and scotch tape was also used to make everything fit snugly together. Each of the components of the bomb, from the lenses to the pit itself, were made as accurately as possible to insure accurate implosion and the highest densities possible in order to have efficient fission of the uranium.

To achieve the most precise detonation synchronization possible in the outer lens system, conventional detonators consisting of an electrically heated wire, and a sequence of primary and secondary explosives were not used. Instead newly invented exploding wire detonators were used. This detonator consists of a thin wire that is explosively vaporized by a surge of current generated by a powerful capacitor. The shock wave of the exploding wire initiates the secondary explosive of the lenses. The discharge of the capacitor, and the generation of initiating shock waves by the exploding wires can be synchronized to +/- 10 nanoseconds. A disadvantage of this system is that large batteries, a high voltage power supply, and a very powerful capacitor bank (the one used weighed 400 lbs.) was needed to explode all 32 detonators simultaneously.

The whole explosive assembly was held together by a shell made of a strong aluminum alloy called dural (or duraluminum). A number of other shell designs had been tried and discarded. This shell design, designated model 1561, was made of an equatorial band bolted together from 5 segments of machined dural castings, with domed caps bolted to the top and bottom to make a complete sphere.

The final bomb design allowed "trap door" assembly. The entire bomb could be assembled ahead of time, except for the pit/initiator. To complete the bomb, one of the domed caps was removed, along with one of the explosive lenses. The initiator was inserted between the plutonium hemispheres, and the assembled pit was inserted in a 40 kg uranium cylinder that slid into the tamper to make the complete core. The explosive lens was replaced, its detonator wires attached, and the cap bolted back into place.

Safety was a serious problem for Fat Man, though in a comparison of worst case accidents, not as serious a problem as it was for Little Boy. Any accidental detonation of the high explosive lenses (in a fire or plane crash for example) would be certain to collapse the uranium core to its supercritical state. The expected yield from the explosion would be on the order of tens of tons, roughly a factor of ten higher than the energy of the high explosive lens itself, but much less than the potential of the bomb. This meant that even if the plane crashed, the bomb would not detonate to full yield. In order to explode to full kilotonage the explosive wires had to be detonated.

For transportation feasibility, as well as safety reasons, Fat man and other implosion bombs were not transported in assembled form but were put together shortly before use. Due to the complexity of the weapon, this was a process that took at least 2 days. Weapons of this design could only be left in the assembled state for a few days due to deterioration of the explosive wire detonation batteries. Fat Man was the more powerful of the two bombs. But this power came at a much greater cost, and a much more fragile design. Little Boy was much more conservative, and technologically easier to complete. The cost of this simplicity was power.

"Little Boy" is the nick name given to the atomic bomb dropped on Hiroshima on August 6, 1945. Little Boy was dropped from the Enola Gay, one of the B-29 bombers that flew over Hiroshima on that day. The design of Little Boy was completely different from Gadget/Fat Man. It was completed in February with only the field preparations necessary.

All of the uranium used in Little Boy had gone through its final stages of enrichment in the Calutron electromagnetic isotope separators in Oak Ridge, Tennessee.

The pit contained 64.1 kg of highly enriched uranium. By the time Little Boy was assembled, 50 kg of uranium was available at Oak Ridge, and an additional 14 kg of was on hand. All of it was used in the bomb. The Uranium used in Little Boy, however, was less pure than that used in Gadget/Fat Man, and consequentially added to the less efficient bomb.

The Uranium of Little boy was divided into two pieces: the bullet and the target. The "bullet" was a cylindrical stack of six Uranium rings about 10 cm wide and 16 cm backed by a tungsten carbide disk and a steel backplate. All of this was contained within a 1/16 inch thick steel can to make a complete projectile. The "target" was a hollow cylinder 16 cm long and wide, weighing 38.4 kg. The target was fabricated as two separate rings that were inserted in the bomb separately. Both the bullet and the target were sheathed in boron in order to absorb neutrons and reduce the chance of a criticality accident.

The bomb was detonated by firing the bullet into the target via a high powered aircraft gun. The gun was a 3" diameter, six feet long anti-aircraft barrel that had been bored out to 4" to accommodate the bullet. It weighed about 450 kg, and used Cordite, a conventional artillery smokeless powder, as propellant.

To reduce the possibility of the bullet being driven into the target by a plane crash, the fit of the barrel was intentionally made very tight. The bullet had to be rammed into the breech to assemble the weapon, and about 70,000 lb were required to drive it forward. Even with this precaution, a plane crash which caused the weapon to strike a hard surface could conceivably produce the 500 Gs of acceleration required to fire the bullet, and implode the weapon.

Little Boy was a terribly unsafe weapon design. Once the gun was loaded with the cordite propellant, anything that ignited it would cause a full yield explosion of the bomb. For this reason, the propellant was not loaded into the weapon on the ground. Instead, one of the bombers placed the cordite in the gun after take-off in case a crash or fire occurred.

The complete weapon was 126 inches long, 28 inches in diameter and weighed 8900 lb. Both Fat man and Little boy were air burst bombs. This meant than an altimeter in the bomb acted as a trigger for the implosion systems.

No other weapon like little boy has ever been detonated. Only five other Little Boy units were built, but none of these entered the US arsenal. The design was too unstable to pursue and, even though cost effective, did not produce the desired destructive force.

The designs of Fat Man and Little Boy were radically different. The differing designs represented the economic, conservative approach, as well as the complicated, costly method. It was shown by these two bombs that the destructive power of nuclear fission could be harnessed in many ways, and that the possibilities for improvement and increased power were inevitable. It was from these basic designs that scientists advanced to develop the modern nuclear arsenal that, if ever utilized, would make its predecessors seem like firecrackers.


Source: The Designs of Fat Man and Little Boy by Marc Stockbauer. Stanford University.