Fat Man and Little Boy: The Design of the Atomic Bombs

In the aftermath of the Hiroshima and Nagasaki bombings, the US tried desperately to keep the “nuclear secret” secret, and released virtually no information about the design and construction of the two atomic bombs–there was not even an acknowledgement that two different methods of ignition had been used. Today, however, although many details of the atomic bombs remain classified, we know a great deal about how the Manhattan Project bombs were constructed.

The simplest and most reliable nuclear bomb design uses two subcritical masses of uranium which are kept separated at either end of a hollow tube until the moment of detonation, when they are driven together to form a supercritical mass which then explodes.  This is known as a “gun-type” design.  The bomb that was dropped on Hiroshima was a gun-type weapon known as “Little Boy”.

Because the gun-type weapon is so inherently simple, most of the work on its design was completed very quickly.  An ordinary anti-aircraft artillery barrel would simply be used to shoot one piece of fissionable material into another.  The basic idea is to place a hollow mass of enriched uranium-235 at one end of a gun barrel (the “target”) and a smaller cylinder of enriched uranium-235 at the other end (the “projectile”).  At the moment of detonation, a charge of ordinary cordite gunpowder would be set off by a primer, driving the projectile down the gun into the target to form a super-critical mass and setting off the nuclear explosion.

In a further refinement, the uranium-235 target was encased in a thick layer of tungsten carbide metal and steel, to serve as a “tamper”, which would reflect escaping neutrons back into the mass and thereby reduce the amount of uranium needed to produce a critical mass.  For a bare sphere of uranium-235, the critical mass is approximately 110 lbs (50 kg).  The use of a tamper can reduce the critical mass down to 50-55 lbs (20-25 kg).  The Hiroshima Little Boy bomb used a total of 140 pounds of uranium-235.  Such a mass would make a sphere measuring about 8 inches in diameter. (The uranium in the Little Boy was enriched to 80% U-235, which, today, would ironically not even be considered “weapons-grade”–today US weapons use uranium that is at least 93% enriched, which requires a smaller critical mass.)

The weight of the heavy tamper (it weighed, by itself, over 2.5 tons) would also help hold the reacting mass together for a longer time during the chain reaction, thus increasing the yield.

In the Hiroshima bomb, the target mass consisted of a series of hollow rings that were mounted next to each other to form a hollow cylinder of approximately 6.5 inches in diameter and length, and weighing around 80 pounds (the hollow cylinder allowed neutrons to escape and thus prevented the heavy mass from going critical while inside the tamper).  The projectile mass consisted of a series of solid discs mounted next to each other to form a cylinder about 4 inches wide and 6.5 inches long, weighing around 60 pounds.  The back of the projectile was fitted with a thick cylindrical piece of tamper material that would, when the projectile entered the target mass, plug the entry hole in the target mass and produce a complete tamper.  When assembled together, the Little Boy target/projectile would contain a little less than 2.5 times the critical mass.

The gun-type design was considered so reliable and so certain to work that no test shots were planned for – the prototype weapon itself would be dropped in combat.  The only delay in constructing the bomb was the amount of time it took for a sufficient amount of enriched uranium-235 to be produced at Oak Ridge.

Schematic diagram of a gun-type atomic bomb.  Not to scale.

The original plans called for the plutonium-239 bomb to be assembled using the gun-type method as well.  However, when the first samples of manufactured plutonium began to arrive from the Hanford reactors, a serious problem was revealed.  The plutonium-239 was produced by the bombardment of uranium-238 by neutrons inside the reactor.  A small portion of the plutonium-239 that was produced, however, would itself capture a neutron to become plutonium-240.  This process could not be avoided, and the Pu-240 impurity could not be removed.  This was a fatal blow to the plutonium-gun design; the Pu-240 isotope had a high rate of spontaneous fission, which would release a large number of neutrons.  These would cause “predetonation”, in which the target mass would begin interacting with the projectile before it had traveled completely down the barrel, producing enough heat to melt the projectile and preventing assembly of the target/projectile mass. (Uranium-235 has a much lower rate of spontaneous fission and therefore did not present this problem.  However, to lessen the chances of predetonation, the projectile mass in Little Boy was covered with a sheath of cadmium, which absorbs neutrons.  This sheath was stripped from the projectile as it entered the target chamber.)

Once the plutonium-gun design had been abandoned, research turned to an alternative method of assembling a critical mass which avoided the spontaneous fission problem and could be used with the plutonium from Hanford.  This method was known as “implosion”.  It was a highly-protected secret (though ultimately the secret failed–the information passed to the Soviets by atomic spy Julius Rosenberg centered around the process of implosion.)

The implosion method is based on the fact that the size of a critical mass is dependent upon the average distance that a neutron will travel before impacting another nucleus and producing another fission.  In ordinary plutonium metal, that distance is about 2.25 inches, producing a spherical critical mass of about 4.5 inches in diameter, weighing around 22 pounds.

If the density of the metal could be increased, however, then the plutonium nuclei would be crowded closer together, and the distance that a neutron would travel before causing another fission would be correspondingly reduced.  This, in turn, would lower the radius  of the necessary critical mass.

It would therefore be possible, in theory, to take a subcritical sphere of plutonium metal (from which the neutrons could easily escape), uniformly compress it very rapidly to increase its density, and thus squeeze it into a supercritical state (in which the nuclei are so close together that the neutrons can no longer escape without hitting one).  This process of very rapid uniform compression was called “implosion”.

The only method of producing such pressures at the time was by using high explosives.  By surrounding a subcritical plutonium sphere with a layer of high explosives, the high-pressure shock waves would move inwards and smash against the plutonium, compressing it and doubling its density.

A simple layer of explosives, however, would not work.  In order to produce a suitable density, the pressure wave had to be absolutely uniform and simultaneous.  The shock waves produced by a layer of explosives, however, would react with each other through wave interference, and produce a series of gaps between high-speed jets, which would reach the surface of the plutonium sphere at different times and fail to produce a uniform increase in density.

To solve this problem, the Los Alamos team planned to produce an “explosive lens”, a combination of different explosives with different shock wave speeds.  When molded into the proper shape and dimensions, the high-speed and low-speed shock waves would combine with each other to produce a uniform concave pressure wave with no gaps.  This inwardly-moving concave wave, when it reached the plutonium sphere at the center of the design, would instantly squeeze the metal to at least twice the density, producing a compressed ball of plutonium that contained about 5 times the necessary critical mass.  A nuclear explosion would then result.

With the theory of implosion, bomb design work at Los Alamos split into two directions.  The original gun-type design would continue under the name “Thin Man”, and it would use the uranium-235 being produced at Oak Ridge.  The implosion design was named “Fat Man”, and it would utilize the plutonium-239 being produced at Hanford.

Shortly after, further work showed that the assembly speed for the gun-type bomb did not need to be as fast as originally thought; the twenty-foot long gun barrel originally planned for Thin Man could be shortened to as little as ten feet.  The smaller design that resulted was called Little Boy.

The most difficult part of the implosion design is the explosive lens assembly that is used to create the inwardly-converging shock wave.  The Los Alamos team performed a long series of experiments with different explosives configurations before they finally found a suitable design.  This utilized Composition B as the fast explosive and Baratol as the slow explosive.  The implosion wave from these lenses was focused onto an inner layer of Composition B, which would be simultaneously detonated on its outer surface by the incoming implosion wave, and add its shock wave to the implosion.

The final explosives component of the Fat Man bomb was almost 18 inches thick and weighed a total of 5500 pounds (2.75 tons).  It consisted of 32 explosive lenses (each with two redundant detonators) surrounding the inner layer of Composition B.  The detonators were specially designed to all explode within 10-billionths of a second of each other – such simultaneity was necessary for a symmetrical implosion wave.  The detonators consisted of special wire that was vaporized by a sudden high electric current from a bank of electrical capacitors.  Each wire set off a primer made of the explosive PETN, which in turn set off the main charge in the explosive lens.

At the center of the spherical explosives assembly was “the pit”, sometimes also referred to as “the physics package”.  This consisted of the plutonium core, a natural uranium-238 tamper, and a neutron initiator.

Schematic diagram of an implosion atomic bomb.  Not to scale.

The core of the Fat Man bomb consisted of approximately 13 pounds of plutonium metal, formed into two solid hemispheres.  The two hemispheres were sealed together by a thin gold gasket, which prevented jets of explosive waves from shooting through the seam and disrupting the implosion.  The complete core was about 7 inches in diameter.
Surrounding the plutonium core was a thick tamper of natural uranium metal, about 2.5 inches thick.  The uranium-238 helped to reflect escaping neutrons back into the core, reducing the amount of plutonium needed for a critical mass.  The heavy momentum of the tamper as it was crushed inwards by the explosive wave also helped to hold the reacting nuclear core together for a few milliseconds, making the reaction more efficient and increasing the explosive yield.  Finally, as the plutonium core fissioned, it would release fast neutrons which would then hit the uranium-238 nuclei, producing additional fissions and increasing the bomb’s yield.  (Uranium could not be used as a tamper for the gun-type design, because the heavy mass that would be necessary for an effective tamper would produce so many spontaneous fissions that it would interact with the target mass and possibly produce predetonation.)

At the center of the Fat Man design, nestled in a small hollow, was the neutron initiator, known as the “Urchin”.  This was a one-inch  nickel ball which contained small amounts of polonium-210 and beryllium, separated by a metal foil.  When the implosion wave reached the center of the core (the point of maximum density), it would crush the initiator and cause the polonium and beryllium to mix.  The rapid alpha radiation from the polonium would kick neutrons out of the beryllium, and these neutrons would travel into the plutonium core and set off the chain reaction.

(The Little Boy was designed without an initiator, since it was calculated that there would be enough background neutrons in the uranium-235 mass to reliably set off a chain reaction.  At the last minute, however, it was decided to add a neutron initiator anyway.  This consisted of pieces of foil-covered polonium attached to the front of the U-235 projectile, which would, when the projectile entered the target mass, contact several pieces of foil-wrapped beryllium and produce a shower of neutrons.)

Since the implosion design was so complex and required absolute precision, it was decided to test–fire the design before assembling an actual weapon.  The “Gadget” was successfully test-detonated at Alamogordo in July 1945.  The test was code-named “Trinity”.

The implosion method was by far a superior design over the gun-type.  It required significantly smaller amounts of fissionable material (implosion U-235 bombs use about 33 lbs of material rather than the 140 lbs used in the Little Boy gun-type), and had a far more compact design.  It was also far more efficient – the Little Boy bomb fissioned about 1.5% of its available nuclear material, while the Fat Man bomb fissioned about 20%.

At the end of World War II, the United States possessed just one unused nuclear weapon, a Fat Man pit that had not yet been assembled into a bomb.  Plutonium production at Hanford, however, was reaching 33 lbs of plutonium per month, enough for two Fat Man bombs.  Production of enriched uranium at Oak Ridge would reach 135 pounds per month by October 1945. This was enough for four implosion uranium bombs (after the prototype was dropped on Hiroshima, the inefficient gun-type design was never put into production by the US).

Los Alamos, meanwhile, had already prepared two improvements to the implosion bomb design.  The first was to use a hollow composite core, consisting of an inner layer of plutonium and an outer layer of uranium-235, surrounded by a natural uranium-238 tamper.  Using a hollow core rather than the Fat Man’s solid core allowed a larger amount of material to be used while still allowing enough neutrons to escape to keep the mass subcritical.  The composite core also allowed supplies of scarce plutonium to be stretched using the more readily available uranium.

In another improvement, this hollow core was “levitated” – a series of small support struts held it a short distance away from the surrounding tamper.  This air space gave the tamper some room to accelerate as it was imploded into the core, building up more momentum and increasing the efficiency of the core by holding it together for a few additional microseconds.  The uranium tamper used in Fat Man was also later replaced by a beryllium tamper, which better reflected neutrons.  Further research found that the U-233 isotope of uranium was also fissionable and could be used to make weapons.

Bomb designers also discovered that using a larger number of individual explosive lens units in the implosion design, 64 instead of the 32 in Fat Man, allowed the lenses to produce the same implosion wave in a shorter distance, making the explosive lens layer thinner and decreasing the size and weight of the bomb.  (Later US designs used 92 separate explosive lenses for even smaller and thinner bombs.)  And work was also finishing up on an improved initiator that was more efficient than the Urchin.

After the war ended, however, priority for nuclear weapons production fell.  The Y-12 uranium-enrichment plant at Oak Ridge proved too costly to operate, and was closed in 1946.  The two plutonium-production reactors at Hanford were suffering from unanticipated radiation damage to their components; they were operated at greatly reduced levels, and were closed as soon as replacements became available.  One year after Hiroshima, the US had a total of 9 Fat Man bombs (but only had initiators available for 7 of these); by July 1947, the entire US nuclear arsenal consisted of just 13 implosion weapons – none of them of the new levitated composite core type. But the rest of the world had no nuclear weapons at all.