Gun-type Fission Weapon - Little Boy

Little Boy

The "gun" method is roughly how the Little Boy weapon, which was detonated over Hiroshima, worked, using uranium-235 as its fissile material. In the Little Boy design, the U-235 "bullet" had a mass of around 86 pounds (39 kg), and it was 7 inches (17.8 cm) long, with a diameter of 6.25 inches (15.9 cm). The hollow cylindrical shape made it subcritical. It was powered by a cordite charge. The uranium target spike was about 57.3 pounds (26 kg). Both the bullet and the target consisted of multiple rings stacked together.

The use of "rings" had two advantages: it allowed the larger bullet to confidently remain subcritical (the hollow column served to keep the material from having too much contact with other material), and it allowed sub-critical assemblies to be tested using the same bullet but with just one ring.

The barrel had an inside diameter of 6.5 inches (16.5 cm). Its length was 70.8 inches (1.8 m), which allowed the bullet to accelerate to its final speed of 984 feet per second (300 m/s) before coming into contact with the target.

When the bullet is at a distance of 9.8 inches (25 cm), the combination becomes critical. This means that some free neutrons may cause the chain reaction to take place before the material could be fully joined (see nuclear chain reaction).

Note that typically the chain reaction takes less than 1 μs (100 shakes), during which time the bullet travels only 0.3 mm. Although the chain reaction is slower when the supercriticality is low, it still happens in a time so short that the bullet hardly moves in that time.

This could cause a fizzle, a predetonation which would blow the material apart before creating much of an explosion. Thus it is important that the frequency at which free neutrons occur is kept low, compared with the assembly time from this point. This also means that the speed of the projectile must be sufficiently high; its speed can be increased but this requires a longer and heavier barrel.

In the case of Little Boy, the 20% U-238 in the uranium had 70 spontaneous fissions per second. With the fissionable material in a supercritical state, each gave a large probability of detonation: each fission creates on average 2.52 neutrons, which each have a probability of more than 1:2.52 of creating another fission. During the 1.35 ms of supercriticality prior to full assembly, there was a 10% probability of a fission, with somewhat less probability of pre-detonation.

Initially the Manhattan Project gun-type effort was directed at making a gun weapon that used plutonium as its source of fissile material, known as the "Thin Man" because of its extreme length. It was thought that if a plutonium gun-type bomb could be created, then the uranium gun-type bomb would be very easy to make by comparison. However, it was discovered in April 1944 that reactor-bred plutonium (Pu-239) is contaminated with another isotope of plutonium, Pu-240, which increases the material's spontaneous neutron-release rate, making pre-detonation inevitable. For this reason, a gun-type bomb is thought to only be usable with an enriched-uranium bomb.

After it was discovered that the "Thin Man" program would not be successful, Los Alamos redirected its efforts into creating the implosion-type plutonium weapon: "Fat Man". The gun program switched completely over to developing a uranium bomb.

Although in Little Boy 132 pounds (60 kg) of 80%-grade U-235 was used (hence 106 pounds / 48 kilograms), the minimum is ca. 44 to 55 pounds (20 to 25 kg), versus 33 pounds (15 kg) for the implosion method.

The scientists who designed the "Little Boy" weapon were confident enough of its likely success that they did not field-test a design before using it in war (though they did perform non-destructive tests with sub-critical assemblies, as part of their experiments—known as tickling the dragon's tail). In any event, it could not be tested before being deployed, as there was only sufficient U-235 available for one device. Even though the design was never proof-tested, there was no risk of the device being captured by an enemy if it malfunctioned: even a "fizzle" would completely disintegrate the device.

For a quick start of the chain reaction at the right moment a neutron trigger/initiator is used. An initiator is not strictly necessary for an effective gun design, as long as the design uses "target capture" (in essence, ensuring that the two subcritical masses, once fired together, cannot come apart until they explode). Considering the 70 spontaneous fissions per second, this only causes a delay of a few times 1/70 second, which in this case does not matter. Initiators were only added to Little Boy late in its design.