A postwar "Little Boy" casing mockup.
"Little Boy" was the codename of the atomic bomb which was dropped on Hiroshima, on August 6, 1945 by the 12-man crew of the B-29 Superfortress Enola Gay, piloted by Lieutenant Colonel Paul Tibbets of the United States Army Air Forces. It was the first atomic bomb ever used as an offensive weapon and came three days before the dropping of "Fat Man" on Nagasaki.
The weapon was developed during the Manhattan Project(World War II), and derived its explosive power from the nuclear fissioning of enriched uranium. The Hiroshima bombing was the second nuclear explosion in history (the first was the "Trinity" test).
Basic weapon design
- Main article: Gun-type fission weapon
The "gun" assembly method. This is not necessarily exactly how the "Little Boy" weapon was designed (which is still classified).
The Mk I "Little Boy" was 10 feet (3 m) in length, 28 inches (71 cm) wide and weighed 8,900 lb (4000 kg). The design used the gun method to explosively force a sub-critical mass of uranium-235 and three U-235 target rings together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by simply shooting one piece of the uranium into the other by means of chemical explosives. It contained 60 kg U-235, of which 0.7 kg underwent nuclear fission.
No full test of a gun-type nuclear weapon had occurred before the "Little Boy" device was dropped over Hiroshima. The only test explosion of a nuclear weapon had been of an implosion-type weapon utilizing plutonium as its fissionable material, on July 16, 1945 at the Trinity test. There were a number of reasons for not testing the "Little Boy" device. A primary one was the fact that there was a very scarce amount of uranium-235 compared with the relatively large amount of plutonium which was expected to be able to be produced monthly from the Hanford reactors. Additionally, the weapon design was simple enough in concept that it was deemed only necessary to conducted limited testing of the gun-type assembly (known during the war as "ticking the dragon's tail"). Unlike the implosion design, which requires very sophisticated coordination of shaped explosive charges, the gun-type design was considered almost guaranteed to work without full testing.
Although used occasionally in later experimental devices, the design was used only once as a weapon because of the extreme danger of a misfire. A simple crash could drive the "bullet" into the "target" and release lethal radiation doses or even a full nuclear detonation. The danger of misfire was even greater over water. Even if the force of a crash did not set the bomb off, if water entered the fail safe system, it would be shorted out, possibly leading to a detonation of the bomb. The British Red Beard nuclear weapon also suffered from this flaw. None of the other five Mark I bombs built on the model of Little Boy were used by the US Army.
Assembly details
The exact specifications of the "Little Boy" bomb have never been declassified for security reasons, though many sources have speculated and relied upon limited photographic evidence to reconstruct its internal dimensions.
In the weapon, the uranium-235 material was divided into two parts, following the gun principle: the "projectile" and the "target". The "projectile" was a cylinder, about 16 cm long and 10 cm wide, with 40% of the total mass (25.6 kg). It was a pile of 6 uranium rings protected by a casing of steel and tungsten. The whole thing was locked in a 2 mm thick steel box. The "target" was a hollowed-out cylinder, with the same length and width, and a mass of 38.4 kg. The most enriched uranium was probably placed in this cylinder to increase the yield of the explosion.
The two parts were protected by bore casings designed to absorb the neutrons. When the projectile reached the target, the bore protection was supposed to explode and be projected into a cavity placed in the bomb nose. The system of reflectors designed to concentrate the neutrons was composed of steel and tungsten. This part, called the tamper, weighted 2.3 tonnes. The "target" was to lodge itself in this part.
The gun (fissile materials and explosives) was surrounded by a thick layer of lead (60 cm) in order to limit emitted radiation and avoid accidents. Its other role was to protect the various mechanical and electronic parts of the bomb from radiation. The tube of the gun was 10 cm wide and 180 cm long, and weighted 450 kg. To launch the uranium at a speed of 300 m/s, cordite was used, an artillery explosive based on nitrocellulose and nitroglycerine.
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- Stabilizing Tail Fins
- Tail cone
- Air inlet tubes
- Air pressure detonator
- Lead Shield container
- Detonator arm
- Detonating head
- Conventional Explosive Charge (cordite)
- Uranium-235 "Bullet" (ca. 24 kg, 16 cm long, 10 cm diameter)
- gun cylinder (not drawn to proportion: it was 180 cm long, with an inner diameter of 10 cm)
- Uranium-235 "Target" (ca. 36 kg) with receptacle (neutron reflector is just above)
- Telemetry monitoring probes
- Fuses (inserted to arm bomb just before dropping it)
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Development of the bomb
- Main article: Manhattan Project
The "Little Boy" bomb was constructed through the massive Manhattan Project during World War II. Because enriched uranium was known to be fissionable, it was the first approach to bomb development pursued (plutonium was, when the project began, still undiscovered). The vast majority of the work in constructing "Little Boy" came in the form of the isotope enrichment of the uranium necessary for the weapon. Enrichment at Oak Ridge, Tennessee began in February 1943, after many years of research.
The development of the first prototypes and the experimental work started during the spring of 1943, at the time when the Los Alamos design laboratory became operational in the framework of the Manhattan Project. Originally gun-type designs were pursued for both a uranium and plutonium weapon, but in April 1944 it was discovered that the spontaneous fission rate for plutonium was too high to use in a gun-type weapon: detonation of the costly weapon would cause an ineffective "fizzle", in which the weapon would destroy itself before creating a large explosion. In July 1944, almost all research at Los Alamos re-oriented around the development of the implosion plutonium weapon. In contrast, the uranium bomb was considered almost a formality.
When plutonium became unnecessary for the gun-type method, the team working on the gun weapon led by A. Francis Birch, faced a new dilemma. The bomb was simple to design, but they lacked the quantity of uranium-235 necessary for its production. The fissile material was not available before mid-1945. Despite these problems, Birch managed to convince others that this concept was the right one, and that in case of a failure of the plutonium bomb, it would still be possible to use the gun principle. His team had heavy responsibilities and even if the technology was less complex than for the other project, a lot of rigorous work was needed. In February 1945, the specifications were completed (model 1850). The bomb, except for the uranium payload, was ready at the beginning of May, 1945.
Most of the uranium necessary for the production of the bomb came from the Shinkolobwe mine and was made available thanks to the foresight of the CEO of the High Katanga Mining Union, Edgar Sengier, who had had 1000 tons of uranium ore transported to a New York warehouse in 1939. The majority of the uranium for Little Boy was enriched in Oak Ridge, Tennessee, primarily by means of electromagnetic separation in calutrons and through gaseous diffusion plants, with a small amount contributed by the cyclotrons at Ernest O. Lawrence's Radiation Laboratory. The core of Little Boy contained 64 kg of uranium, of which 50 kg were enriched to 89%, and the remaining 14 kg at 50%. With enrichment averaging 80%, it could reach about 2.5 critical masses. "Fat Man" and the Trinity "gadget", by way of comparison, had five critical masses.
Handling
Little Boy, before being loaded into the plane's cargo hold (a part of the hatch can be seen on the top right)
Handling "Little Boy" was extremely dangerous. Once cordite was placed in the right spot, any firing of the explosive would at worst cause a nuclear chain reaction and at best a contamination of the explosion zone. The simple contact of the two uranium masses could have caused an explosion with dire consequences (from the simple fizzle explosion, to the destruction of Tinian Island). Water was also a risk, since it could serve as a moderator between the fissile materials and cause a violent dispersal of the nuclear material. The uranium projectile could only be inserted with an apparatus using a force of 300,000 newtons (67,000 lbf, over 30 tons). For safety reasons, the weaponeer, Captain William Sterling Parsons, decided to set the cordite in place only after take-off.
After the launch, the bomb used an altimeter and pressure captors to trigger itself. At high altitudes, the air pressure was small but it increased with the fall. A thin metallic membrane increasingly deformed with pressure and once the bomb reached the correct altitude, closed a contact which caused the explosion of the cordite. About 10 milliseconds later, the chain reaction started and nothing could stop it from there on.
The bombing of Hiroshima
Main articles: Enola Gay and atomic bombings of Hiroshima and Nagasaki
The bomb was armed in flight 9600 m (31,000 feet) above the city, then dropped at approximately 8:15 a.m. (JST). The detonation happened at an altitude of 580 m. With a power of 13 to 16 kilotons (estimations vary), it was less powerful than "Fat Man," which was dropped on Nagasaki (21–23 kt). The official yield estimate of "Little Boy" was about 15 kilotons of TNT equivalent in explosive force, i.e. 6.3 × 1013 joules = 63 TJ (terajoules).[1]. However, the damage and the number of victims were much higher, as Hiroshima was on flat terrain, while the hypocenter of Nagasaki lay in a small valley.
Approximately 70,000 people were killed as a direct result of the blast, and a similar number were injured. A great number more would later die as a result of nuclear fallout and cancer.[2] Unborn babies died or were born with deformities.[3],[4] Clothing was burned into the skin.
The success of the bombing was reported with great enthusiasm in the United States, and there were many commentators who thought it would end the war before a drawn-out and likely bloody invasion of the Japanese home islands would have to take place.
Nazi origins of uranium?
Several historians have conjectured that some of the source uranium used for the "Little Boy" or (after conversion to plutonium) the "Fat Man" bombs may have been produced in Nazi Germany. Uranium was reportedly secured by Manhattan Project scientific director Robert Oppenheimer from the surrendering German submarine U-234. The German U-boat had been on its way to deliver the uranium and other top secret German warfare technology to the then Empire of Japan. However U-234 surrendered following the end of hostilities in the European war theatre and Germany's unconditional surrender and was led on May 19, 1945 to Portsmouth, New Hampshire. Two Japanese military officials on board the German vessel committed suicide and were buried at sea.
There are conflicting assessments of the importance of the German material to the Manhattan Project. The German uranium was likely to have been unenriched uranium oxide which would have yielded a small fraction of the amount of fissionable material used in the Little Boy: it is estimated that with technology available at the time, it was possible to obtain 4 kg of enriched uranium out of 560 kg of uranium oxide. Compare this to the 64 kg of uranium used in Little Boy. Conversely if the uranium was fully enriched, it seems unlikely that Germany would be exporting enough material to make a number of nuclear weapons to Japan. Recent literature has claimed that uranium oxide recovered from Germany had been used for the Soviet nuclear program. Furthermore, Japan had only 50 scientists working on its atomic bomb program and no known means of enriching uranium as the United States did at Oak Ridge.
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