Little Boy

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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 Colonel Paul Tibbets of the United States Army Air Forces. It was the first atomic bomb ever used as a weapon and was dropped three days before the "Fat Man" bomb was used against Nagasaki.

The weapon was developed during the Manhattan Project during World War II. It 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), and it was the first uranium-based detonation ever. It exploded with a power of 13 to 16 kilotons (estimations vary).

1 Basic weapon design
2 Assembly details
3 Development of the bomb
4 Handling
5 The bombing of Hiroshima
6 Possible Nazi origins of uranium
7 See also
8 References
9 External links

[edit] Basic weapon design

Main articles: Gun-type fission weapon and Nuclear weapon design

The "gun" assembly method. This is not necessarily exactly how the "Little Boy" weapon was designed (as it is still classified).

The Mk I "Little Boy" was 10 feet (3 m) in length, 28 inches (71 cm) in diameter 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 64 kg of uranium, 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 several reasons for not testing the "Little Boy" device. Primarily, scarcity of uranium-235 compared with the relatively large amount of plutonium which, it was expected, could be produced monthly from the Hanford reactors. Additionally, the weapon design was conceptually simple enough that it was only deemed necessary to do laboratory tests with the gun-type assembly (known during the war as "tickling the dragon's tail"). Unlike the implosion design, which required very sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work without full testing.

Although occasionally used in later experimental devices, the design was only used once as a weapon because of the extreme danger of accidental detonation. Little Boy's design was highly unsafe by modern standards, due to the risk that an accidental detonation could occur. For example, a simple crash could drive the "bullet" into the "target" resulting in a massive release of radiation, or possibly full nuclear detonation. The danger of misfire was even greater over water. Even if the force of a crash did not trigger the bomb, the resulting leakage of water into the unprotected system would short it out, again possibly leading to accidental detonation. The British Red Beard nuclear weapon also suffered from this design flaw. A lightning strike or fire could also trigger the bomb. None of the other five Mark I bombs built on the model of Little Boy were used by the US Army.

[edit] Assembly details

The exact specifications of the "Little Boy" bomb remain classified because they can still be used to create a viable nuclear weapon. Even so, many sources have speculated as to the design, relying on limited photographic evidence, interviews with former Manhattan Project personnel, and piecing together information from declassified sources to reconstruct its internal dimensions.

According to one source considered reliable,^[1] inside 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 stack of 6 uranium rings protected by a casing of steel, with a tungsten carbide and steel backing plate at the back end. The entire "projectile" assembly was locked in a 2 mm thick steel box. The "target" was a hollowed-out cylinder, 16 cm long and in diameter, with a 10 cm diameter hole in the middle for the bullet, with a uranium mass of 38.4 kg. The most enriched uranium was probably placed in the projectile to increase the yield of the explosion.

The two parts were protected by boron casings designed to absorb the neutrons, one inside the target and one surrounding the projectile (described as a sabot). When the projectile reached the target, the boron protection was supposed to be removed; the segment inside the target pushed forwards into a cavity at the nose, and the sabot around the projectile stripped off just as it reached the target assembly. The system of neutron reflectors was composed of steel and tungsten. This part, called the tamper, weighed 2,300 kg. The "target" was to lodge itself in this part.

The barrel was made from a modified artillery gun barrel and breech assembly. After machining operations were completed it measured 10 cm wide and about 180 cm long, and weighed 450 kg. To launch the uranium at a speed of 300 m/s, a bag of cordite was used as the propellant.

Box tail fins
Steel gun breech assembly
Detonator
Cordite (conventional) explosives
Uranium-235 "projectile", six rings (26 kg) in a thin can of steel
Baro sensing ports and manifold
Bomb casing wall
Arming and fusing equipment
Gun barrel, steel, around 10 cm diameter, 200 cm length
Arming wires
Tamper assembly, steel
Uranium-235 "target", two rings (38 kg)
Tamper/reflector assembly, tungsten carbide
Neutron initiator
Archie fuzing radar antennas
Recess for the boron safety plug (not shown) to be ejected into

[edit] Development of the bomb

Uranium for "Little Boy" was enriched in calutrons and by gaseous diffusion at Oak Ridge, Tennessee.

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 (the "Thin Man" design), 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.

As part of Project Alberta, Commander A. Francis Birch (left) numbers the bomb while physicist Norman Ramsey watches. This is one of the rare photos where the inside of the bomb can be seen.

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 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.

[edit] Handling

Little Boy, before being loaded into the plane's cargo hold. Part of the hatch can be seen on the top right.

Handling "Little Boy" was particularly dangerous. Once cordite was loaded in the breech, any firing of the explosive would at worst cause a nuclear chain reaction and at best a contamination of the explosion zone. The mere contact of the two uranium masses could have caused an explosion with dire consequences (from a simple fizzle explosion to an explosion large enough to destroy 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 that produced a force of 300,000 newtons (67,000 lbf, over 30 tons). For safety reasons, the weaponeer, Captain William Sterling Parsons, decided to load the bag of cordite only after take-off.

After the launch, the bomb used four radar altimeters and a barometric fuse to trigger detonation, and a timing clock to activate the radars fifteen seconds after release. At high altitudes, the air pressure was small but it increased with the fall. A thin metallic membrane was gradually deformed due to pressure. Once the bomb approached the intended height (reportedly 8,000 feet) the membrane closed the circuit and activated the radars for final arming. This mechanism was a safety feature to block a firing voltage from prematurely reaching the trigger and thus prevent radar emissions received from external sources from detonating the bomb during its fall. Then, once two of the radar altimeters sensed the correct height, they would close the firing switch to ignite the cordite. This launched the uranium projectile towards the other end of the gun barrel at a velocity of ~300 meters per second. Approximately 10 milliseconds later an irreversible chain reaction began.

[edit] The bombing of Hiroshima

The mushroom cloud over Hiroshima after the dropping of "Little Boy".

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).^[2]. However, the damage and the number of victims at Hiroshima 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.[1] Unborn babies died or were born with deformities.[2],[3] Clothing was burned into the skin.

The success of the bombing was reported with great enthusiasm in the United States, and it ended the war before a drawn-out and likely bloody invasion of the Japanese home islands would have taken place. However, see Atomic bombings of Hiroshima and Nagasaki for discussion of contemporary opposition to the bombings, both on ethical and military grounds.

[edit] Possible Nazi origins of uranium

It has been assumed that most of the uranium enriched for the bomb came from the Shinkolobwe mine in what was then the Belgian Congo, operated by the Union Minière du Haut Katanga, whose Director-General, Edgar Sengier had sent a stock of the element to New York for the Manhattan Project. He was later awarded the Presidential Medal for Merit for his aid to the victory of the Allies. Other uranium came from sources in the United States (especially the Four Corners region), and from Port Radium, Canada.

However, 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 actually 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.^[3]

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". While it is possible that the uranium could have been added into the overall Manhattan Project materials development effort, as Lt. Col. John Lansdale, Jr., head of Manhattan Project intelligence and security said years later,^[4] it would have only added a relatively small amount of material to either of the bombs, making its final disposition ironic but not essential.

Conversely if the uranium was fully enriched, it would have been over eight times as much enriched uranium than had been developed by the U.S. during the Manhattan Project, and it seems unlikely that Germany would be exporting enough material to make a number of nuclear weapons to Japan, especially since their own nuclear program is known to have been a failure. 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.

Between 240 and 340 tons of uranium oxide was recovered from Germany for use in the Soviet nuclear program, however—the head Soviet scientist on the effort, Igor Kurchatov, said that it sped up the development of their first experimental nuclear reactor by at least a year, as the Soviet Union had very poor uranium reserves at the time.^[5]

[edit] See also

[edit] References

^ Much of this account is taken from the description of the "Little Boy" by Carey Sublette in Section 8 of his "Nuclear Weapons Frequently Asked Questions", available online at http://nuclearweaponarchive.org/Nwfaq/Nfaq8.html.
^ Los Alamos National Laboratory report LA-8819, The yields of the Hiroshima and Nagasaki nuclear explosions by John Malik, september 1985. Available online at http://www.mbe.doe.gov/me70/manhattan/publications/LANLHiroshimaNagasakiYields.pdf
^ Wolfgang Hirschfeld, U-234's signal officer on its final sortie, describes packages labelled "U 235" stored aboard the sub, as well as the suicide of the Japanese on surrender, in his war memories, Feindfahrten: Das Logbuch eines U-Boot-Funkers (German, Neff Verlag 1982)
^ Lansdale is reported to have said that: "It went to the Manhattan District. ... It certainly went into the Manhattan District supply of uranium." Anahad O'Connor, "John Lansdale Jr., 91, Is Dead" New York Times (31 August 2003), copy online at http://www.mindfully.org/Nucs/2003/John-Lansdale-Jr31aug03.htm.
^ David Holloway, Stalin and the bomb (New Haven, CT: Yale University Press, 1994): 111.