Little Boy Totally Explained

Everything about Little Boy totally explained

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 in the 393d Bombardment Squadron, Heavy 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 by the Manhattan Project during World War II. It derived its explosive power from the nuclear fission of uranium 235. The Hiroshima bombing was the second man-made nuclear explosion in history (the first was the "Trinity" test), and it was the first uranium-based detonation ever. Approximately 600 milligrams of mass were converted into energy. It exploded with a destructive power equivalent to between 13 and 16 kilotons of TNT (estimates vary) and killed approximately 140,000 people including associated effects. Little boy was named after Robert Oppenheimer the scientific head of the project.

Basic weapon design

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 hollow sub-critical mass of uranium-235 and a solid target spike together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of chemical explosives. It contained 64 kg of uranium, of which 0.7 kg underwent nuclear fission, and of this mass only 0.6 g was transformed into energy.
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 using 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 when compared to modern nuclear weapons, which incorporate many safety features, designed to anticipate various accident scenarios. The main design objectives of Little Boy were to create a nuclear weapon that was absolutely guaranteed to work. As a result, Little Boy incorporated only the most basic safety mechanisms, so an accidental detonation could easily occur during one or more of the following scenarios:

a simple crash could drive the "bullet" onto the "target" resulting in a massive release of radiation, or possibly nuclear detonation.
an electrical short circuit of some sort.
the danger of misfire was even greater over water. Even if the force of a crash didn't trigger the bomb, the resulting leakage of water into the unprotected system could short it out, again possibly leading to accidental detonation. The British Red Beard nuclear weapon also suffered from this design flaw.
Fire.
Lightning strike.

None of the other five Mark I bombs built on the model of Little Boy were used by the U.S. Army.

Assembly details

The exact specifications of the "Little Boy" bomb remain classified because they could 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 the website Nuclear Weapon Archive, 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 hollow cylinder with 60% of the total mass (38.5 kg). It consisted of a stack of 9 uranium rings, each in diameter with a 4-inch-diameter hole in the middle, pressed together into a thin-walled canister long. At detonation, it would be pushed down a short section of smooth-bore gun barrel by a tungsten carbide and steel plug. The target was a 4-inch-diameter solid spike, 7 inches long, with 40% of the total mass (25.6 kg). Made of a stack of 6 washer-like uranium rings somewhat thicker than the projectile rings, it was held in place by a 1-inch-diameter steel bolt that ran through the rings and out the front end of the bomb casing.
   When the projectile and plug reached the target, the assembled super-critical mass of uranium would be completely surrounded by a tamper and neutron reflector of tungsten carbide and steel. Neutron generators at the base of the spike would be activated by the impact.
   The projectile rings were delivered to Tinian Island on July 26, 1945, by the cruiser USS Indianapolis. The target rings arrived two days later by air.

Counter-intuitive design

For the first fifty years after 1945, every published description and drawing of the Little Boy mechanism assumed that a small, solid projectile was fired into the center of a larger target.

Physical effects of the bomb

Hiroshima was spared conventional bombing in order to serve as a pristine target, one where the effects of a nuclear bomb on a previously undamaged city could be observed. While damage could be studied later, the energy yield of the untested Little Boy design could be determined only at the moment of detonation, using instruments dropped by parachute from a plane flying in formation with the one that dropped the bomb. Radio-transmitted data from these falling instruments indicated a yield of about a dozen kilotons.
Comparing this yield to the observed damage produced a rule of thumb called the 5 psi (1pounds per square inch = 6.894 kilopascal = 0.069 bar) lethal area rule. The number of prompt fatalities will approximately equal the number of people inside the lethal area.
The damage came from three main effects: blast, fire, and radiation.

Blast

The blast from a nuclear bomb is the result of x-ray-heated air (the fireball) sending a shock/pressure wave in all directions at the speed of sound, analogous to thunder generated by a bolt of lightning. Studies of Little Boy at Hiroshima have given us most of what we know about urban blast destruction from nuclear weapons. Nagasaki was less useful in that respect because hilly terrain deflected the blast and generated a more complicated pattern of destruction. At Hiroshima, severe structural damage to buildings extended about one mile (1.6 km) in every direction from ground zero, making a circle of destruction two miles (3 km) in diameter. There was little or no structural damage outside a two-mile (3 km) radius. At one mile (1.6 km), the force of the blast wave was 5 psi, with enough duration to implode houses and reduce them to kindling as it passed. 5 psi is 720 pounds (324 kilogrammes) per square foot (1 sq foot = 0.09 square meters).
Later test explosions of nuclear weapons, with houses and other test structures placed nearby, confirmed that 5 psi is an important threshold figure. Ordinary urban buildings close enough to experience it'll be crushed, toppled, or gutted by the force of air pressure. The picture at right shows the effects of a nuclear-bomb-generated 5 psi pressure wave on a test structure in Nevada in 1953.
The most important effect of this kind of structural damage was that it created fuel for a firestorm. For this reason, the 5 psi contour defines the lethal area for blast and fire.

Fire

The first effect of a nuclear explosion is blinding light, accompanied by radiant heat from the fireball. (The Hiroshima fireball was in diameter.) Near ground zero, everything flammable burst into flame, and any humans were instantly vaporized. One famous, anonymous Hiroshima victim left only a shadow, permanently etched into stone steps near a bank building. Some of the fires started by the fireball's heat were probably blown out by the following blast wave. The blast wave would have started additional fires through overturned stoves, wrecked vehicles, electrical shorts, etc. These numerous small fires quickly merged into a single firestorm which consumed everything inside the 5 psi lethal area.
   The Hiroshima firestorm was thus two miles (3 km) in diameter, corresponding closely to the severe blast damage zone. (See the USSBS map, right.) Blast-damaged buildings provided ideal fuel for the fire. Structural lumber and furniture were splintered and scattered about. Debris-choked roads prevented entry by fire fighters. Broken gas pipes fueled the fire, and broken water pipes rendered hydrants useless.
   As the map shows, the firestorm easily jumped the natural firebreaks (river channels) as well as prepared firebreaks. The spread of fire stopped only when it reached the edge of the blast-damaged area and ran out of easily available fuel.
   Accurate casualty figures are impossible because so many victims were cremated by the firestorm. For the same reason, the portion of firestorm victims who survived the blast and died of fire can never be known. Casualty figures are based on estimates of how many people were inside the lethal area when the bomb went off.

Radiation

Because Little Boy was detonated above the ground, as an air burst, there was no bomb crater and no local radioactive fallout. Local Fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls back to the ground downwind of the crater and can easily produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products remain in aerosol form until they rise into the stratosphere, where they dissipate and become part of the global, rather than the local environment.
However, an intense flux of neutron and gamma radiation came directly from the fireball. Most people close enough to receive lethal doses of that direct radiation died in the firestorm before their radiation injuries could become apparent. But survivors on the edge of the lethal area and beyond suffered injuries from radiation as well as from blast and fire.
Some temporary survivors died soon afterward due to the effects of acute radiation sickness, but most of the radiation effects show up statistically, as increases in cancer rates, birth defects, etc., over the lifetimes of the survivors and their descendants.

Development of the bomb

Manhattan Project during World War II. Because enriched uranium was known to be fissionable, it was the first approach to bomb development pursued. 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 in early 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 from the Hanford enrichment plant was too high to use in a gun-type weapon. 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 almost trivial to design.
   With plutonium found unsuitable for the gun-type design, the team working on the gun weapon (led by A. Francis Birch), faced another problem: the bomb was simple, but they lacked the quantity of uranium-235 necessary for its production. Enough fissile material wasn't going to be available before mid-1945. Despite this, Birch managed to convince others that this concept was worth pursuing, 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 though the technology was less complex than for the other project, a lot of rigorous work was still 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. A small amount may have come from a captured German submarine, U-234, after the German surrender in May 1945. 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.

Construction and delivery

On July 14 1945 a train left Los Alamos carrying several "bomb units" (the major non-nuclear parts of a gun-type bomb) together with a single completed uranium projectile; the uranium target was still incomplete. The consignment was delivered to the San Francisco Naval Shipyard at Hunters Point in San Francisco, California . With all the necessary components delivered to Tinian, bomb unit L11 was chosen, and the final Little Boy weapon was assembled and ready by August 1

The bombing of Hiroshima

The bomb was armed in flight 31,000ft (9600m) above the city, then dropped at approximately 8:15 a.m. (JST). The detonation happened at an altitude of 1980ft (580m). With a power of 13 to 16 kilotons, 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, for example 6.3 × 10¹³ joules = 63 TJ (terajoules). 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 later died as a result of radiation sickness and cancer. Unborn babies died or were born with deformities.
   Recent studies point to different conclusions, based on results of studies conducted by American and Japanese researchers since the war. Data collected for over 86,000 people suggests that roughly 700 people died from long-term effects of radiation in Hiroshima, significantly less than the over 100,000 cited in numerous previously published sources.
   The success of the bombing was reported with great enthusiasm in the United States. See Atomic bombings of Hiroshima and Nagasaki for discussion of contemporary opposition to the bombings, on both moral and military grounds.

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