Little Boy

Little Boy
A post-war "Little Boy" model.
Type	Nuclear weapon
Place of origin	United States
Specifications
Mass	4,000 kg
Length	3.0 m
Diameter	0.7 m
Blast yield	13 to16 kilotons

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 (Tibbets, age 92, dies Nov. 1, 2007) 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 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.

Basic weapon design

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

File:Gun-Type Fission Weapon.gif

The "gun" assembly method. When the hollow uranium projectile was driven onto the target spike, a nuclear explosion resulted.

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 became 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 did not 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 was 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 6.25 inches in diameter with a 4-inch-diameter hole in the middle, pressed together into a thin-walled canister 7 inches 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.

File:Little Boy Internal Components.gif

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.

Critical mass considerations dictated that in Little Boy the larger, hollow piece would be the projectile. For the assembled fissile core to have more than two critical masses of U-235, one of the two pieces would need to have more than one critical mass, and to avoid criticality by means of shape, namely a hole in the middle. The larger (outer) surface area allows more fission neutrons to escape and hence not cause a new fission.

It was also important for the larger piece to have minimal contact with the tamper of neutron-reflecting tungsten carbide until the moment of detonation. As the projectile, it would have only its back end in contact with tungsten carbide (see drawing above). The rest of the tungsten carbide could be installed around the target spike, called the "insert" by designers, where an air space would keep it away from the sides of the insert. This is the only way to pack the maximum amount of fissile material into a gun-assembly design.

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 (pounds per square inch) 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.

Frame house in 1953 nuclear test, 5 psi overpressure.

At Hiroshima, severe structural damage to buildings extended about one mile in every direction from ground zero, making a circle of destruction two miles in diameter. There was little or no structural damage outside a two-mile radius. At one mile, 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 per square foot.

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 will 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 1200 feet in diameter.) Near ground zero, everything flammable burst into flame, including human flesh. One famous, anonymous Hiroshima victim left only a shadow, permanently etched into stone steps near a bank building.

Hiroshima blast and fire damage, U.S. Strategic Bombing Survey map.

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 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 1900 feet 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 of acute radiation illness, 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

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 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.

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.

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 was not 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

Little Boy in the bomb pit on Tinian, before being loaded into Enola Gay's bomb bay. A section of the bomb bay door is visible on the top right

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. There, two hours before the successful test of Little Boy's plutonium-implosion brother at the Trinity test in New Mexico, the bomb units and the projectile were loaded aboard the heavy cruiser USS Indianapolis. Indianapolis steamed, at a record pace, to the airbase at Tinian island in the Mariana Islands, delivering them ten days later on the 26th. While returning from this mission Indianapolis was sunk by a Japanese submarine, with great loss of life due to shark attacks. Also on the 26th the three sections of the uranium target assembly were shipped from Kirtland Air Force Base near Albuquerque, New Mexico in three C-54 Skymaster aircraft operated by the 509th Composite Group's Green Hornet squadron . 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.

Handling the completed 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 (including the 500 B-29s based there, and their supporting infrastructure and personnel). 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 bags of cordite only after take-off.

Fuse system

The bomb employed a fuse system worthy of a device whose total development cost was approximately $1,000,000,000 ($11 billion in 2006 dollars) to build, and was designed to detonate at the most destructive altitude. Calculations showed that for the largest destructive effect, the bomb should explode at an altitude of 580 meters. The resultant fuse design was a three-stage interlock system:

• A timer ensured that the bomb would not explode until at least fifteen seconds after release, one-quarter of the predicted fall rate, to ensure safety of the aircraft. The timer was activated when the electrical pullout plugs connecting it to the airplane were pulled loose by the drop of the bomb, switching it to internal (24V battery) power and starting the timer. At the end of the 15 seconds the batteries then powered the radar system and passed on responsibility to a barometric stage.
• The purpose of the barometric stage was to delay activating the final radar altimeter firing command circuit until near detonation altitude. A thin metallic membrane was gradually deformed as ambient air pressure increased during descent. The barometric fuse was not in itself considered accurate enough to be used to detonate the bomb at the precise ignition height, because air pressure varies with local weather conditions. When the bomb reached the design height for this stage (reportedly 2,000 meters) the membrane closed a circuit, activating the circuit between the radar altimeter and the projectile gun. The barometric stage was added because of a worry that radar signals from external sources might detonate the bomb too early to be effective.
• The doubly-redundant radar system employed four radar altimeters that independently detected altitude directly from radar reflections off the ground. When any two of the four altimeters sensed the correct height, the firing switch closed, igniting the cordite charge. This launched the uranium projectile towards the other end of the gun barrel at an eventual muzzle velocity of ~300 meters per second. Approximately 10 milliseconds later the chain reaction took place, lasting less than 1 μs.

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 (1900 feet). 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, i.e. 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 nuclear fallout 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.Spiegel

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.

See also

• White Light/Black Rain: The Destruction of Hiroshima and Nagasaki
• Enola Gay
• Fat Man
• Manhattan Project
• Trinity test
• The gadget
• Atomic bombings of Hiroshima and Nagasaki
• List of nuclear weapons
• Upshot-Knothole Grable

External links

• White Light/Black Rain Official Website (film)
• Video of Little Boy and Fat Man
• Little Boy description at Carey Sublette's NuclearWeaponArchive.org
• Nuclear Files.org Definition and explanation of 'Little Boy'
• The Nuclear Weapon Archive
• History of Enola Gay
• A little known episode of the Manhattan Project Factitious tests of bombs and problems in aerodynamism
• Development of Little Boy and Fat Man (fr)
• Functioning de Little Boy et Fat Man (fr)
• Little Boy : a macabre irony (fr)
• Little Boy 3D Model
• Hiroshima Remembered Information about preparation and dropping the Little Boy bomb
• Models of Hiroshima city center before and after In the Hiroshima Peace Memorial Museum

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