Little Boy

A post-war "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 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 simply 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 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 when compared to modern nuclear weapons, which incorporate many different safety features embedded in them, 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 were used by the US Army.

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, 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

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 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. 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 who's total development cost 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. The timer then passed on responsibility to a barometric stage.
• The purpose of the barometric stage was to delay activating the final radar altimeter fuse until the bomb was far enough from the airplane that the radar fuze, which was originally developed to warn bombers of approaching fighters, would not detonate the bomb prematurely. A thin metallic membrane was gradually deformed as ambient air pressure naturally 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 moment-to-moment 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 final ground radar altimeter fuse. The barometric stage was added because of a real 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. 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). 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. Unborn babies died or were born with deformities.

The success of the bombing was reported with great enthusiasm in the United States, and it contributed to ending the war before a drawn-out and likely bloody invasion of the Japanese home islands would have taken place (see the surrender of Japan and the U.S. occupation and Japan chose not to surrender ). However, see Atomic bombings of Hiroshima and Nagasaki for discussion of contemporary opposition to the bombings, on both moral and military grounds.

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. U-234 surrendered following the end of hostilities in the European war theater 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". 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, 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.

See also

• White Light/Black Rain: The Destruction of Hiroshima and Nagasaki
• 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