Little Boy

Last updated

Little Boy
Little boy.jpg
A post-war Little Boy model
Type Nuclear weapon
Place of originUnited States
Production history
Designer Los Alamos Laboratory
ManufacturerLos Alamos
No. builtonly 26 were ever made
Mass9,700 pounds (4,400 kg)
Length10 feet (3.0 m)
WidthAbout 1 foot
HeightAbout 1 foot
Diameter28 inches (71 cm)

Filling Uranium-235
Filling weight64 kg
Blast yield15 kilotons of TNT (63 TJ)

"Little Boy" was the codename for the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II. It was the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress Enola Gay piloted by Colonel Paul W. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces and Captain Robert A. Lewis. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and caused widespread death and destruction throughout the city. The Hiroshima bombing was the second man-made nuclear explosion in history, after the Trinity test.


Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a reworking of their unsuccessful Thin Man nuclear bomb. Like Thin Man, it was a gun-type fission weapon, but it derived its explosive power from the nuclear fission of uranium-235, whereas Thin Man was based on fission of plutonium-239. Fission was accomplished by shooting a hollow cylinder of enriched uranium (the "bullet") onto a solid cylinder of the same material (the "target") by means of a charge of nitrocellulose propellant powder. It contained 64 kg (141 lb) of enriched uranium, although less than a kilogram underwent nuclear fission. Its components were fabricated at three different plants so that no one would have a copy of the complete design.

After the war ended, it was not expected that the inefficient Little Boy design would ever again be required, and many plans and diagrams were destroyed. However, by mid-1946, the Hanford Site reactors began suffering badly from the Wigner effect, the dislocation of atoms in a solid caused by neutron radiation, and plutonium became scarce, so six Little Boy assemblies were produced at Sandia Base. The Navy Bureau of Ordnance built another 25 Little Boy assemblies in 1947 for use by the Lockheed P2V Neptune nuclear strike aircraft which could be launched from the Midway-class aircraft carriers. All the Little Boy units were withdrawn from service by the end of January 1951.


Physicist Robert Serber named the first two atomic bomb designs during World War II based on their shapes: Thin Man and Fat Man. The "Thin Man" was a long, thin device and its name came from the Dashiell Hammett detective novel and series of movies about The Thin Man . The "Fat Man" was round and fat so it was named after Kasper Gutman, a rotund character in Hammett's 1930 novel The Maltese Falcon, played by Sydney Greenstreet in the 1941 film version. Little Boy was named by others as an allusion to Thin Man since it was based on its design. [1]


Because uranium-235 was known to be fissionable, it was the first material pursued in the approach to bomb development. As the first design developed (as well as the first deployed for combat), it is sometimes known as the Mark I. [2] The vast majority of the work came in the form of the isotope enrichment of the uranium necessary for the weapon, since uranium-235 makes up only 1 part in 140 of natural uranium. [3] Enrichment was performed at Oak Ridge, Tennessee, where the electromagnetic separation plant, known as Y-12, became fully operational in March 1944. [4] The first shipments of highly enriched uranium were sent to the Los Alamos Laboratory in June 1944. [5]

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 1,200 short tons (1,100  t ) of uranium ore transported to a New York warehouse in 1940. [6] At least part of the 1,200 short tons (1,100 t) in addition to the uranium ore and uranium oxide captured by the Alsos Mission in 1944 and 1945 went to Oak Ridge for enrichment, [7] as did 1,232 pounds (559 kg) of uranium oxide captured on the Japan-bound German submarine U-234 after Germany's surrender in May 1945. [8]

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

Little Boy was a simplification of Thin Man, the previous gun-type fission weapon design. Thin Man, 17 feet (5.2 m) long, was designed to use plutonium, so it was also more than capable of using enriched uranium. The Thin Man design was abandoned after experiments by Emilio G. Segrè and his P-5 Group at Los Alamos on the newly reactor-produced plutonium from Oak Ridge and the Hanford site showed that it contained impurities in the form of the isotope plutonium-240. This has a far higher spontaneous fission rate and radioactivity than the cyclotron-produced plutonium on which the original measurements had been made, and its inclusion in reactor-bred plutonium (needed for bomb-making due to the quantities required) appeared unavoidable. This meant that the background fission rate of the plutonium was so high that it would be highly likely the plutonium would predetonate and blow itself apart in the initial forming of a critical mass. [9]

In July 1944, almost all research at Los Alamos was redirected to the implosion-type plutonium weapon. Overall responsibility for the uranium gun-type weapon was assigned to Captain William S. Parsons's Ordnance (O) Division. All the design, development, and technical work at Los Alamos was consolidated under Lieutenant Commander Francis Birch's group. [10] In contrast to the plutonium implosion-type nuclear weapon and the plutonium gun-type fission weapon, the uranium gun-type weapon was straightforward if not trivial to design. The concept was pursued so that in case of a failure to develop a plutonium bomb, it would still be possible to use the gun principle. The gun-type design henceforth had to work with enriched uranium only, and this allowed the Thin Man design to be greatly simplified. A high-velocity gun was no longer required, and a simpler weapon could be substituted. The simplified weapon was short enough to fit into a B-29 bomb bay. [11]

The design specifications were completed in February 1945, and contracts were let to build the components. Three different plants were used so that no one would have a copy of the complete design. The gun and breech were made by the Naval Gun Factory in Washington, D.C.; the target case and some other components by the Naval Ordnance Plant in Center Line, Michigan; and the tail fairing and mounting brackets by the Expert Tool and Die Company in Detroit, Michigan. [12] The bomb, except for the uranium payload, was ready at the beginning of May 1945. [13] Manhattan District Engineer Kenneth Nichols expected on 1 May 1945 to have uranium-235 "for one weapon before August 1 and a second one sometime in December", assuming the second weapon would be a gun-type; designing an implosion bomb for uranium-235 was considered, and this would increase the production rate. [14] The uranium-235 projectile was completed on 15 June, and the target on 24 July. [15] The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the heavy cruiser USS Indianapolis, arriving on 26 July. [16] The target inserts followed by air on 30 July. [15]

Although all of its components had been tested, [15] no full test of a gun-type nuclear weapon occurred before the Little Boy was dropped over Hiroshima. The only test explosion of a nuclear weapon concept had been of an implosion-type device employing plutonium as its fissile material, and took place on 16 July 1945 at the Trinity nuclear test. There were several reasons for not testing a Little Boy type of device. Primarily, there was little uranium-235 as compared with the relatively large amount of plutonium which, it was expected, could be produced by the Hanford Site reactors. [17] Additionally, the weapon design was simple enough that it was only deemed necessary to do laboratory tests with the gun-type assembly. Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun-type design was considered almost certain to work. [18]

Though Little Boy incorporated various safety mechanisms, an accidental detonation was nonetheless possible. For example, should the bomber carrying the device crash then the hollow "bullet" could be driven into the "target" cylinder, detonating the bomb or at least releasing massive amounts of radiation; tests showed that this would require a highly unlikely impact of 500 times the force of gravity. [19] Another concern was that a crash and fire could trigger the explosives. [20] If immersed in water, the uranium components were subject to a neutron moderator effect, which would not cause an explosion but would release radioactive contamination. For this reason, pilots were advised to crash on land rather than at sea. [19]


The "gun" assembly method. When the hollow uranium projectile was driven onto the target cylinder, a nuclear explosion resulted. Gun-type fission weapon en-labels thin lines.svg
The "gun" assembly method. When the hollow uranium projectile was driven onto the target cylinder, a nuclear explosion resulted.

The Little Boy was 120 inches (300 cm) in length, 28 inches (71 cm) in diameter and weighed approximately 9,700 pounds (4,400 kg). [21] The design used the gun method to explosively force a hollow sub-critical mass of enriched uranium and a solid target cylinder together into a super-critical mass, initiating a nuclear chain reaction. [22] This was accomplished by shooting one piece of the uranium onto the other by means of four cylindrical silk bags of cordite powder. This was a widely used smokeless propellant consisting of a mixture of 65 percent nitrocellulose, 30 percent nitroglycerine, 3 percent petroleum jelly, and 3 percent carbamite that was extruded into tubular granules. This gave it a high surface area and a rapid burning area, and could attain pressures of up to 40,000 pounds per square inch (280,000 kPa). Cordite for the wartime Little Boy was sourced from Canada; propellant for post-war Little Boys was obtained from the Picatinny Arsenal. [23] The bomb contained 64 kg (141 lb) of enriched uranium. Most was enriched to 89% but some was only 50% uranium-235, for an average enrichment of 80%. [22] Less than a kilogram of uranium underwent nuclear fission, and of this mass only 0.6 g (0.021 oz) was transformed into several forms of energy, mostly kinetic energy, but also heat and radiation. [24]

Assembly details

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 (85 lb)). It consisted of a stack of nine uranium rings, each 6.25-inch (159 mm) in diameter with a 4-inch (100 mm) bore in the center, and a total length of 7 inches (180 mm), pressed together into the front end of a thin-walled projectile 16.25 inches (413 mm) long. Filling in the remainder of the space behind these rings in the projectile was a tungsten carbide disc with a steel back. At ignition, the projectile slug was pushed 42 inches (1,100 mm) along the 72-inch (1,800 mm) long, 6.5-inch (170 mm) smooth-bore gun barrel. The slug "insert" was a 4 inches (100 mm) cylinder, 7 inches (180 mm) in length with a 1 inch (25 mm) axial hole. The slug comprised 40% of the total fissile mass (25.6 kg or 56 lb). The insert was a stack of six washer-like uranium discs somewhat thicker than the projectile rings that were slid over a 1 inch (25 mm) rod. This rod then extended forward through the tungsten carbide tamper plug, impact-absorbing anvil, and nose plug backstop, eventually protruding out of the front of the bomb casing. This entire target assembly was secured at both ends with locknuts. [25] [26]

When the hollow-front projectile reached the target and slid over the target insert, the assembled super-critical mass of uranium would be completely surrounded by a tamper and neutron reflector of tungsten carbide and steel, both materials having a combined mass of 2,300 kg (5,100 lb). [27] Neutron initiators at the base of the projectile were activated by the impact. [28]

Little Boy Internal Components.png

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, stationary target. [29] However, critical mass considerations dictated that in Little Boy the larger, hollow piece would be the projectile. The assembled fissile core had more than two critical masses of uranium-235. This required one of the two pieces to have more than one critical mass, with the larger piece avoiding criticality prior to assembly by means of shape and minimal contact with the neutron-reflecting tungsten carbide tamper.

A hole in the center of the larger piece dispersed the mass and increased the surface area, allowing more fission neutrons to escape, thus preventing a premature chain reaction. [30] But, for this larger, hollow piece to have minimal contact with the tamper, it must be the projectile, since only the projectile's back end was in contact with the tamper prior to detonation. The rest of the tungsten carbide surrounded the sub-critical mass target cylinder (called the "insert" by the designers) with air space between it and the insert. This arrangement packs the maximum amount of fissile material into a gun-assembly design. [30]

Fuze system

Arming plugs for a Little Boy type atomic bomb on display at the National Air and Space Museum's Steven F. Udvar-Hazy Center. Little Boy arming plugs in October 2009.JPG
Arming plugs for a Little Boy type atomic bomb on display at the National Air and Space Museum's Steven F. Udvar-Hazy Center.

The fuzing system was designed to trigger at the most destructive altitude, which calculations suggested was 580 meters (1,900 ft). It employed a three-stage interlock system: [31]


Little Boy in the bomb pit on Tinian island, before being loaded into Enola Gay's bomb bay. A section of the bomb bay door is visible on the top right. Atombombe Little Boy 2.jpg
Little Boy in the bomb pit on Tinian island, before being loaded into Enola Gay's bomb bay. A section of the bomb bay door is visible on the top right.

The Little Boy pre-assemblies were designated L-1, L-2, L-3, L-4, L-5, L-6, L-7, and L-11. L-1, L-2, L-5, and L-6 were expended in test drops. The first drop test was conducted with L-1 on 23 July 1945. It was dropped over the sea near Tinian in order to test the radar altimeter by the B-29 later known as Big Stink , piloted by Colonel Paul W. Tibbets, the commander of the 509th Composite Group. Two more drop tests over the sea were made on 24 and 25 July, using the L-2 and L-5 units in order to test all components. Tibbets was the pilot for both missions, but this time the bomber used was the one subsequently known as Jabit . L-6 was used as a dress rehearsal on 29 July. The B-29 Next Objective , piloted by Major Charles W. Sweeney, flew to Iwo Jima, where emergency procedures for loading the bomb onto a standby aircraft were practiced. This rehearsal was repeated on 31 July, but this time L-6 was reloaded onto a different B-29, Enola Gay , piloted by Tibbets, and the bomb was test dropped near Tinian. L-11 was the assembly used for the Hiroshima bomb. [32] [33]

Bombing of Hiroshima

Enola Gay after Hiroshima mission, entering hardstand. It is in its 6th Bombardment Group livery, with victor number 82 visible on fuselage just forward of the tail fin. Enola Gay (plane).jpg
Enola Gay after Hiroshima mission, entering hardstand. It is in its 6th Bombardment Group livery, with victor number 82 visible on fuselage just forward of the tail fin.

Parsons, the Enola Gay's weaponeer, was concerned about the possibility of an accidental detonation if the plane crashed on takeoff, so he decided not to load the four cordite powder bags into the gun breech until the aircraft was in flight. After takeoff, Parsons and his assistant, Second Lieutenant Morris R. Jeppson, made their way into the bomb bay along the narrow catwalk on the port side. Jeppson held a flashlight while Parsons disconnected the primer wires, removed the breech plug, inserted the powder bags, replaced the breech plug, and reconnected the wires. Before climbing to altitude on approach to the target, Jeppson switched the three safety plugs between the electrical connectors of the internal battery and the firing mechanism from green to red. The bomb was then fully armed. Jeppson monitored the bomb's circuits. [34]

The mushroom cloud over Hiroshima after the dropping of Little Boy Atomic cloud over Hiroshima - NARA 542192 - Edit.jpg
The mushroom cloud over Hiroshima after the dropping of Little Boy

The bomb was dropped at approximately 08:15 (JST) on 6 August 1945. After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism. The detonation happened at an altitude of 1,968 ± 50 feet (600 ± 15 m). It was less powerful than the Fat Man, which was dropped on Nagasaki, but 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. According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees. [35] Of those deaths, 20,000 were members of the Imperial Japanese Army. [36]

The exact measurement of the yield was problematic since the weapon had never been tested. President Harry S. Truman officially announced that the yield was 20 kilotons of TNT (84 TJ). This was based on Parsons's visual assessment that the blast was greater than what he had seen at the Trinity nuclear test. Since that had been estimated at 18 kilotons of TNT (75 TJ), speech writers rounded up to 20 kilotons. Further discussion was then suppressed, for fear of lessening the impact of the bomb on the Japanese. Data had been collected by Luis Alvarez, Harold Agnew, and Lawrence H. Johnston on the instrument plane, The Great Artiste , but this was not used to calculate the yield at the time. [37]

After hostilities ended, a survey team from the Manhattan Project that included William Penney, Robert Serber, and George T. Reynolds was sent to Hiroshima to evaluate the effects of the blast. From evaluating the effects on objects and structures, Penney concluded that the yield was 12 ± 1 kilotons. [38] Later calculations based on charring pointed to a yield of 13 to 14 kilotons. [39] In 1953, Frederick Reines calculated the yield as 15 kilotons of TNT (63 TJ). [37] This figure became the official yield. [40]

Project Ichiban

In 1962, scientists at Los Alamos created a mockup of Little Boy known as "Project Ichiban" in order to answer some of the unanswered questions, but it failed to clear up all the issues. In 1982, Los Alamos created a replica Little Boy from the original drawings and specifications. This was then tested with enriched uranium but in a safe configuration that would not cause a nuclear explosion. A hydraulic lift was used to move the projectile, and experiments were run to assess neutron emission. [41] Based on this and the data from The Great Artiste, the yield was estimated at 16.6 ± 0.3 kilotons. [42] After considering many estimation methods, a 1985 report concluded that the yield was 15 kilotons of TNT (63 TJ) ± 20%. [40]

Physical effects

The General Effects of the Atomic Bombs on Hiroshima and Nagasaki, a US Air Force film.

After being selected in April 1945, Hiroshima was spared conventional bombing to serve as a pristine target, where the effects of a nuclear bomb on an undamaged city could be observed. [43] 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 instruments indicated a yield of about 15 kilotons. [40]

Comparing this yield to the observed damage produced a rule of thumb called the 5 pounds per square inch (34  kPa ) lethal area rule. Approximately all the people inside the area where the shock wave carried such an overpressure or greater would be killed. [44] At Hiroshima, that area was 3.5 kilometres (2.2 mi) in diameter. [45]

The damage came from three main effects: blast, fire, and radiation. [46]


The blast from a nuclear bomb is the result of X-ray-heated air (the fireball) sending a shock wave or pressure wave in all directions, initially at a velocity greater than the speed of sound, [47] analogous to thunder generated by lightning. Knowledge about urban blast destruction is based largely on studies of Little Boy at Hiroshima. Nagasaki buildings suffered similar damage at similar distances, but the Nagasaki bomb detonated 3.2 kilometres (2.0 mi) from the city center over hilly terrain that was partially bare of buildings. [48]

Frame house in 1953 nuclear test, 5 psi overpressure House 1953 Nevada Nuclear Test 5 psi.jpg
Frame house in 1953 nuclear test, 5 psi overpressure

In Hiroshima, almost everything within 1.6 kilometres (1.0 mi) of the point directly under the explosion was completely destroyed, except for about 50 heavily reinforced, earthquake-resistant concrete buildings, only the shells of which remained standing. Most were completely gutted, with their windows, doors, sashes, and frames ripped out. [49] The perimeter of severe blast damage approximately followed the 5 psi (34 kPa) contour at 1.8 kilometres (1.1 mi).

Later test explosions of nuclear weapons with houses and other test structures nearby confirmed the 5 psi overpressure threshold. Ordinary urban buildings experiencing it were 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. [50]

A major effect of this kind of structural damage was that it created fuel for fires that were started simultaneously throughout the severe destruction region.


The first effect of the explosion was blinding light, accompanied by radiant heat from the fireball. The Hiroshima fireball was 370 metres (1,200 ft) in diameter, with a surface temperature of 6,000 °C (10,830 °F). [51] Near ground zero, everything flammable burst into flame. One famous, anonymous Hiroshima victim, sitting on stone steps 260 metres (850 ft) from the hypocenter, left only a shadow, having absorbed the fireball heat that permanently bleached the surrounding stone. [52] Simultaneous fires were started throughout the blast-damaged area by fireball heat and by overturned stoves and furnaces, electrical shorts, etc. Twenty minutes after the detonation, these fires had merged into a firestorm, pulling in surface air from all directions to feed an inferno which consumed everything flammable. [53]

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

The Hiroshima firestorm was roughly 3.2 kilometres (2.0 mi) in diameter, corresponding closely to the severe blast damage zone. (See the USSBS [54] map, right.) Blast-damaged buildings provided fuel for the fire. Structural lumber and furniture were splintered and scattered about. Debris-choked roads obstructed firefighters. Broken gas pipes fueled the fire, and broken water pipes rendered hydrants useless. [53] At Nagasaki, the fires failed to merge into a single firestorm, and the fire-damaged area was only one fourth as great as at Hiroshima, due in part to a southwest wind that pushed the fires away from the city. [55]

As the map shows, the Hiroshima firestorm jumped 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, encountering less available fuel. [56]

Accurate casualty figures are impossible to determine, because many victims were cremated by the firestorm, along with all record of their existence. The Manhattan Project report on Hiroshima estimated that 60% of immediate deaths were caused by fire, but with the caveat that "many persons near the center of explosion suffered fatal injuries from more than one of the bomb effects." [57] In particular, many fire victims also received lethal doses of nuclear radiation.


Local fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products rise into the stratosphere, where they dissipate and become part of the global environment. Because Little Boy was an air burst 580 metres (1,900 ft) above the ground, there was no bomb crater and no local radioactive fallout. [58]

However, a burst of intense neutron and gamma radiation came directly from the fireball. Its lethal radius was approximately 1.3 kilometres (0.8 mi), [45] covering about half of the firestorm area. An estimated 30% of immediate fatalities were people who received lethal doses of this direct radiation, but died in the firestorm before their radiation injuries would have become apparent. Over 6,000 people survived the blast and fire, but died of radiation injuries. [57] Among injured survivors, 30% had radiation injuries [59] from which they recovered, but with a lifelong increase in cancer risk. [60] [61] To date, no radiation-related evidence of heritable diseases has been observed among the survivors' children. [62] [63] [64]

Conventional weapon equivalent

Although Little Boy exploded with the energy equivalent of 16,000 tons of TNT, the Strategic Bombing Survey estimated that the same blast and fire effect could have been caused by 2,100 tons of conventional bombs: "220 B-29s carrying 1,200 tons of incendiary bombs, 400 tons of high-explosive bombs, and 500 tons of anti-personnel fragmentation bombs." [65] Since the target was spread across a two-dimensional plane, the vertical component of a single spherical nuclear explosion was largely wasted. A cluster bomb pattern of smaller explosions would have been a more energy-efficient match to the target. [65]


One of five casings built for the Little Boy bomb used on Hiroshima on display at the Imperial War Museum in London during 2015 Spare Little Boy atomic bomb casing at the Imperial War Museum in London in November 2015.jpg
One of five casings built for the Little Boy bomb used on Hiroshima on display at the Imperial War Museum in London during 2015

When the war ended, it was not expected that the inefficient Little Boy design would ever again be required, and many plans and diagrams were destroyed. However, by mid-1946 the Hanford Site reactors were suffering badly from the Wigner effect. Faced with the prospect of no more plutonium for new cores and no more polonium for the initiators for the cores that had already been produced, the Director of the Manhattan Project, Major General Leslie R. Groves, ordered that some Little Boys be prepared as an interim measure until a cure could be found. No Little Boy assemblies were available, and no comprehensive set of diagrams of the Little Boy could be found, although there were drawings of the various components, and stocks of spare parts. [66] [67]

At Sandia Base, three Army officers, Captains Albert Bethel, Richard Meyer, and Bobbie Griffin attempted to re-create the Little Boy. They were supervised by Harlow W. Russ, an expert on Little Boy who served with Project Alberta on Tinian, and was now leader of the Z-11 Group of the Los Alamos Laboratory's Z Division at Sandia. Gradually, they managed to locate the correct drawings and parts, and figured out how they went together. Eventually, they built six Little Boy assemblies. Although the casings, barrels, and components were tested, no enriched uranium was supplied for the bombs. By early 1947, the problem caused by the Wigner effect was on its way to solution, and the three officers were reassigned. [66] [67]

The Navy Bureau of Ordnance built 25 Little Boy assemblies in 1947 for use by the nuclear-capable Lockheed P2V Neptune aircraft carrier aircraft (which could be launched from but not land on the Midway-class aircraft carriers). Components were produced by the Naval Ordnance Plants in Pocatello, Idaho, and Louisville, Kentucky. Enough fissionable material was available by 1948 to build ten projectiles and targets, although there were only enough initiators for six. [68] All the Little Boy units were withdrawn from service by the end of January 1951. [69] [70]

The Smithsonian Institution displayed a Little Boy (complete, except for enriched uranium), until 1986. The Department of Energy took the weapon from the museum to remove its inner components, so the bombs could not be stolen and detonated with fissile material. The government returned the emptied casing to the Smithsonian in 1993. Three other disarmed bombs are on display in the United States; another is at the Imperial War Museum in London. [29]


  1. Serber & Crease 1998, p. 104.
  2. Hansen 1995, p. V-105.
  3. Jones 1985, p. 9.
  4. Jones 1985, p. 138.
  5. Jones 1985, p. 143.
  6. Jones 1985, pp. 64–65.
  7. Rhodes 1995, pp. 160–161.
  8. "The Sensational Surrender of Four Nazi U-boats at the Portsmouth Naval Shipyard". New England Historical Society. 15 May 2015. Retrieved 19 September 2018.
  9. Hoddeson et al. 1993, p. 228.
  10. Hoddeson et al. 1993, pp. 245–249.
  11. Rhodes 1986, p. 541.
  12. Hoddeson et al. 1993, p. 257.
  13. Hoddeson et al. 1993, p. 262.
  14. Nichols 1987, pp. 175–176.
  15. 1 2 3 Hoddeson et al. 1993, p. 265.
  16. Coster-Mullen 2012, p. 30.
  17. Hansen 1995, pp. 111–112.
  18. Hoddeson et al. 1993, p. 293.
  19. 1 2 Hansen 1995, p. 113.
  20. Hoddeson et al. 1993, p. 333.
  21. Gosling 1999, p. 51.
  22. 1 2 Coster-Mullen 2012, p. 18.
  23. Coster-Mullen 2012, p. 27.
  24. Glasstone & Dolan 1977, p. 12.
  25. Sublette, Carey. "Nuclear Weapons Frequently Asked Questions, Section 8.0: The First Nuclear Weapons" . Retrieved 29 August 2013.
  26. Coster-Mullen 2012, pp. 18–19, 27.
  27. Bernstein 2007, p. 133.
  28. Hoddeson et al. 1993, pp. 263–265.
  29. 1 2 Samuels 2008.
  30. 1 2 Coster-Mullen 2012, pp. 23–24.
  31. 1 2 3 4 Hansen 1995a, pp. 2–5.
  32. Campbell 2005, pp. 46, 80.
  33. Coster-Mullen 2012, pp. 100–101.
  34. Coster-Mullen 2012, pp. 34–35.
  35. The Manhattan Engineer District (29 June 1946). "The Atomic Bombings of Hiroshima and Nagasaki". Project Gutenberg (Public Domain). p. 3.
  36. Alan Axelrod (6 May 2008). The Real History of World War II: A New Look at the Past . Sterling. p.  350.
  37. 1 2 Hoddeson et al. 1993, p. 393.
  38. Malik 1985, pp. 18–20.
  39. Malik 1985, p. 21.
  40. 1 2 3 Malik 1985, p. 1.
  41. Coster-Mullen 2012, pp. 86–87.
  42. Malik 1985, p. 16.
  43. Groves 1962, p. 267, "To enable us to assess accurately the effects of the [nuclear] bomb, the targets should not have been previously damaged by air raids." Four cities were chosen, including Hiroshima and Kyoto. War Secretary Stimson vetoed Kyoto, and Nagasaki was substituted. p. 275, "When our target cities were first selected, an order was sent to the Army Air Force in Guam not to bomb them without special authority from the War Department.".
  44. Glasstone 1962, p. 629.
  45. 1 2 Glasstone & Dolan 1977, p. Nuclear Bomb Effects Computer.
  46. Glasstone & Dolan 1977, p. 1.
  47. Diacon 1984, p. 18.
  48. Glasstone & Dolan 1977, pp. 300, 301.
  49. The Atomic Bombings of Hiroshima and Nagasaki, 1946, p. 14.
  50. Glasstone & Dolan 1977, p. 179.
  51. Nuclear Weapon Thermal Effects 1998.
  52. Human Shadow Etched in Stone.
  53. 1 2 Glasstone & Dolan 1977, pp. 300–304.
  54. D'Olier 1946, pp. 22–25.
  55. Glasstone & Dolan 1977, p. 304.
  56. The Atomic Bombings of Hiroshima and Nagasaki, 1946, pp. 21–23.
  57. 1 2 The Atomic Bombings of Hiroshima and Nagasaki, 1946, p. 21.
  58. Glasstone & Dolan 1977, p. 409 "An air burst, by definition, is one taking place at such a height above the earth that no appreciable quantities of surface material are taken up into the fireball. ... the deposition of early fallout from an air burst will generally not be significant. An air burst, however, may produce some induced radioactive contamination in the general vicinity of ground zero as a result of neutron capture by elements in the soil." p. 36, "at Hiroshima ... injuries due to fallout were completely absent.".
  59. Glasstone & Dolan 1977, pp. 545, 546.
  60. Richardson RR 2009.
  61. "The ongoing research into the effects of radiation". Radio Netherlands Archives. 31 July 2005. Retrieved 16 December 2018.
  62. Genetic Effects.
  63. Izumi BJC 2003.
  64. Izumi IJC 2003.
  65. 1 2 D'Olier 1946, p. 24.
  66. 1 2 Coster-Mullen 2012, p. 85.
  67. 1 2 Abrahamson & Carew 2002, pp. 41–42.
  68. Hansen 1995, pp. 116–118.
  69. Hansen 1995, p. 3.
  70. "Chart of Strategic Nuclear Bombs".

Related Research Articles

Fat Man Codename for the type of atomic bomb that was detonated over the Japanese city of Nagasaki on 9 August 1945

"Fat Man" is the codename for the type of nuclear bomb that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation marked the third nuclear explosion in history. It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site, and it was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney.

Manhattan Project Research and development project that produced the first atomic bombs.

The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the actual bombs. As engineer districts by convention carried the name of the city where they were located, the Army component of the project was designated the Manhattan District; Manhattan gradually superseded the official codename, Development of Substitute Materials, for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion. Over 90 percent of the cost was for building factories and to produce fissile material, with less than 10 percent for development and production of the weapons. Research and production took place at more than thirty sites across the United States, the United Kingdom, and Canada.

Nuclear fission Nuclear reaction splitting an atom into multiple parts

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

Trinity (nuclear test) United States historic event

Trinity was the code name of the first detonation of a nuclear device. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, on what was then the USAAF Alamogordo Bombing and Gunnery Range, now part of White Sands Missile Range. The only structures originally in the vicinity were the McDonald Ranch House and its ancillary buildings, which scientists used as a laboratory for testing bomb components. A base camp was constructed, and there were 425 people present on the weekend of the test.

Nuclear technology Technology that involves the reactions of atomic nuclei

Nuclear technology is technology that involves the nuclear reactions of atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in smoke detectors and gun sights.

Nuclear weapon design Process by which nuclear WMDs are designed and produced

Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types:

Operation Crossroads 1946 nuclear weapon tests at Bikini Atoll

Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the atomic bombing of Nagasaki on August 9, 1945. The purpose of the tests was to investigate the effect of nuclear weapons on warships.

History of nuclear weapons

Nuclear weapons possess enormous destructive power from nuclear fission or combined fission and fusion reactions. Building on scientific breakthroughs made during the 1930s, the United States, the United Kingdom, Canada, and free France collaborated during World War II, in what was called the Manhattan Project, to build a fission weapon, also known as an atomic bomb. In August 1945, the atomic bombings of Hiroshima and Nagasaki were conducted by the United States against Japan at the close of that war, standing to date as the only use of nuclear weapons in hostilities. The Soviet Union started development shortly after with their own atomic bomb project, and not long after, both countries were developing even more powerful fusion weapons known as hydrogen bombs. Britain and France built their own systems in the 1950s, and the list of states with nuclear weapons has gradually grown larger in the decades since.

Effects of nuclear explosions Type and severity of damage caused by nuclear weapons

The effects of a nuclear explosion on its immediate vicinity are typically much more destructive and multifaceted than those caused by conventional explosives. In most cases, the energy released from a nuclear weapon detonated within the lower atmosphere can be approximately divided into four basic categories:

Mushroom cloud Mushroom-shaped flammagenitus cloud of debris/smoke resulting from a large or nuclear explosion

A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke and usually condensed water vapor resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce the same effect. They can be caused by powerful conventional weapons, like thermobaric weapons, including the ATBIP and GBU-43/B Massive Ordnance Air Blast. Some volcanic eruptions and impact events can produce natural mushroom clouds.

Operation Sandstone

Operation Sandstone was a series of nuclear weapon tests in 1948. It was the third series of American tests, following Trinity in 1945 and Crossroads in 1946, and preceding Ranger. Like the Crossroads tests, the Sandstone tests were carried out at the Pacific Proving Grounds, although at Enewetak Atoll rather than Bikini Atoll. They differed from Crossroads in that they were conducted by the Atomic Energy Commission, with the armed forces having only a supporting role. The purpose of the Sandstone tests was also different: they were primarily tests of new bomb designs rather than of the effects of nuclear weapons. Three tests were carried out in April and May 1948 by Joint Task Force 7, with a work force of 10,366 personnel, of whom 9,890 were military.

Soviet atomic bomb project Soviet program to develop nuclear weapons during World War II

The Soviet atomic bomb project was the classified research and development program that was authorized by Joseph Stalin in the Soviet Union to develop nuclear weapons during World War II.

Timeline of the Manhattan Project

The Manhattan Project was a research and development project that produced the first atomic bombs during World War II. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army Corps of Engineers. The Army component of the project was designated the Manhattan District; "Manhattan" gradually became the codename for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion. Over 90% of the cost was for building factories and producing the fissionable materials, with less than 10% for development and production of the weapons.

Nuclear weapon yield Energy released in nuclear weapons explosions

The explosive yield of a nuclear weapon is the amount of energy released when that particular nuclear weapon is detonated, usually expressed as a TNT equivalent (the standardized equivalent mass of trinitrotoluene which, if detonated, would produce the same energy discharge), either in kilotons (kt—thousands of tons of TNT), in megatons (Mt—millions of tons of TNT), or sometimes in terajoules (TJ). An explosive yield of one terajoule is equal to 0.239 kilotonnes of TNT. Because the accuracy of any measurement of the energy released by TNT has always been problematic, the conventional definition is that one kiloton of TNT is held simply to be equivalent to 1012 calories.

Gun-type fission weapon

Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another. Although this is sometimes pictured as two sub-critical hemispheres driven together to make a supercritical sphere, typically a hollow projectile is shot onto a spike which fills the hole in its center. Its name is a reference to the fact that it is shooting the material through an artillery barrel as if it were a projectile. Other potential arrangements may include firing two pieces into each other simultaneously, though whether this approach has been used in actual weapons designs is unknown.

Nuclear explosion Explosion that occurs as a result of rapid release of energy from a nuclear reaction (fission or fusion)

A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, though to date all fusion-based weapons have used a fission device to initiate fusion, and a pure fusion weapon remains a hypothetical device.

Thin Man (nuclear bomb) Nuclear weapon

"Thin Man" was the code name for a proposed plutonium gun-type nuclear bomb using plutonium-239 which the United States was developing during the Manhattan Project. They aborted its development when they discovered that the spontaneous fission rate of their nuclear reactor-bred plutonium was too high for use in a gun-type design, due to the high concentration of the isotope plutonium-240.

Upshot-Knothole Grable 1953 nuclear artillery test at the Nevada Test Site, United States

Upshot–Knothole Grable was a nuclear weapons test conducted by the United States as part of Operation Upshot–Knothole. Detonation of the associated nuclear weapon occurred 19 seconds after its deployment at 8:30am PDT on May 25, 1953, in Area 5 of the Nevada Test Site. The codename Grable was chosen because the letter Grable is phonetic for G, as in "gun", since the warhead was a gun-type fission weapon. As a shell, or artillery-fired atomic projectile (AFAP), the device was the first of its kind. The test remains the only nuclear artillery shell ever actually fired in the U.S. nuclear weapons test program.

Atomic bombings of Hiroshima and Nagasaki Use of nuclear weapons in World War II

The United States detonated two nuclear weapons over the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August 1945, respectively. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the only use of nuclear weapons in armed conflict.

Project Y Secret laboratory established by the Manhattan Project

The Los Alamos Laboratory, also known as Project Y, was a secret laboratory established by the Manhattan Project and operated by the University of California during World War II. Its mission was to design and build the first atomic bombs. Robert Oppenheimer was its first director, serving from 1943 to December 1945, when he was succeeded by Norris Bradbury. In order to enable scientists to freely discuss their work while preserving security, the laboratory was located in a remote part of New Mexico. The wartime laboratory occupied buildings that had once been part of the Los Alamos Ranch School.