Fat Man

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Fat Man
A post-war "Fat Man" model.
Type	Nuclear weapon
Place of origin	United States
Specifications
Weight	10,200 lbs (4,630 kg)
Length	10.6 Feet (3.25m)
Diameter	5 Feet (1.52m)
Blast yield	21 kilotons

"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the two nuclear weapons to be used in warfare and was the third man-made nuclear explosion at that time. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core. ^[1]

Fat Man was possibly named after Winston Churchill^[2], though Robert Serber said in his memoirs that as the "Fat Man" bomb was round and fat, he named it after Sidney Greenstreet's character in The Maltese Falcon. During development, however, there was little uniformity, and both it and the shotgun design, now commonly known as Little Boy, were variously called Fat Boy, Thin Man (allegedly after FDR), S-1, and "the gadget".^[3]

"Fat Man" was detonated at an altitude of about 1,800 feet (550 m) over the city, and was dropped from a B-29 bomber Bockscar, piloted by Major Charles Sweeney of the 393d Bombardment Squadron, Heavy. The bomb had a yield of about 21 kilotons of TNT, or 8.78×10¹³ joules = 88 TJ (terajoules).^[4] Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 39,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured.^[5] Thousands more would die later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiations. The aerial bombing raid on Nagasaki had the third highest fatality rate^[6] in World War II after the nuclear strike on Hiroshima^{[7][8][9][10]} and the March 9/10 1945 fire bombing raid on Tokyo^[11].

[edit] Technology

The weapon was 10 feet 8 inches (3.25 m) long, five feet (1.52 m) in diameter, and weighed 10,200 pounds (4,630 kg). In accordance with the name, it was more than twice as wide as Little Boy, which was dropped on Hiroshima three days earlier; however, the mass was only 10% more than that of Little Boy.

"Fat Man" was an implosion-type weapon using plutonium 239. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.

The difficulty in the design lay primarily in properly compressing the plutonium core into a near-perfect sphere; if the compression was not symmetrical it would cause the plutonium to be ejected from the weapon, making it an inefficient "dirty bomb". In order to accomplish the compression, the high explosive system had to be carefully designed as a series of explosive lenses which used alternating fast- and slow-burning explosives to shape the explosive shockwave into the desired spherical shape. An early idea of this sort had been raised by physicist Richard Tolman during early discussions of possible bomb designs, specifically in having many pieces of fissile material attached to explosives that would then assemble them in a spherical fashion. This idea was further developed by Seth Neddermeyer, who attempted to find a way to collapse a hollow sphere of plutonium onto a solid sphere of it inside itself. Neither of these ideas relied on compression of the plutonium, and neither would assemble the device fast enough to avoid predetonation (see discussion below).^[12]

The idea of using shaped charges came from James L. Tuck^[13] and was developed by mathematician John von Neumann^[14], and the idea that under such pressures the plutonium metal itself would be compressed may have come about from conversations with Edward Teller, whose knowledge of how dense metals behaved under heavy pressure was influenced by his theoretical studies of the Earth's core with George Gamov.^[12] Von Neumann and George Kistiakowsky eventually became the principal architects behind the lens system. Robert Christy is generally credited with the insight that a solid subcritical sphere of plutonium could be compressed to a critical state greatly simplifying the task since earlier efforts had attempted the more difficult compression of 3d shapes like spherical shells. Because Christy's insight made the feasibility of a plutonium bomb much more likely, the weapon tested at Alamogordo and used at Nagasaki is sometimes referred to as the "Christy Gadget."

The "Trinity" device was similar to the "Fat Man" bomb.

At first it was thought that two pieces of subcritical plutonium (Pu-239) could simply be shot into one another to create a nuclear explosion, and a plutonium gun-type design of this sort (known as the "Thin Man" bomb) was worked on for some time during the Manhattan Project. However in April 1944 it was discovered by Emilio Segrè that plutonium created for the bomb in the nuclear reactors at Hanford, Washington—even though it was supergrade weapon plutonium containing only about 0.9% Pu-240—was not as pure as the initial samples of plutonium developed at the cyclotrons at Ernest O. Lawrence's Radiation Laboratory in Berkeley, California. Because of the presence of the Pu-240 isotope, reactor-bred plutonium had a much higher rate of spontaneous neutron emission than was previously thought, and if a gun-type device was used it would most likely pre-detonate and result in a messy and costly "fizzle." The spontaneous fission rate of Pu-240 is 40,000 times greater than that of Pu-239, so that in a gun-type plutonium device of the sort planned during the Manhattan Project, the last few centimeters would have to be traveled in less than 40 microseconds.

After this problem was realized, the entire Los Alamos laboratory re-organized around the problem of the implosion bomb, the "Fat Man" starting in June 1944.

The gun-type method could still be used for highly enriched uranium, though, and was employed in the "Little Boy" weapon, which was used against Hiroshima. The implosion method is more efficient than the gun-type method, and also far safer, as a perfect synchronization of the explosion lenses is required for the core to properly detonate, greatly reducing the chances of an accidental nuclear detonation.

As a result of espionage information procured by Klaus Fuchs, the first Soviet bomb, "RDS–1" (above) closely resembled Fat Man, even in its external shape.

Because of its complicated firing mechanism, and the need for previously untested synchronization of explosives and precision design, it was felt that a full test of the concept was needed before the scientists and military representatives could be confident it would perform correctly under combat conditions. On July 16, 1945, a device utilizing a similar mechanism (called the "gadget" for security reasons) detonated in a test explosion at a remote site in New Mexico, known as the "Trinity" test. It gave somewhere around 20 kt (80 TJ), 2 to 4 times the expected yield.

The Soviet Union's first nuclear weapon detonated at Operation First Lightning (known as "Joe 1" in the West) was closely based on the "Fat Man" device, on which they had obtained detailed information from the spy Klaus Fuchs.

The names for all three projects ("Fat Man", "Thin Man", and "Little Boy") were created by Robert Serber, a former student of Los Alamos director Robert Oppenheimer's who worked on the project. According to his later memoirs, he chose them based on their design shapes; the "Thin Man" would be a very long device, and the name came from the Dashiell Hammett detective novel and series of movies by the same name; the "Fat Man" bomb would be round and fat and was named after Sidney Greenstreet's character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.^[15]

[edit] Interior of bomb

The original blueprints of the interior of both Fat Man and Little Boy are still classified. However, much information about the main parts is available in the unclassified public literature. Of particular interest is a description of Fat Man sent to Moscow by Soviet spies at Los Alamos in 1945. It was released by the Russian government in 1992.^[16]

Below is a diagram of the main parts of the "Fat Man" bomb itself, followed by a more detailed look at the different materials used in the physics package of the bomb (the part responsible for the nuclear detonation).

AN 219 contact fuse (four) Archie radar antenna Plate with batteries (to detonate charge surrounding nuclear components) X-Unit, a firing set placed near the charge Hinge fixing the two ellipsoidal parts of the bomb Physics package (see details below) Plate with instruments (radars, baroswitches and timers) Barotube collector California Parachute tail assembly (.20-inch aluminium sheet)

[edit] Physics package

[edit] Assembly

Jim Sanborn's "Critical Assembly." The bottom hemisphere of the pusher, with pieces of the boron shell, tamper, pit, and urchin.

The physics package getting its shell.

Fat Man on its transport carriage.

To allow insertion of the plutonium pit as late as possible in the bomb's assembly, the spherical U-238 tamper had a 4" diameter cylindrical hole running through it, like the hole in a cored apple. The missing cylinder, containing the plutonium pit, could be slipped in through a hole in the surrounding aluminium pusher. In August 1945, it was assembled on Tinian Island. When the physics package was fully assembled and wired, it was placed inside its ellipsoidal aerodynamic bombshell and wheeled to the bomb bay of a B-29 named "Bockscar" for its flight to Nagasaki on August 9.

In 2003, these concentric spheres and cylinder were recreated as the centerpiece of an art installation called "Critical Assembly" by sculptor Jim Sanborn. Using non-nuclear materials, he replicated the internal components of the "Trinity" bomb, which had the same design as Fat Man. Critical Assembly was first displayed at the Corcoran Gallery of Art, in Washington, DC.^[17]

[edit] Detonation sequence

The plutonium must be compressed to twice its normal density before free neutrons are added to start the fission chain reaction:

•      An exploding-bridgewire detonator simultaneously starts a detonation wave in each of the 32 tapered high explosive columns (pentagons and hexagons arranged as on a soccer ball - in a truncated icosahedron).
•      The detonation wave (arrows) is initially convex in the
•      faster explosive, Composition B: 60% RDX, 39% TNT, 1% wax. Its shape becomes concave in the
•      slower explosive (Baratol). The 32 waves merge into a single spherical implosive wave before they hit the
•      faster explosive, Composition B, of the inner charges.
•      The medium-density aluminum "pusher" transfers the imploding shock wave from low-density explosive to high-density uranium, minimizing undesirable turbulence; the shock wave then compresses the inner components. At the very center, the

•      beryllium–polonium-210 "initiator" (the "urchin") is crushed, bringing the two metals in contact to release a burst of neutrons into the compressed
•      "pit" of plutonium-239–plutonium-240–gallium delta-phase alloy (96%–1%–3% by molarity). A fission chain reaction starts. The tendency of the fissioning pit to prematurely blow itself apart is retarded by the inward momentum of the
•      natural-uranium "tamper" (inertial containment). The tamper also reflects neutrons back into the pit, speeding up the chain reaction.
•      The boron plastic shell was intended to protect the pit from stray neutrons, but was later deemed unnecessary.

The result is the fissioning of about two and a half of the thirteen pounds of plutonium in the pit, and the release of twenty-one kilotons of energy (21,000 tons of TNT).

[edit] Nagasaki casualties

According to published US Army figures 39,000 people were killed as a direct result of the Nagasaki blast, and 25,000 were injured to varying degrees. ^[18] Most sources refer to a great number more later dying as a result of radiation sickness and cancer or unborn babies that died before birth or were born with deformities ^[19] but this appears to be just commonly accepted urban myth and unsupported by the actual events and studies.

The survivors of both the Hiroshima and Nagasaki nuclear explosions have subsequently been some of the closest monitored survivors of the Second World War. Both Japanese and American medical institutes launched a massive and thorough epidemiological study after the war. The study included ALL residents of both Hiroshima and Nagasaki who had survived the atomic explosion within a 10 kilometre (6.2 mile) radius. As previously noted the bombs were exploded as airbursts and there was no residual radioactive fallout, so anyone outside of 10 kilometres would have received no radioactive contamination.

Investigators questioned the residents to identify their precise locations when the bombs exploded, and used this information to calculate a personal radiation dose for each resident. Data was collected for all 86,572 survivors of the two bursts.

Sixty three years later the results from the extensive study are clear. The post event deaths from nuclear effects are not the alarmist tens of thousands up to 105,000 claimed in several sources. The subsequent deaths are actually less than 1,000 from the combined cities. In fact, in the years since 1945 just 777 eventually died as a result of radiation received from the atomic attacks:

87 have died from leukemia

440 died of radiation induced tumors;

and 250 died of radiation induced heart attacks.

In addition it should be noted that from the pregnant women irradiated by the Hiroshima and Nagasaki bursts only 30 fetuses subsequently developed mental disabilities after they were born and no physical deformities were noted. There have also been no significant birth problems in the years following the explosions as there was no residual radiation at either site, the initial radiations dissipating and decaying in the days and weeks following the bursts. ^[20]

[edit] See also

Wikimedia Commons has media related to:

Fat Man

• Atomic bombings of Hiroshima and Nagasaki
• Little Boy
• The gadget
• Trinity test
• Thin Man nuclear bomb
• Fat Man and Little Boy, a 1989 film that reenacts the Manhattan Project
• List of nuclear weapons
• Manhattan Project

[edit] References

[edit] External links

• thorough descriptions of Gadget ("a great deal of tissue paper and Scotch tape were used to make everything fit snugly") and Fat Man at Nuclear Weapons Archive
• The Design of Gadget, Fat Man, and "Joe 1" (RDS-1)
• Fat Man Model in QuickTime VR format
• Naming of "Fat Man" & "Thin Man" after Churchill, Roosevelt
• The final bomb design, with pictures

v • d • e Manhattan Project Project Sites Hanford Engineer Works · Oak Ridge, Tennessee · Los Alamos, New Mexico · Berkeley Radiation Laboratory · Chicago Metallurgical Laboratory · Trinity Test Site · Project Alberta · Project Ames · Dayton Project People Robert Oppenheimer · Leslie Groves · Enrico Fermi · David Bohm · Emilio Segrè · Edward Teller Related Articles Timeline of the Manhattan Project · History of nuclear weapons · Nuclear weapons and the United States · S-1 Uranium Committee · Chicago Pile-1 · X-10 Graphite Reactor · Y-12 National Security Complex · Operation Alsos · Smyth Report · 509th Operations Group · Fat Man · Little Boy · Atomic bombings of Hiroshima and Nagasaki (debate)