Fat Man

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This article is about the nuclear weapon used in World War II. For other uses, see Fat Man (disambiguation).
A post-war "Fat Man" model.
A post-war "Fat Man" model.

"Fat Man" is the codename of 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. 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.

"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. The bomb had a yield of about 21 kilotons of TNT, or 8.78×1013 joules = 88 TJ (terajoules).[1] Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 40,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured.[2] Many thousands more would die later from related injuries, and radiation sickness from nuclear fallout.

Contents

Technology

Illustration of the implosion concept.

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.

"Fat Man" was an implosion-type weapon using plutonium. 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 simply ejected from the weapon, making it an inefficient and un-impressive "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, though Seth Neddermeyer is credited for developing the idea further. The idea of using shaped charges came from mathematician John von Neumann, and both he and George Kistiakowsky eventually ended up being the principal architects behind the lens system.

The "Trinity" device was similar to the "Fat Man" bomb.
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 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 isotope Pu-240, 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 415,000 fissions/(s·kg), and the amount was 0.9% of 6.2 kg, is 56 g. Thus there were 23,000 spontaneous fissions per second. This means that the last few centimeters would have to be travelled in preferably much less than 40 microseconds.

It is theoretically possible to build a plutonium gun-type device, but it would need to be 19 feet long in order to accelerate the subcritical masses sufficiently to be fused into a critical mass before a fizzle occurs. The mass of a plutonium gun-type device would have been beyond the payload of the B-29.

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

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. In the end, 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 more or less a purposeful copy of 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.[3] Scientists said that "Thin Man" was meant to represent Roosevelt, "Fat Man" after Churchill.

Interior of bomb

The original blueprints of the interior of both Fat Man and Little Boy are still classified. However, there is some information about the main parts in the public domain available. 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).
  1. AN 219 contact fuse (four)
  2. Archie radar antenna
  3. Plate with batteries (to detonate charge surrounding nuclear components)
  4. X-Unit, a firing set placed near the charge
  5. Hinge fixing the two ellipsoidal parts of the bomb
  6. High explosive pentagonal lens (12 units around the core, made of high and low velocity explosive), together with the next item forming the pattern of a truncated icosahedron
  7. High explosive hexagonal lens (20 units around the core, made of high and low velocity explosive)
  8. California Parachute tail assembly (.20-inch aluminium sheet)
  9. Dural casing, ~140 cm inner diameter
  10. Cones that contained the whole sphere
  11. Explosive lenses (low and high velocity)
  12. Nuclear material (see figure below for details about the different layers)
  13. Plate with instruments (radars, baroswitches and timers)
  14. Barotube collector

Physics package

Schematic cross-section of the "gadget" used in the Trinity test on which the "Fat Man" design was based; some boundaries are approximate. From left to right (outside inward):

Schematic cross-section of the "gadget"
  •      dural casing, ~140 cm inner diameter
  •      exploding-bridgewire detonator (allows for instantaneous detonation of explosives)
  •      faster explosive, Composition B: 60% RDX, 39% TNT, 1% wax
  •      slower explosive (Baratol)
  •      faster explosive, "amplifier"
  •      aluminum–boron "pusher" (absorbs stray neutrons and widens/smooths implosion pulse)
  •      natural-uranium "tamper" (neutron reflector, inertial containment, improves efficiency, reduces the amount of fission material needed)
  •      the "pit"; plutonium-239–plutonium-240–gallium delta-phase alloy (96%–1%–3% by molarity)(fissionable material); sphere with a diameter of 9 cm, with a 2.5 cm cavity and a plutonium plug to allow initiator insertion; mass 6.2 kg
  •      air gap
  •      beryllium–polonium-210 "initiator" (the "urchin"), neutron source

See also

Wikimedia Commons has media related to:

Notes

  1. ^ http://www.warbirdforum.com/hiroshim.htm
  2. ^ http://www.yale.edu/lawweb/avalon/abomb/mp10.htm
  3. ^ Robert Serber, Peace & War: Reminiscences of a Life on the Frontiers of Science (New York: Columbia University Press, 1998): 104.

External links

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