Encyclopedia
"Fat Man" was 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.
It was detonated at an altitude of about 1,800 feet over the city. It was dropped from the
B-29 bomber
Bockscar, piloted by Major Charles Sweeney. The bomb had a yield of about 21 kilotons of
TNT, or 8.78×10
13 joules = 88 TJ . 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. Many thousands more would die later from related injuries, and
radiation sickness from
nuclear fallout.
Technology
The weapon was 7 feet 8 inches long, five feet in diameter, and weighed 10,200 pounds . 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 unefficient 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 designed, 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 principle architects behind the lens system.
At first it was thought that two pieces of subcritical plutonium could simply be shot into one another to create a nuclear explosion, and a plutonium gun-type design of this sort 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/, 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 allow the subcritical masses 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.
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 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 , 2 to 4 times the expected yield.
The
Soviet Union's first nuclear weapon detonated at Operation First Lightning 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 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 is a detective novel by Dashiell Hammett [i] that has been adapted several times ...
. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.
The bombing of Nagasaki
- Main Article: Atomic bombings of Hiroshima and Nagasaki
By August 1, two nuclear weapons had been produced and a third nearly completed. The
U.S. Army Air Forces had trained and deployed to
Tinian a unit to drop the bombs. Components of the uranium bomb "Little Boy" were delivered by the USS
Indianapolis along with its U-235 projectile, and the target rings for "Little Boy" and all components of the "Fat Man" bomb were delivered by air. On August 6, 1945, "Little Boy" was dropped on
Hiroshima.
On the morning of August 9 1945, the U.S.
B-29 Superfortress Bockscar, sometimes called
Bock's Car or
Bocks Car, is the name of a U.S. Army Air Force [i]...
, flown by the crew of 393rd Squadron commander Major Charles W. Sweeney, carried the "Fat Man" bomb, with
Kokura as the primary target and
Nagasaki the secondary target. The mission plan for the second attack was nearly identical to that of the Hiroshima mission, with two B-29's flying an hour ahead as weather scouts and two additional B-29's in Sweeney's flight for instrumentation and photographic support of the mission. Sweeney also took off with his weapon already armed but with the electrical safety plugs still engaged.
Observers aboard the weather planes reported both targets clear. When Sweeny's aircraft arrived at the assembly point for his flight off the coast of Japan, the third plane failed to make the rendezvous.
Bockscar and the instrumentation plane circled for forty minutes without locating Hopkins. Already thirty minutes behind schedule, Sweeney decided to fly on without Hopkins. The resulting explosion had a blast yield equivalent to 21 kilotons of TNT.
According to most estimates, about 70,000 of Nagasaki's 240,000 residents were killed instantly, and up to 60,000 were injured. The radius of total destruction was about 1.6 km , followed by fires across the northern portion of the city to 3.2 km south of the bomb. The total number of residents killed is believed to be as many as 80,000, including those who died from radiation poisoning in the following months.
Immediately after the bombings of Japan, the United States produced a technical history of the Manhattan Project, known as the
Smyth Report, that did not disclose the information that the "Fat Man" device was different from the "Little Boy" device, and did not imply that a different method was required for plutonium weapons. The "implosion" design was considered top-secret in the United States until it was declassified and released during the testimony of
David Greenglass against
Ethel and Julius Rosenberg in 1951. Photographs of the casings of "Little Boy" and "Fat Man" were not released publicly until the 1960s.
The United States produced a small stockpile of "Fat Man" bombs after the war, but they were highly idiosyncratic and extremely delicate. It was eventually reworked in the MK 4 Fat Man bomb, which was similar in principle but was appropriate for long-term stockpiling and use by nonexperts, and used a more efficient implosion system .
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 .
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- AN 219 contact fuse
- Archie radar antenna
- Plate with batteries
- X-Unit, a firing set placed near the charge
- Hinge fixing the two ellipsoidal parts of the bomb
- High explosive pentagonal lens , together with the next item forming the pattern of a truncated icosahedron
- High explosive hexagonal lens
- California Parachute tail assembly
- Dural casing, ~140 cm inner diameter
- Cones that contained the whole sphere
- Explosive lenses
- Nuclear material
- Plate with instruments
- 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 :
*****
See also
Notes
External links
- and at Nuclear Weapons Archive
- in QuickTime VR format
