Fat Man Totally Explained

Everything about Fat Man totally explained

"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. Fat Man was possibly named after Winston Churchill, 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".
"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). 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 exposure to the bomb's initial radiations. The aerial bombing raid on Nagasaki had the third highest fatality rate in World War II after the nuclear strike on Hiroshima and the March 9/10 1945 fire bombing raid on Tokyo.

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 wasn't 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).
   The idea of using shaped charges came from James L. Tuck and was developed by mathematician John von Neumann, 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.

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.
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)

Physics package

Assembly

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.

Detonation sequence

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

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).

Further Information

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