Fat Man
Replica of the original weapon
Type	Nuclear weapon
Place of origin	United States of America
Specifications
Mass	10,265 pounds (4,656 kg)
Length	128 inches (3.3 m)
Diameter	60 inches (1.5 m)
Filling	plutonium
Filling weight	6.2 kilograms (14 lb)
Blast yield	21 kt (88 TJ)

"Fat Man" was the codename for the type of atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of two nuclear weapons to be used in warfare to date (the other being "Little Boy"), and its detonation caused the third man-made nuclear explosion. The name also refers more generically to the early nuclear weapon designs weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core, similar to "The Gadget", the experimental device detonated in the Trinity nuclear test less than a month earlier on July 16 at the Alamogordo Bombing and Gunnery Range, New Mexico.

Early decisions

In 1942, prior to the Army taking over wartime atomic research, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California, in July at which various engineers and physicists discussed nuclear bomb design issues. A gun-type design was chosen, in which two sub-critical masses would be brought together by firing a "bullet" into a "target".^[1] The idea of an implosion-type nuclear weapon was suggested by Richard Tolman but attracted scant consideration.^[2]

The feasibility of a plutonium bomb was questioned in 1942. James Conant heard on November 14 from Wallace Akers, the director of the British Tube Alloys project, that James Chadwick had "concluded that plutonium might not be a practical fissionable material for weapons because of impurities."^[3] Conant consulted Ernest Lawrence and Arthur Compton, who acknowledged that their scientists at Berkeley and Chicago respectively knew about the problem, but could offer no ready solution. Conant informed the director of the Manhattan Project, Brigadier General Leslie R. Groves, Jr., who in turn assembled a special committee consisting of Lawrence, Compton, Oppenheimer and McMillan to examine the issue. The committee concluded that any problems could be overcome simply by requiring higher purity.^[4]

Oppenheimer, reviewing his options in early 1943, gave priority to the gun-type weapon,^[2] but as a hedge against the threat of pre-detonation, he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion. Implosion-type bombs were determined to be significantly more efficient in terms of explosive yield per unit mass of fissile material in the bomb, because compressed fissile materials react more rapidly and therefore more completely. Nonetheless, it was decided that the plutonium gun would receive the bulk of the research effort, since it was the project with the least amount of uncertainty involved. It was assumed that the uranium gun-type bomb could be easily adapted from it.^[5]

The gun-type and implosion-type designs were codenamed "Thin Man" and "Fat Man" respectively. These code names were created by Robert Serber, a former student of Oppenheimer's who worked on the Manhattan Project. 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 The Thin Man and series of movies by the same name; the "Fat Man" bomb would be round and fat and was named after Sydney Greenstreet's character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.^[6]

Development

Neddermeyer discarded Server and Tolman's initial concept of implosion as assembling a series if pieces in favor of a concept in which a hollow sphere was imploded by an explosive shell. He was assisted in this work by Hugh Bradner, Charles Critchfield and John Streib. L.T.E. Thompson was brought in as a consultant, and discussed the problem with Neddermeyer in June 1943. Thompson was sceptical that an implosion could be made sufficiently symmetric. Oppenheimer arranged for Neddermeyer and Edwin McMillan to visit the National Defense Research Committee's Explosives Research Laboratory near the laboratories of the Bureau of Mines in Bruceton, Pennsylvania, where they spoke to George Kistiakowsky and his team. But Neddermeyer's effects in July and August at imploding tubes to produce cylinders produced objects that resembled rocks. Neddermeyer was the only person who believed that implosion was practical, and only his enthusiasm kept the project alive.^[7]

Oppenheimer arranged for John von Neumann to visit Los Alamos in September to look at implosion with a fresh set of eyes. After reviewing Neddermeyer's studies, and discussing the matter with Edward Teller, von Neumann suggested the use of high explosive in shaped charges to implode a sphere, which he showed could not only result in a faster assembly of fissile material than was possible with the gun method, but which could greatly reduce the amount of material required. The prospect of more efficient nuclear weapons impressed Oppenheimer, Teller and Hans Bethe, but they decided that an expert on explosives would be required. Kistiakowsky's name was immediately suggested, and Kistiakowsky was brought into the project as a consultant in October 1943.^[8]

The implosion project remained a backup until April 1944, when experiments by Emilio G. Segrè and his P-5 Group at Los Alamos on the newly reactor-produced plutonium from the X-10 Graphite Reactor at Oak Ridge and the B Reactor at 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 plutonium-239. The cyclotron-produced isotopes on which the original measurements had been made has much lower traces of plutonium-240. Its inclusion in reactor-bred plutonium appeared unavoidable. This meant that the spontaneous fission rate of the reactor plutonium was so high that it would be highly likely that it would predetonate and blow itself apart during the initial formation of a critical mass.^[9] The distance required to accelerate the plutonium to speeds where predetonation would be less likely would need a gun barrel too long for any existing or planned bomber. The only way to use plutonium in a workable bomb was thus implosion.^[10]

The impracticability of a gun-type bomb using plutonium was agreed at a meeting in Los Alamos on July 17, 1944. All gun-type work in the Manhattan Project was directed at the Little Boy enriched uranium gun design, and almost all of the research at the Los Alamos Laboratory was re-oriented around the problems of implosion for the Fat Man bomb.^[10] The idea of using shaped charges as three-dimensional explosive lenses came from James L. Tuck and was developed by von Neumann.^[11] 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 Gamow.^[12] To overcome the difficulty of synchronizing multiple detonations, Luis Alvarez came up with the idea of replacing the primacord detonators with exploding-bridgewire detonators.^[11] Robert Christy is credited with doing the calculations that showed 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 a hollow spherical shell.^[13] After Christy's report, the solid-plutonium core weapon was referred to as the "Christy Gadget".^[14]

The task of the metallurgists was to determine how to cast plutonium into a sphere. The difficulties became apparent when attempts to measure the density of plutonium gave inconsistent results. At first contamination was believed to be the cause, but it was soon determined that there were multiple allotropes of plutonium.^[15] The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures. Attention then shifted to the even more malleable δ phase that normally exists in the 300 °C to 450 °C range. It was found that this was stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with alpha particles, which would exacerbate the pre-ignition problem. The metallurgists then hit upon a plutonium-gallium alloy, which stabilized the δ phase and could be hot pressed into the desired spherical shape. As plutonium was found to corrode readily, the sphere was coated with nickel.^[16]

Because of its complicated firing mechanism, and the need for previously untested synchronization of explosives and precision design, it was thought 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 Fat Man called "the gadget" for security reasons was detonated in a test explosion at a remote site in New Mexico, known as the "Trinity" test. It gave a yield of about 20 kilotonnes of TNT (84 TJ)Jones 1985, pp. 465, 514–517

Interior of bomb

The bomb was 128 inches (3,300 mm) long, 60 inches (1,500 mm) in diameter, and weighed 10,200 pounds (4,600 kg). As suggested by 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 15% more than that of Little Boy.^{[citation needed]}

AN 219 contact fuze (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 (0.20-inch (5.1 mm) aluminium sheet)

Assembly

To allow insertion of the 3.62 inch (92 mm) diameter plutonium pit, containing the 0.83 inch (21 mm) diameter "Urchin" initiator, as late as possible in the device's assembly, the spherical 8.75 inch (222 mm) diameter U-238 tamper surrounded by a 0.125 inch (3.2 mm) thick shell of boron impregnated plastic had a 5 inch (130 mm) diameter cylindrical hole running through it, like the hole in a cored apple. The missing U-238 tamper cylinder, containing the plutonium pit, could be slipped in through a hole in the surrounding 18.5 inch (470 mm) diameter aluminium pusher.^{[citation needed]}

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" device, which had the same design as Fat Man. Critical Assembly was first displayed at the Corcoran Gallery of Art, in Washington, DC.^[17]

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

•   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—a truncated icosahedron).
•   The detonation wave (arrows) is initially convex in the
•   faster explosive, Composition B: 60% RDX, 39% TNT, 1% wax. The wavefront 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 aluminium "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 was that in the Fat Man bomb, about 1 kilogram (2.2 lb) of the 6.2 kilograms (14 lb) of plutonium in the pit (about 17%) fissioned. In this process 1 gram (0.035 oz) of matter in the bomb was converted into the active energy of heat and radiation (see mass-energy equivalence for detail), releasing the energy equivalent of 21 kilotons of TNT or 88 terajoules.^{[citation needed]}

Deployment

The first plutonium core, encased in its insertion capsule, along with its polonium-beryllium urchin initiator, was transported in the custody of Project Alberta couriers. It departed from Kirtland Army Air Field on a C-54 transport aircraft of the 509th Composite Group's 320th Troop Carrier Squadron on 26 July, and arrived at North Field on Tinian on 28 July. Three Fat Man high explosive pre-assemblies designated F31, F32, and F33 were picked up at Kirtland on 28 July by three B-29s, two, Luke the Spook and Laggin' Dragon, from the 509th Composite Group's 393d Bombardment Squadron, and one from the 216th AAF Base Unit, and transported to North Field, arriving 2 August. On arrival, F31 was partly disassembled in order to check all its components. F33 was expended near Tinian during a final rehearsal on 8 August, and F31 was the bomb dropped on Nagasaki. F32 presumably would have been used for a third attack or its rehearsal.^[18]

Bombing of Nagasaki

The original target for the bomb was the city of Kokura, but obscuring clouds necessitated changing course to the alternative target, Nagasaki. "Fat Man" was dropped from the Boeing B-29 bomber Bockscar, piloted by Major Charles Sweeney of the 393rd Bombardment Squadron, Heavy, and following a 43-second duration free fall, exploded at 11:02 local time, at an altitude of about 1,650 feet (500 m), with a yield of about 21 kilotons of TNT or 88 terajoules.^[19] The Mitsubishi-Urakami Ordnance Works, the factory that manufactured the type 91 torpedoes released in the attack on Pearl Harbor, was destroyed in the blast. Because of poor visibility due to cloud cover, the bomb missed its intended detonation point by almost two miles, and damage was somewhat less extensive than that in Hiroshima. An estimated 40,000 people were killed outright by the bombing at Nagasaki, and a further 25,000 were injured.^[20] Thousands more died later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiation. The bombing raid on Nagasaki had the third highest fatality rate in World War II,^[21] after the nuclear strike on Hiroshima^{[22][23][24][25]} and the March 9/10 1945 Operation Meetinghouse firebombing raid on Tokyo.^[26]

Post-war development

After the war, the Fat Man (technically the model 1561 Fat Man) was modified—improved detonators, a more reliable firing system, and other minor changes. It thus emerged as the Mark III (or Mark 3) atomic bomb. Approximately 100 units were added to the arsenal before retirement by 1950.

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 spies Klaus Fuchs, Theodore Hall, and David Greenglass.^[27][28]