Fat Man
Fat Man was the codename for the plutonium-based implosion-type atomic bomb developed by the United States during the Manhattan Project and detonated over the Japanese city of Nagasaki on August 9, 1945.[1][2] Designed at Los Alamos National Laboratory to overcome plutonium's higher rate of spontaneous fission, which precluded a simpler gun-type assembly like that used in the uranium-based Little Boy bomb dropped on Hiroshima three days earlier, Fat Man employed a sophisticated implosion mechanism involving explosive lenses to achieve symmetric compression of the fissile core.[2][3][4] Weighing approximately 10,000 pounds and released from a B-29 Superfortress bomber named Bockscar, the device exploded at an altitude of about 1,650 feet with a yield estimated at 21 kilotons of TNT equivalent, devastating over two square miles of the city and killing around 39,000 people instantly, with tens of thousands more injured or dying later from radiation and burns.[5][1][6] The bombing, alongside the prior Hiroshima attack, prompted Japan's surrender on August 15, 1945, thereby ending World War II in the Pacific theater and averting a costly Allied invasion of the Japanese home islands, though it ignited enduring controversies over the ethics of deploying weapons of mass destruction against civilian populations and the long-term implications of unleashing nuclear warfare.[7][8]Background and Development Decisions
Manhattan Project Origins and Early Choices
The Manhattan Project originated amid fears that Nazi Germany was developing nuclear weapons, spurred by émigré physicists' concerns over fission research in Europe. On August 2, 1939, physicist Leo Szilard drafted a letter signed by Albert Einstein and delivered to President Franklin D. Roosevelt, warning that Germany might construct atomic bombs and recommending U.S. investigation of uranium chain reactions for military applications.[9][10] This prompted the formation of the Advisory Committee on Uranium under Lyman Briggs, which evolved into broader efforts under the National Defense Research Committee and, by June 1941, Vannevar Bush's Office of Scientific Research and Development (OSRD).[7] In June 1942, the U.S. Army Corps of Engineers established the Manhattan Engineer District to oversee large-scale atomic research, with Colonel (later Brigadier General) Leslie Groves appointed director on September 17, 1942.[11] Early organizational choices emphasized parallel production of fissile materials to hedge against technical uncertainties: uranium-235 enrichment via gaseous diffusion, electromagnetic separation, and thermal diffusion at Oak Ridge, Tennessee (Clinton Engineer Works), and plutonium-239 production through nuclear reactors at Hanford, Washington.[12] Plutonium development accelerated after Enrico Fermi's December 1942 Chicago Pile-1 reactor demonstrated controlled chain reactions, confirming Pu-239's fissile potential; by early 1943, Groves selected Hanford and contracted DuPont to build full-scale reactors based on Metallurgical Laboratory designs initiated in June 1942.[12][13] These decisions prioritized industrial-scale output over proven methods, committing billions in resources despite untested reactor operations.[8] Bomb design work centralized at Los Alamos Laboratory under J. Robert Oppenheimer from 1943, initially favoring simple gun-type assembly—firing one fissile subcritical mass into another—for both uranium and plutonium weapons.[14] For plutonium, this led to the "Thin Man" concept, a long-barreled gun design, but reactor-produced Pu-239 contained higher Pu-240 impurities causing spontaneous neutrons and predetonation risks, rendering gun assembly unreliable as confirmed by April 1944 criticality experiments at Los Alamos.[2] In response, project leaders pivoted to implosion: compressing a plutonium sphere symmetrically with precisely timed explosives, a concept pioneered by Seth Neddermeyer in 1943 and refined by John von Neumann's shock-wave modeling later that year.[15][16] By July 1944, Oppenheimer halted Thin Man development and reorganized Los Alamos to prioritize implosion for the plutonium bomb (later Fat Man), allocating over 1,000 personnel despite the method's complexity and unproven status, as uranium gun designs progressed more straightforwardly but slower in material production.[17][18] This shift, driven by empirical tests revealing plutonium's neutron emission flaws, committed the project to a high-risk, resource-intensive path essential for timely weaponization.[7]Transition to Plutonium Implosion Design
The Manhattan Project pursued parallel paths for uranium-235 enrichment at Oak Ridge and plutonium-239 production at Hanford, initially envisioning gun-type assembly for both fissile materials to achieve supercriticality by firing one subcritical mass into another.[12] This approach, dubbed "Thin Man" for the plutonium variant, relied on the simplicity of conventional explosives propelling components together at high velocity, as demonstrated feasible for uranium in early tests.[19] However, reactor-produced plutonium inevitably incorporated significant Pu-240 due to neutron capture on Pu-239 during irradiation, with Pu-240's high spontaneous fission rate—about 1.0 × 10^6 fissions per gram per second—triggering premature chain reactions and fizzle yields in gun-type designs before full assembly.[20] Glenn Seaborg's team identified this isotopic impurity issue by early 1944 through metallurgical analysis, rendering the plutonium gun-type unreliable despite Hanford's ramp-up to produce kilograms of material; predetonation risks exceeded acceptable odds for weapon reliability, prompting abandonment of Thin Man by April 1944 after subscale tests confirmed inconsistencies.[19][17] Implosion, an alternative compression method using symmetrically arranged high-explosive lenses to uniformly squeeze a plutonium pit to supercritical density, had been theoretically proposed by Seth Neddermeyer in April 1943 as a means to assemble hollow fissile spheres without mechanical motion, addressing potential predetonation by minimizing assembly time to microseconds.[21] Neddermeyer's group conducted initial explosive experiments by July 1943, demonstrating inward radial compression of metal targets, though early results suffered from asymmetric shocks and instabilities.[22] J. Robert Oppenheimer reorganized Los Alamos' Project Y in June-August 1944, elevating implosion to priority by recruiting explosives expert George Kistiakowsky and reallocating resources from gun-type efforts, as plutonium yields outpaced uranium and implosion offered the only viable path to a plutonium bomb by mid-1945.[17] This pivot, driven by empirical reactor chemistry and hydrodynamic simulations, culminated in the Fat Man design, retaining uranium's gun-type for Little Boy to hedge against implosion uncertainties.[23]Technical Design and Components
Implosion Mechanism and Core Assembly
The implosion mechanism of Fat Man employed precisely shaped high-explosive lenses to symmetrically compress a subcritical plutonium core to supercritical density, initiating a nuclear chain reaction. This design was necessitated by the presence of plutonium-240 impurities in reactor-produced plutonium-239, which caused excessive spontaneous fission and rendered gun-type assembly infeasible due to predetonation risks.[24][25] The core consisted of approximately 6.15 kilograms of plutonium-gallium alloy in delta phase, formed into two hollow hemispheres coated with nickel plating, surrounding a central cavity for the neutron initiator. A natural uranium tamper, weighing 108 kilograms and 6.56 centimeters thick, encased the core to reflect neutrons and sustain the fission reaction post-compression. The initiator, known as "Urchin," was a polonium-beryllium device designed to release neutrons precisely at peak compression, triggered by the imploding shock wave deforming its components.[26] Surrounding the core and tamper were 32 explosive lens assemblies—20 hexagonal and 12 pentagonal—totaling about 2,400 kilograms of high explosives, arranged in a soccer-ball-like polyhedron. Each lens combined fast-detonating Composition B (60% RDX, 39% TNT, 1% wax, velocity ~8,000 m/s) with slower baratol (25-33% TNT, barium nitrate, 1% wax, velocity ~6,000 m/s), cast to tolerances of ±0.8 millimeters to shape detonation waves into a converging spherical front that compressed the core to over twice its original density, achieving 3-4 critical masses in microseconds.[26] Initiation relied on 32 exploding-bridgewire detonators, fired simultaneously by the X-Unit timing system to within ±10 nanoseconds, ensuring uniform detonation across all points. Core assembly involved mating the plutonium hemispheres around the Urchin initiator, inserting the assembly into the tamper, and integrating it into the explosive package via a trap-door mechanism that allowed the fissile "capsule" to be loaded into the pre-assembled high-explosive sphere. This process was finalized on Tinian Island, where plutonium components arrived separately for security.[26][27]Physical Specifications and Naming
The Fat Man atomic bomb featured a distinctive implosion design that necessitated a more compact and spherical form compared to the elongated Little Boy. It measured 128 inches (10 feet 8 inches) in length and 60 inches in diameter, with an overall weight of approximately 10,300 pounds (4,670 kilograms).[26][28] The bomb's exterior consisted of a steel ballistic case enclosing the plutonium core, high-explosive lenses, and tamper assembly, designed for aerial delivery from a modified B-29 Superfortress bomber.[5]| Specification | Value |
|---|---|
| Length | 128 inches (3.25 m) |
| Diameter | 60 inches (1.52 m) |
| Weight | 10,300 lb (4,670 kg) |