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FAT MAN (model “Y-1561” / Mk.3)

Javier
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Although the first operational nuclear bombs emerged in 1945, in June 1942 Robert Oppenheimer was already theorizing about this type of weapon. In 1943, Oppenheimer was working on a “gun” type trigger, like the one installed in the Little Boy bomb dropped on Hiroshima, but he was also investigating the possibility of installing an implosion type trigger, which was considered much more effective in terms of destructive power. In addition, thanks to this quality, much less fissile material was used in the bombs. However, although priority was given to the “gun” type trigger for the plutonium bomb, (due to the uncertainties of the implosion trigger type), everything was prepared to be able to install the “gun” type trigger in a plutonium bomb if necessary.
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Despite the work to use a gun type trigger in the plutonium bomb, in July 1944 this system was abandoned because it was discovered that plutonium-239 contains impurities in the form of plutonium-240 isotopes that tend to spontaneous fission. If this were to occur inside the bomb, it could pre-detonate the plutonium before the chain reaction takes place and greatly diminish the destructive effectiveness. Furthermore, to accelerate the sub-critical mass of plutonium-239 to the speed required in the gun type trigger, it would take a barrel too long to fit a bomb that could be carried by existing bombers. These were the reasons why it was finally decided to install the implosion type trigger on Fat Man bomb.
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The mass of plutonium-239 had to be molded into a sphererical shape, which was not easy. The sphere was built from plutonium-gallium alloy, which could be hot pressed into the desired spherical shape and its density was stable and consistent enough. To prevent the constant corrosion of the plutonium, the sphere was coated with nickel. Afterwards, a system had to be manufactured to carry out the implosion that would generate the chain reaction, and it was decided to use shaped charges as three-dimensional explosive lenses activated by exploding-bridgewire detonators that very precisely synchronized the detonations of the explosive charges. These charges would compress the plutonium mass into a sphere to go from a sub-critical state to a critical state and generate the chain reaction.
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The dimensions and weight of the bomb were limited by the size of the bomb bay of the bomber that was going to launch it. At that time only the USAF’s B-29 Superfortress bomber and the British Avro Lancaster aircraft were the only ones in service capable of carrying Fat Man. The maximum weight should not exceed 9.1 tons and the dimensions should not exceed 3.40 meters long by 1.70 wide. Despite respecting these limits, the B-29 participants in the launch tests had to be modified due to the weight of the bomb. This, in turn, also had to be modified, since its tail fins were deformed by the weight and made the descent erratic. Different bomb casings were manufactured, selecting for the operational bomb the Y-1561 together with a rear tail box nicknamed “California Parachute” that gave the bomb sufficient stability during the descent.
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The core of the implosion type trigger consisted of two plutonium-239 hemispheres designed as “pit” of 92mm in diameter and weighing 6.19 kg in total, together with a “Urchin” modulated neutron initiator installed inside a depleted uranium tamper surrounded by a shell of boron-impregnated plastic. The “pit” was inside a cylinder that was slipped in through a hole in the surrounding aluminum pusher and then the bomb was armed. Subsequently, the detonation of the explosive lenses symmetrically compressed the plutonium and the neutron initiator added free neutrons to initiate a fission chain reaction. In addition to the destructive capacity of plutonium, uranium tamper itself undergoes fission, increasing the destructive power by 20%.
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On July 16, 1945, a Fat Man bomb nicknamed “The Gadget” was successfully detonated during the “Trinity” test. Later, as a result of this test, some minor modifications would be included in the Fat Man bomb that would be launched on Nagasaki. Finally, on July 28, the plutonium core arrived on the Tinian Island along with 3 bomb casings designated as F31, F32 and F33. On August 7, the 11th was chosen as date to drop Fat Man, but as bad weather was expected on that date, all preparations could be advanced and everything was ready for the next August 9.
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At 03:47 on August 9, 1945, a B-29 bomber named “Bockscar” took off bound for Kokura as the primary target and Nagasaki as the secondary target. After arming the bomb and after 3 unsuccessful attempts to bomb Kokura, it was decided to change course towards the secondary target. Poor target visibility and fuel pump problems that left the bomber without reserve fuel, forced a target change. In addition, the antiaircraft defense was more and more precise and the arrival of fighters was feared at any moment, which would greatly expose the B-29 and its deadly load. Finally at 11:02 a.m. Fat man was released and after 43 seconds of descent, it exploded about 500 meters above the ground with a power equivalent to 22,000 tonnes of TNT explosive (22 kilotons). The result, 39,000 people killed by a single bomb.

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