The Story of ‘Fat Man’: Unraveling the Mechanics Behind Japan’s Encounter with Its Second Atomic Bomb
Seventy years ago, Japan experienced the devastating impact of its second atomic bomb, famously known as “Fat Man.” Despite being smaller in size compared to its predecessor, “Little Boy,” Fat Man packed a more powerful punch.
How did engineers achieve such a feat in this insane realm of physics?
The key lay in the unstable nature of atomic nuclei, where a higher concentration of protons and neutrons led to increased instability. Engineers opted to utilize plutonium as the fissile material for both nuclear fuel and weapons due to its relatively poor nuclear stability. Upon colliding with a free neutron, plutonium atoms undergo nuclear fission, releasing 2 to 3 new neutrons and copious amounts of energy. This cascading process, known as a fission chain reaction, instantaneously unleashes tremendous energy.
At the core of Fat Man lies a mere 2-centimeter neutron initiator, essential for the initial neutron production. Encasing it is a 9.2-centimeter-diameter plutonium sphere, initially in a subcritical state. Achieving a critical mass of 6.19 kilograms is necessary to trigger the chain reaction. To compress the plutonium sphere to this critical state, engineers resorted to a crude yet effective method: implosion.
Normally, the neutrons released in fission lose some of their energy before reacting with other atomic nuclei, causing gaps to widen between atoms, hindering sustained fission reactions. However, compressing the plutonium sphere reduces these gaps significantly, increasing the probability of neutron collisions with other atoms, thus sustaining the fission reaction.
However, compressing the plutonium sphere to criticality proved immensely challenging, requiring pressures equivalent to 300,000 atmospheres, akin to the force exerted by 500,000 elephants, and maintaining symmetry in force application to achieve the target shape. Engineers resolved this by surrounding the plutonium sphere with explosives, carefully layered to modify the curvature of the blast wave, ensuring even compression and criticality. Additionally, a neutron initiator within, composed of beryllium, triggered the reaction.
Surrounding the plutonium core is a uranium-238 shell that reflects scattered neutrons back into the plutonium, enhancing fission efficiency. A boron plastic casing absorbs stray neutrons to prevent premature detonation, followed by an aluminum shell to efficiently transmit the explosive shockwave. The outermost layer comprises 32 blocks of high-speed explosives, each equipped with a detonator connected to a capacitor unit, essential for initiating the explosion.
On August 8, 1945, Fat Man was loaded onto a renowned B29 bomber for deployment. Adverse weather conditions forced a change of plans, redirecting the aircraft to Nagasaki, 155 kilometers away. As Fat Man plummeted from the aircraft, its countdown commenced. Within seconds, the pressure sensor and radar system activated. At 43 seconds, as the radar calculated Fat Man’s altitude at 500 meters, the capacitor unit triggered, igniting the detonators. The resulting explosion, exceeding 300,000 atmospheres, propelled the plutonium into a supercritical state, initiating a catastrophic chain reaction.
Within microseconds, a colossal fireball erupted, releasing an intense shockwave, leaving Nagasaki in ruins and claiming the lives of over 40,000 people, with hundreds of thousands succumbing to radiation-related illnesses in the following decades. The legacy of Fat Man serves as a harrowing reminder of the destructive power and ethical implications of nuclear technology.
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