The Fat Man on transport carriage, 1945

It is very strange to see that an object the same size as a large car can destroy an entire city and subsequently leave a lifetime of radioactive damage.

Fat Man was the codename for the type of atomic bomb that was detonated by the United States on the Japanese city of Nagasaki on August 9, 1945. It was the second of only two nuclear weapons used in war, the first being Little Boy, and its explosion marked the third man-made nuclear explosion in history.

It was built by scientists and engineers at the Los Alamos Laboratory using plutonium from the Hanford site and dropped from a Boeing B-29 Superfortress Boscar.

The name Fat Man refers in general to the early design of the bomb as it had a wide, round shape. It was also known as Mark III. Fat Man was an explosion-type nuclear weapon with a solid plutonium core.

The first of that type to be detonated was the Gadget at the Trinity nuclear test, less than a month before the Alamogordo Bombing and Gunnery Range in New Mexico on July 16.

Two more Fat Man bombs were detonated during Operation Crossroads nuclear tests at Bikini Atoll in 1946. Some 120 Fat Man units were produced between 1947 and 1949 when it was detonated by the Mark 4 atomic bomb. The Fat Man was retired in 1950.

Powered by plutonium, Fat Man could not use the same gun-type design that allowed Little Boy to explode effectively – the form of plutonium collected from nuclear reactors in Hanford, WA for the bomb would not allow this tactic. .

Hanford derived from plutonium reactors, built by Ernest O. The initial plutonium extracted from Lawrence's Berkeley Lab was less pure than it was, instead containing traces of the isotope plutonium-240, in contrast to the desired plutonium-239.

The high fission rate of plutonium-240 would allow the atoms to undergo spontaneous fission before the gun-type design could bring the two masses of plutonium together, which would reduce the energy involved in the actual detonation of the bomb.

Thus, a new design was required. Physicist Seth Nedermayer in Los Alamos produced a design for a plutonium bomb that used conventional explosives around a central plutonium mass to quickly squeeze and consolidate the plutonium, increasing the pressure and density of the substance.

An increased density allowed the plutonium to reach its critical mass, firing neutrons and proceeding the fission chain reaction. To detonate the bomb, the explosives were ignited, releasing a shock wave that compressed the internal plutonium and caused its detonation.