In a remote laboratory at Los Alamos, scientists raced to build a new kind of weapon during World War II. Behind locked doors, two bomb designs were taking shape—one would destroy Hiroshima, the other Nagasaki.
But before either was ready, engineers had pinned their hopes on a long, narrow device called Thin Man. What followed was a sharp turn in nuclear history, dictated not by theory, but by the unpredictable nature of plutonium.
A Gun-Type Bomb and a Silent Rival
The Manhattan Project began in 1942 after warnings from Albert Einstein and others that Germany might be developing atomic weapons. By then, nuclear fission had been demonstrated in Germany, and U.S. scientists proved both uranium-235 and plutonium-239 could fuel a bomb. J. Robert Oppenheimer supported a gun-type design called Thin Man.
It aimed to shoot one subcritical mass into another to create a chain reaction. At the same time, a second group tested a riskier implosion concept—compressing plutonium into critical mass using explosive lenses.
A Flaw Inside the Plutonium
The Hanford Site in Washington State began producing plutonium in 1943. But by mid-1944, tests showed reactor-made plutonium emitted excess neutrons due to the presence of plutonium-240. This led to premature initiation—known as pre-detonation—if used in a gun-type design.
According to historian Barton Hacker, this rendered the Thin Man concept unusable with plutonium. The gun would fail before reaching critical mass, producing a fizzle instead of a detonation. With uranium in limited supply, Thin Man was quickly dropped for all but one bomb.
From Thin Man to Little Boy
Engineers reworked the gun design into a smaller version: Little Boy. It used uranium-235 and was successfully deployed over Hiroshima on August 6, 1945. Meanwhile, the implosion design, though initially seen as uncertain, became the foundation for the plutonium bomb Fat Man, dropped on Nagasaki three days later.
Despite Little Boy being less efficient, its blast was more devastating due to Hiroshima’s geography. Fat Man’s implosion method proved about ten times more efficient and became the standard for postwar nuclear weapons.
The End of Gun-Type Weapons
After 1945, implosion became the dominant bomb design. According to Hacker, the only other gun-type weapon ever detonated was a nuclear artillery shell tested in Nevada in 1953. Thin Man, once central to the Manhattan Project, was never completed.
The shift to implosion not only made better use of fissile materials but also shaped the direction of global arsenals for decades. The decision to abandon Thin Man had far-reaching consequences, determined not by preference, but by plutonium’s unpredictable physics.
During World War II, scientists raced to build the first atomic bomb.
In secret labs, two bomb designs competed for success—one would be dropped on Japan, while the other was suddenly scrapped.
What caused the U.S. to abandon its original design just months before use? 🧵👇 pic.twitter.com/3uQaWu7FBj
— Detective Tiger's Stories (@TigerDetective) June 12, 2025