"Why is plutonium bomb bigger than the unanium bomb dropped on Japan?

Question

"Why is plutonium bomb bigger than the unanium bomb dropped on Japan?

Answer 1

Because the explosive mechanism to join the 2 Uranium halves together (to create a supercritical mass) was much much simpler than the explosive mechanism necessary to implode the Plutonium.

Basically, Little Boy was a gun barrel with a small charge of explosive on one end.  

Fat Man was a complex set of shaped charges, carefully aligned, with timing circuits (tubes -- no transistors back then), etc.  It took up much more space.

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