What element was used as fuel for the "Little Boy" bomb?

What element was used as fuel for the "Little Boy" bomb?

• A. Uranium-235
• B. Plutonium-239
• C. Uranium-238
• D. Thorium-232

Answer: (A)

Fission fuel must be fissile, meaning it can sustain a rapid chain reaction when struck by neutrons. Little Boy used highly enriched uranium-235 because it fissions readily with neutrons and can be assembled into a supercritical mass with a simple gun-type design: one piece of uranium-235 is fired into another, creating a single, large mass that undergoes a fast, runaway reaction. Plutonium-239 is also fissile and was used in the Fat Man device with a more complex implosion design, partly to manage issues like spontaneous fission. Uranium-238 isn’t easily fissioned by slow neutrons and isn’t suitable as a standalone weapon fuel, while thorium-232 is fertile and would need conversion to a fissile form in a reactor. So the fuel for Little Boy is uranium-235.

Fission fuel must be fissile, meaning it can sustain a rapid chain reaction when struck by neutrons. Little Boy used highly enriched uranium-235 because it fissions readily with neutrons and can be assembled into a supercritical mass with a simple gun-type design: one piece of uranium-235 is fired into another, creating a single, large mass that undergoes a fast, runaway reaction. Plutonium-239 is also fissile and was used in the Fat Man device with a more complex implosion design, partly to manage issues like spontaneous fission. Uranium-238 isn’t easily fissioned by slow neutrons and isn’t suitable as a standalone weapon fuel, while thorium-232 is fertile and would need conversion to a fissile form in a reactor. So the fuel for Little Boy is uranium-235.