Chain reaction initiated by neutrons

Rumors and pyramid schemes are chain reactions.

A chain reaction in Uranium²³⁵, Plutonium²³⁹ etc., relies on neutrons released by occasional spontaneous fission to initiate more fission and so on. A moderator of light nuclei (deuterium in heavy-water) slows the neutrons and make it more likely that they will be captured by a fissionable nucleus and not escape to the surroundings.

Sectors of a plutonium sphere were compressed together by an explosive charge of dynamite, to suddenly assemble a critical mass, to initiate an uncontrolled chain reaction. The bomb was called FAT MAN (Nagasaki, 1945).

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