Why is large-scale nuclear fission a rare phenomenon on Earth?

Large-scale nuclear is fission a rare phenomenon on Earth because

atoms that undergo fission aren't concentrated enough to sustain a chain reaction.

Atoms that undergo fission aren't concentrated enough to sustain a chain reaction. For example, the decay chain of U-238 is called the uranium series. ... Uranium is find in ores on Earth with very law mass percentage in it, so fission is not possible

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