Why is large-scale nuclear fission a rare phenomenon on Earth? A. There isn't enough energy to start a chain reaction in nature.
B. There are no naturally occurring atoms that can undergo fission left on Earth. C. Atoms that undergo fission aren't concentrated enough to sustain a chain reaction. D. Atoms that undergo fission tend to decay more slowly on Earth than in stars.
The radioisotopes in the interior of the earth are not in pure form and have many embedded impurities and are interspersed. Therefore when a neutron is released by a decaying atom, its chances of hitting another radioisotope atom (to continue the chain reaction) are lower than in a manmade nuclear reactor. This way, the radioactive chain reaction in the earth’s interior is not well sustained.