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将下列段落翻译 For a neutron - initiated chain- reaction, there must be a critical mass of the element present in a certain space under certain conditions. The conditions for the smallest critical mass require the conservation of the emitted neutrons and also their slowing or moderation .So there is a greater cross-section or probability of them initiating another fission. In two regions of Oklo , Gabon, Africa, natural nuclear fission reactors were active over 1 .5 billion years ago .Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the earth’s core results from radioactive decay. However, it is not known if any of this results from fission chain-reactions.