Most of the neutrons released during the fission of uranium nuclei have an energy of 1–2 MeV, that is, they are fast. Since in natural uranium the nuclei of the 235 (92) U isotope is small, most of these neutrons are absorbed by the 238 (92) U nuclei without fission, and the chain reaction does not develop. In this regard, slow (thermal) neutrons are used in nuclear chain reactions, i.e. neutrons with low kinetic energy (no more than 100 keV). Such neutrons are captured by 235 (92) U nuclei 500 times more efficiently than fast ones.
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