Background
Of the 33 known radioactive primordial nuclides, two of them (235U and 238U) are isotopes of uranium. These two isotopes are similar in many ways, except that only 235U is fissile (capable of sustaining a nuclear chain reaction of nuclear fission with thermal neutrons). In fact, 235U is the only naturally occurring fissile nucleus. Because natural uranium is only about 0.72% 235U by weight, it must be enriched to a concentration of 2–5% to be able to support a continuous nuclear chain reaction when normal water is used as the moderator. The product of this enrichment process is called enriched uranium.
Read more about this topic: Gaseous Diffusion
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