There is both a natural conversion of U-238 to U-235, and enrichment. Both are tiny effects.
U-238 may capture a neutron. Then it becomes U-239, which beta decays first to Np-239 then to Pu-239. Pu-239 then alpha decays to U-235.
Neutrons are rare on Earth outside reactors. They may come from cosmic rays, and in U deposits, they may come from spontaneous fission of U-238, or from nuclei that collide with alpha particles. How common are these effects? Spontaneous fission of U-238 happens roughly once per 2 million initial alpha decays of U-238... how frequent is neutron generation by alpha decay in nature, seeing that an U-238 emits 8 alphas in the chain to Pb-206?
And of course U is enriched or depleted in nature, just like H, C-12, N-14, O-16 or S-32 are enriched or depleted. U undergoes chemical reactions - it is sometimes oxidized and dissolved into UO2(2+) cation, sometimes reduced and precipitated as UO2. All these reactions have nonzero isotope effects.
What is the size of natural isotope effects of U isotope compositions?
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