how does U 238 change to U 235 in nature earth
or how does uranium enrichment in nature
U-238 doesn't change into U-235. Uranium ore contains about 0.72% U-235. To enrich uranium, you need to separate out U-235 from U-238.
The original stellar nucleosynthesis created U235 and U238 in about equal quantities.
Since they have different decay rates the U235 is much scarcer today.
For an interesting story read about the natural nuclear reactor at Oklo, Gabon:
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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