Throughout geological time natural Uranium has been composed to within a fraction of a percent of only two isotopes, U-235 and U-238. In this note, the abundance fractions of U-235 and U-238 and the ratio of U-235 to U-238 are computed throughout the earth’s history. This computation is of interest since the U-235 isotope, which sustains nuclear reactions unlike the more copious U-238, was significantly more abundant in the past and induced natural operating reactors long ago in the earth.
http://deepblue.lib.umich.edu/bitstream/2027.42/163723/1/ScholardoxB1.pdf
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