a a Uranium-235 has a half-life of 700 million years. The current abundance of this nuclide found in nature is 0.71%. Calculate how many billions (10%) of years ago the concentration of Uranium-235 was 5%. Describe a plausible scenario under which the uranium deposit at Oklo in Gabon, West Africa became a natural fission reactor. Assume this reactor started at the time the U-235 abundance was 5% and operated for 100 million years. If, after 100 million years of operation the U-235 abundance was reduced by 30% of its original value, what would be the current abundance of U-235 in the uranium deposits at Oklo