Occurrence of naturally enriched 235U: Implications for plutonium behavior in natural environments

It is generally accepted that uranium and most of the fission products, with the exception of the alkalis, alkaline earths and rare gases, remained in the irradiated uranium oxides during the nuclear reactions that took place 2.0 Ga ago in the Oklo uranium deposit (Gabon). New isotope investigations show that clay minerals from argillaceous rocks neighboring the natural fission reactor 10 at Oklo have depleted 235U with 235U/238U ratios ranging between 0.00560 and the common natural value of 0.00725. One sample, however, is enriched in 235U with a 235U/238U ratio of 0.007682. Leach experiments of this sample with dilute 1N HCl revealed that the 235U enrichment is actually restricted to the insoluble residue (235U/238U = 0.010511), whereas the leachate remains depleted in 235U. This unique discovery of very enriched uranium, together with samarium, neodymium, rubidium, and strontium isotopic analyses, indicate that a small amount of plutonium could have been more mobile than uranium in the reactor 10, and it is suggested that plutonium was incorporated in the crystallographic structure of clay minerals such as the chlorites. 28 refs., 3 figs., 1 tab