OCCURRENCE OF NATURALLY ENRICHED U-235 - 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 neighbouring the natural fission reactor 10 at Oklo have depleted U-235 with U-235 / U-238 ratios ranging between 0.00560 and the common natural value of 0.00725. One sample, however, is enriched in U-235 with a U-235/U-238 ratio of 0.007682. Leaching experiments of this sample with dilute 1 N HCI revealed that the U-235 enrichment is actually restricted to the insoluble residue (U-235 / U-238 = 0.0 1 051 1 ), whereas the leachate remains depleted in U-235. 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.