Abstract: Favorable Criteria for Radioactive-Waste-Disposal Sites in Shales Based on Geologic, Geochemical, and Theoretical Study of Oklo Uranium Mine, Gabon

Douglas G. Brookins

The 1.8 b.y.-old uranium deposit at the Oklo Mine, Gabon, sustained a low-power, nuclear-fission reaction lasting about 500,000 years. This "fossil nuclear reactor" allows comment quantitatively on retention or migration of the fissionogenic elements produced in a setting without benefit of other than natural-rock shielding. The geologic evidence indicates that the high-grade "reactor ore" was formed in a partly conglomeratic shale forced downward into a tilted, broken, mineralized sandstone layer. The nuclear reactions were made possible by a necessary combination of favorable uranium/water ratio, 235-U content (3.16% at 1.8 b.y. ago), high total U (to 70%), and lack of neutron-absorbing elements. Although 235-U fission produced most of the 10,000 megawatt-years energy, about 7% was ue to 238-U and 239-Pu fission.

Many of the fissionogenic elements in the atomic weight range from 80 to 180 have been retained. Eh-pH diagrams and other theoretical considerations allow prediction that Y, Zr, Nb, Tc, Ru, Rh, Pd, Ag, In, Te, Ba(?), and the rare-earth elements (REE) should have been retained and that Rb, Sr, Mo, Cd, I, Xe, and Cs should have migrated to various degrees. These predictions are consistent with measurement. Further, that even small amounts of fissionogenic Rb, Sr, Ba, and Cs(?) are still present suggests that these elements have been redistributed only locally.

Of special interest are the studies of U and the transuranic elements Np, Pu, Am, and some of their intermediate radioactive daughters because of the production of these elements in man-made reactors. Eh-pH diagrams indicate that Pu, Am, and Np should have been retained at Oklo, as should 209-Bi formed from 241-Pu. Measurements agree with predictions. Further, 226-Ra has not migrated because of probable fixation by sulfates. Collectively the Oklo data support the feasibility of radioactive-waste disposal in tectonically favorable shale sites at a depth of 1 to 2 km with an outside container packing of high CEC K-bentonite mixed with sulfate. These criteria can be met easily with existing technology.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC