Shale as a repository for radioactive waste: The evidence from Oklo

Abstract

Study of the 1.8 billion-year-old “fossil nuclear reactor” zones at the Oklo Mine in the Republic of Gabon shows that many of the elements produced by fission have been almost completely retained, as evidenced by proper budgets of stable daughter elements. Plutonium, ruthenium, the rare earth elements, zirconium, and palladium have been effectively retained while most chalcophile elements exhibit some degree of remobilization. The alkali and alkaline earth elements have migrated to varying degrees but their presence in gangue affected by younger periods of alteration suggests redistribution not far removed from sites of formation. More important, such migration may not have started until some 25,000,000 years after the reactor shut down. The noble gases xenon and krypton escaped with apparent ease during the 500,000 years the reactor was operative, and iodine seems to have been mobile.

The Oklo reactor ores, significantly, occur in shale infilled into a fracture system in organo-argillaceous sandstone. So many of the fission-produced elements retained in this shale along with evidence that most others may have been only locally redistributed lends support to considering shales in geologically stable areas for radioactive waste disposal.