Fission-product retentivity in peripheral rocks at the Oklo natural fission reactors, Gabon

Abstract

Thermal ionisation mass spectrometry has been used to measure the isotopic and elemental abundances of Mo, Ru, Pd, Ag, Cd, Sn and Te in a number of sandstones and shales in the immediate vicinity of Reactor Zone 9 at the Oklo mine site. The mass spectrometric isotope dilution technique was used to measure the elemental abundances of these elements. The data show that considerable amounts of the fissiogenic Mo, Ru, Pd, Ag and Te which escaped from the reactor zone, have been retained in the peripheral rocks, although Cd and Sn were not contained to any significant extent in these samples. Some information on the retention of Sb and Tc was also obtained. Evidence of element fractionation has also been demonstrated in that 99Tc and Sn were fractionated from Ru and Te, respectively within 1 Ma of the end of reactor criticality. The presence of excess 125Te in some of the peripheral rock samples indicates that Sb was mobilised from Reactor Zone 9 within tens of years of the completion of reactor criticality. The implications of these results to the storage of radioactive wastes in natural geological repositories is discussed.