MIGRATION OF FISSION-PRODUCTS INTO MICRO-MINERALS OF THE OKLO NATURALREACTORS

Isotopic characterization of two kinds of unique micro inclusions, met allic aggregate and apatite, in reactor core samples at the Oklo urani um ore in Republic of Gabon was studied. In order to investigate the m igration and retention mechanism of fission products, in-situ isotopic analyses by secondary ion mass spectrometry (SIMS), and precise isoto pic measurements by thermal ionization mass spectrometry (TLMS) and in ductively coupled plasma mass spectrometry (ICP-MS) were carried out. The results of isotopic analyses led to the following obser vations: ( 1) fissiogenic Ru, Pd and Te are enriched in the micrometallic inclusi ons, and (2) micro apatites contain fissiogenic alkaline, alkaline ear th, and rare earth elements (REE). Moreover, the geochemical behaviour of long-lived radionuclides Tc-99 and Cs-137 could be estimated from the precise isotopic determination of fissiogenic Ru and Ba, respectiv ely. Fissiogenic REE have fractionated chemically during the migration processes from the uraninite matrix into an apatite structure.