Tc/Ru fractionation in the Oklo and Bangombe natural fission reactors, Gabon

Ruthenium isotopic compositions of eleven samples selected from four kinds of natural fission reactors at Oklo and Bangombe uranium deposits were dete rmined by inductively coupled plasma mass spectrometry (ICP-MS). The detect able Ru isotopes in all samples were only fissiogenic origin. The fission c ontribution of U-235, U-238 and Pu-239 in each reactor was estimated by usi ng Ru-102/Ru-101 and Ru-104/Ru-101 isotopic ratios. Some of the samples sho w unusual Ru isotopic compositions, possibly derived from fast neutron indu ced fission. Depletion or enrichment of isotopic abundance of fissiogenic R u-99, defined as Delta(99)Ru, reveals the occurrence of a chemical fraction ation between Ru and Tc in the reactors. A major part of the fissiogenic Ru has been incorporated into micro metallic aggregates containing Ru, Rh, Te , Pb, As and S in the reactors. In situ analyses of metallic aggregates by secondary ion mass spectrometry (SIMS) and electron probe micro analysis (E PMA) were performed to obtain information on formation process of the aggre gates. A correlation between Delta(99)Ru and isotopic abundance of Ru-100 w as observed in the metallic aggregates. This correlation suggests that the chemical fractionation between Tc and Ru in the reactors had occurred at th e first stage of criticality, and that Ru-100 was produced by successive ne utron capture during criticality. The chemical compositions indicate that t he aggregates were formed as a mixture of PbS and fissiogenic Ru-Rh-Te comp onents. (C) 1999 Elsevier Science B.V. All rights reserved.