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

Abstract

Ruthenium isotopic compositions of eleven samples selected from four kinds of natural fission reactors at Oklo and Bangombé uranium deposits were determined by inductively coupled plasma mass spectrometry (ICP-MS). The detectable Ru isotopes in all samples were only fissiogenic origin. The fission contribution of 235 U , 238 U and 239 Pu in each reactor was estimated by using 102 Ru/ 101 Ru and 104 Ru/ 101 Ru isotopic ratios. Some of the samples show unusual Ru isotopic compositions, possibly derived from fast neutron induced fission. Depletion or enrichment of isotopic abundance of fissiogenic 99 Ru , defined as Δ 99 Ru , reveals the occurrence of a chemical fractionation 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 (EPMA) were performed to obtain information on formation process of the aggregates. A correlation between Δ 99 Ru and isotopic abundance of 100 Ru was observed in the metallic aggregates. This correlation suggests that the chemical fractionation between Tc and Ru in the reactors had occurred at the first stage of criticality, and that 100 Ru was produced by successive neutron capture during criticality. The chemical compositions indicate that the aggregates were formed as a mixture of PbS and fissiogenic Ru–Rh–Te components.