The purpose of the present study is the determination of the different fissiogenic ways of isotopic production which occured during the nuclear reactions of the Oklo natural reactor (Gabon). This study was achieved by using isotopic ratios of fissiogenic platinum group elements PGE (Ru, Rh, Pd). On the other hand, the mobility of fission products has been determined as well as the retention of these differents products by mineral phases existing in the paragenesis of the uranium deposit.
The study is dealing with the SF 29 bore-hole (Oklo mine, reaction zone 10).
The sample was attacked by melting with a mixture of sodium peroxyde and potassium and sodium carbonate in a vitreous carbon crucible. The product of the melting is dissolved in water and acidified with HCl. The PGE are extracted with a procedure developped in our Laboratory for determination of PGE in silicated rocks. The reduction in solution is achieved by SnCl2 and the PGE are recovered by coprecipitation with Se + Te used as carriers. Ru is separated by distillation of RuO4 after oxydation by HClO4+HCl. Rh and Pd are recovered in the residue of distillation by the same reduction process using SnCl2.
Contents of elements was obtained by ICP-AES (Jobin Yvon JY 24 High Resolution) and isotopic ratios by ICP-MS (Fisons, VG Plasmaquad Turbo 2+).
The PGE contents in the reactor show that the 235U fission is the major way observed in this zone of the reactor. Nevertheless the Ru/Rh, Rh/Pd and Ru/Pd ratios point out the presence in the reactor of the fission way 239Pu and 238U but at a minor extend. In the sandstone lying under the reaction zone the Ru/Rh and Rh/Pd ratios are different as those encountered in the reaction zone. These anomalous ratios are reported to differential mobility processes in a fracturation zone. We suppose that palladium has a strong mobility in percolation fluids with a high chlorine content. As, S, Se and Te are also involved in these processes.
The palladium fission yields have been used to calculate the 235U, 238U and 239Pu fissions in the reactor. Ruthenium can be expected as an unsoluble element in the fluids involved during the hydrothermal event. Ru metallic agregates was observed in uranium ore depleted in 235U. From the Ru content values, the theoretical fissiogenic production of Pd and Rh can be calculated. Comparison of these values with "normal" content which can be expected from the fissiogenic production points out a Pd depletion which reachs 50%. Fissiogenic Pd seems to have been dissolved during hydrothermal phase related to the nuclear reactions or accompanying a doleritic intrusion which has been recognized in the mine. The observation of a complex sulfide of palladium confirms this hypothesis.
Fissiogenic PGE can be used as well as Rare Earth Elements to evaluate the mobility, the dispersion and the retentivity of the fissiogenic elements in uranium ore and also in the geological environment of Oklo. Mobility of palladium can be attributed to the presence of Cl-, As, Te ,..... in the hydrothermal fluid phase which affects the mineralisation and the reaction zones.