Geochemical and Neutronic Characteristics of the Natural Fossil Fission Reactors at Oklo and Bangombé, Gabon

Abstract

Isotopic studies have been completed on samples from the natural fission reactors at Oklo and Bangombé in order to determine the conditions under which they functioned when critical and to evaluate the retention and migration of fissiogenic radionuclides. The abundances and isotopic compositions of the elements Rb, Sr, Zr, Ru, Pd, Ag, Te, Ba, rare earth elements (REEs), and U have been measured by thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). Isotopic analyses and in situ ion imaging have also been performed by using an ion microprobe. Seven samples were taken from the SF84 borehole (zone 10), one from the S2 borehole in gallery SD37 (zone 13), both being zones in the Oklo deposit, and one from the BA145 borehole in the Bangombé deposit. The isotopic data allow for a detailed description of the functional conditions of these reactors, and based on these results, we have calculated the retention rates of the fissiogenic nuclides and nucleogenic Bi and Th. The nuclear parameters of the natural fission reactors are characterized by the isotopic abundances of Ru, Nd, Sm, Gd, Er, Yb, Lu, and U: neutron fluence (n/cm 2), fission proportions of 235U, 238U, and 239Pu, the restitution factor of 235U resulting from 239Pu decay, average temperature (°C) in the reactor, and duration of functioning (yr). In the 70 cm thick reactor core encountered by borehole SF84, the neutron fluence is in the range from 5.3x10 20 to 8.0x10 20 (n/cm 2). The variation in 235U depletion shows a strong positive correlation with the restitution factor and an inverse correlation with neutron fluence, which demonstrates the stability of the reaction zone since the period of criticality. Large depletions of 149Sm, 155Gd, and 157Gd have been detected in a sample of sandstone from 60 cm below this reactor core which also had a normal uranium isotopic ratio ( 235U/ 238U = 0.007254); this resulted from neutron capture reactions. The neutron fluence calculated from these isotopic anomalies is relatively high (6.2x10 18 n/cm 2) and probably shows that nuclear reactions began, but that criticality could never be sustained due to an excess of neutron poisons (e.g., Sm and Gd). The results obtained from SD37 reveal that reactor zone 13 is not similar to the other reactor zones. The proportion of 238U fission as calculated from the isotopic composition of Ru is extremely high (18% of the total), while that of SF84 (zone 10) is at most 5.0% of total fission. This result implies that the duration of criticality in reactor zone 13 was much shorter than in other reactor zones. In the Bangombé reactor zone BA145, the chemical and nuclear characteristics are close to those of SF84. The retentivities of many fission products as compared with fissiogenic Nd have been assessed for the reactor core samples. From the measured and calculated relative retentions, more than 90% of fissiogenic Ru, Rh, Pd, Te, and REEs have been retained in SF84 and SD37. In these same zones, however, the relative retentions of fissiogenic alkaline and alkaline earth elements are less than 20%. The retentions of long-lived radioisotopes, such as 90Sr, 99Tc, 137Cs, 236U, and 237Np have been calculated by reference to their radiogenic daughters 90Zr, 99Ru, 137Ba, 232Th, and 209Bi, respectively. The excess or depletion of isotopic abundances measured in the daughter nuclides has allowed the prediction of the rate of chemical fractionation between the parent and daughter nuclides in the reactor during criticality. These results greatly improve the understanding of the Oklo phenomenon and provide important data for the evaluation of the concept of long-term storage of radioactive wastes in geological formations.