GEOCHEMICAL AND NEUTRONIC CHARACTERISTICS OF THE NATURAL FOSSIL FISSION REACTORS AT OKLO AND BANGOMBE, GABON

Isotopic studies have been completed on samples from the natural fissi on reactors at Oklo and Bangombe in order to determine the conditions under which they functioned when critical and to evaluate the retentio n and migration of fissiogenic radionuclides. The abundances and isoto pic 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 spectrome try (ICP-MS). Isotopic analyses and in situ ion imaging have also been performed by using an ion microprobe. Seven samples were taken from t he 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 Bangombe deposit. The isotopic data allow for a detai led description of the functional conditions of these reactors, and ba sed on these results, we have calculated the retention rates of the fi ssiogenic nuclides and nucleogenic Bi and Th. The nuclear parameters o f the natural fission reactors are characterized by the isotopic abund ances of Ru, Nd, Sm, Gd, Er, Yb, Lu, and U: neutron fluence (n/cm(2)), fission proportions of U-235, U-238, and Pu-239, the restitution fact or of U-235 resulting from Pu-239 decay, average temperature (degrees C) in the reactor, and duration of functioning (yr). In the 70 cm thic k 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 U -235 depletion shows a strong positive correlation with the restitutio n factor and an inverse correlation with neutron fluence, which demons trates the stability of the reaction zone since the period of critical ity. Large depletions of Sm-149, Gd-155, and Gd-157 have been detected in a sample of sandstone from 60 cm below this reactor core which als o had a normal uranium isotopic ratio (U-235/U-238 = 0.007254); this r esulted 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., S m and Gd). The results obtained from SD37 reveal that reactor zone 13 is not similar to the other reactor zones. The proportion of U-238 fis sion as calculated from the isotopic composition of Ru is extremely hi gh (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 i n,reactor zone 13 was much shorter than in other reactor zones. In the Bangombe reactor zone BA145, the chemical and nuclear characteristics are close to those of SF84. The retentivities of many fission product s as compared with fissiogenic Nd have been assessed for the reactor c ore samples. From the measured and calculated relative retentions, mor e than 90% of fissiogenic Ru, Rh, Pd, Te, and REEs have been retained in SF84 and SD37. In these same zones, however, the relative retention s of fissiogenic alkaline and alkaline earth elements are less than 20 %. The retentions of long-lived radioisotopes, such as Sr-90, Tc-99, C s-137, U-236, and Np-237 have been calculated by reference to their ra diogenic daughters Zr-90, Ru-99, Ba-137, Th-232, and Bi-209, respectiv ely. The excess or depletion of isotopic abundances measured in the da ughter nuclides has allowed the prediction of the rate of chemical fra ctionation between the parent and daughter nuclides in the reactor dur ing criticality. These results greatly improve the understanding of th e Oklo phenomenon and provide important data for the evaluation of the concept of long-term storage of radioactive wastes in geological form ations. Copyright (C) 1998 Elsevier Science Ltd.