H. Hidaka et P. Holliger, GEOCHEMICAL AND NEUTRONIC CHARACTERISTICS OF THE NATURAL FOSSIL FISSION REACTORS AT OKLO AND BANGOMBE, GABON, Geochimica et cosmochimica acta, 62(1), 1998, pp. 89-108
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.