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Using selective laser extraction technique combined with sensitive ion-counting mass spectrometry, we have analyzed the isotopic structure of fission noble gases in U-free La-Ce-Sr-Ca aluminous hydroxy phosphate associated with the 2 billion yr old Oklo natural nuclear reactor. In addition to elevated abundances of fission-produced Zr, Ce, and Sr, we discovered high (up to 0.03 cm(3) STP/g) concentrations of fission Xe and Kr, the largest ever observed in any natural material. The specific isotopic structure of xenon in this mineral defines a cycling operation for the reactor with 30-min active pulses separated by 2.5 h dormant periods. Thus, nature not only created conditions for self-sustained nuclear chain reactions, but also provided clues on how to retain nuclear wastes, including fission Xe and Kr, and prevent uncontrolled runaway chain reaction.
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Record of Cycling Operation of the Natural Nuclear Reactor
in the Oklo/Okelobondo A rea in Gabon
A. P. Meshik, C. M. Hohenberg, and O.V. Pravdivtseva
Physics Depar tment, Washington University, St. Louis, Missouri 63130, USA
(Received 13 May 2004; published 27 October 200 4)
Using selective laser extract ion technique combined with sensitive ion-counting mass spect rometr y,
we have analyzed the isotopic str ucture of fission noble gases in U-free La-Ce-Sr-Ca aluminous hydroxy
phosphate associated with the 2 billion yr old Oklo natural nuclea r reactor. In addition to elevated
abundances of fission-produced Zr, Ce, and Sr, we discovered high (up to 0:03 cm3STP=g) concen-
trations of fission Xe and Kr, the largest ever observed in any nat ural material. The specific isotopic
structure of xenon in this minera l defines a cycling operation for the reactor with 30-min active pulses
separated by 2.5 h dormant periods. Thus, nature not only created conditions for self-susta ined nuclear
chain reactions, but also provided clues on how to retain nuclea r wastes, including fission Xe and Kr,
and prevent uncontrolled runaway chain reaction.
DOI: 10.1103/PhysRevLet t.93.182302 PACS numbers: 28.50.–k, 28.41.Kw, 28.41.My, 91.90.+p
A natural nuclear chain reaction was predicted by
Kuroda [1] 20 years before the remnants of the natural
reactor were actually discovered [2 4]. So far, 16 indi-
vidual reactor zones have been found in the Oklo/
Okelobondo area in Gabon. Aside from being a fascinat-
ing natural phenomenon, the occurrence of self-
sustaining natural nuclear reactors has several impor tant
implications, ranging from the verification of variability
of the long-term fundamental physical constants [5,6] to
storage of nuclea r wastes in geological environments [7].
Many elements extracted from the reactor material still
car ry clear isotopic signatures of 235Uand 239Pu fission
and neutron capture reactions. Isotopic compositions of
these elements allowed for reconstruction of the effective
neutron fluence (up to 1021 n=cm2), the amount 235U
consumed (>5 tons), the energy released (15 GW yr).
Also, using fission products of 24 000 yr 239Pu, an esti-
mate was made of the effective duration of this nuclear
fission chain reaction (150 000 yr). The average power,
therefore, was only about 100 kW, equivalent to a small
research reactor. The fact that natural reactors did not
explode and dissipate t hemselves right after they went
critical was evidently due to some self-regulation mecha-
nism providing a negative feedback. It is not clear, how-
ever, whether the reactor was operated continuously or
in pulses. This would depend on the mechanism of self-
regulation, and/or the time constant for the negative
feedback which prevented a runaway chain reaction.
One proposed mechanism was related to the burning up
of highly neutron absorbing impurities, such as rare ea rth
isotopes or boron, both of which have been detected in
Oklo [8]. As the strong absorbers were burned up at one
edge of the active reactor zone and uranium was burned at
the other, the active zone perhaps shifted along the U
vein, like a flame over a wet log. Therefore, different
par ts of the natural reactor could have operated at differ-
ent times [8]. Another potential self-regulation mecha-
nism could have involved water, which acts as a neutron
moderator [9]. As the temperature of the reactor in-
creased, all unbounded water was converted into steam.
This would reduce the neutron thermalization and shut
down the chain reaction.The chain reaction could resume
only when the reactor cooled down and the water concen-
tration increased again. But until recently, there was no
strong evidence in favor of any of these mechanisms.
Amazingly, isotopically anomalous xenon we found in
Oklo Al phosphate carries the fingerprint of a specific
cycling operation with a time scale, which suggests that
the self-regulation must indeed involve water.
The material from the nat ural nuclea r reactor was
acquired from a Pixie drill with a double swindler in
drill hole S2 in the SD.37 gallery on the east face of
reactor zone 13. It consisted mainly of massive lustrous
uranium oxide grains with numerous 0.1–0.5 mm-sized
La-Ce-Sr-Ca aluminous hydroxyl phosphate inclusions
[10]. A polished slice (34mm,1mm thick) was
prepared from this sample and placed into vacuum ex-
traction cell where heavy noble gases were extracted using
a slightly defocused beam from acoustically Q-switched
Nd-YAG laser (this extraction technique described in de-
tails in [11,12]). A typical diameter of the extraction
crater was about 25 mm far smaller than the investi-
gated m ineral grai ns — ensur ing m ineral specific analy-
sis. All stable Xe and Kr isotopes (except 78 Kr) from 28
individual extraction spots on U-bearing minerals and 13
spots on Al phosphates have been analyzed using a high
transmission ion-counting mass spectrometer [13]. In all
experiments, the amount of extracted gases was sufficient
for precise measurement of their isotopic compositions.
Va rious U oxides contained from 105to 103cm3
STP=gof 136Xe (Fig. 1), while U-free alumophosphates
had even more fission Xe, up to 0:03 cm3STP=g,the
highest Xe concentration ever found in natural material.
Evidently, fission Xe migrated from the U-bearing phase,
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where it has been produced, to the adjacent Al phosphate.
86Kr=136 Xe ratios tend to be lower in minerals, which
have less fission gases (Fig. 1), providing further evidence
for losses of fission products from U oxides. Less than 1%
of all fission Xe (as calculated from 235 Uburnout) is
retained in the U-rich phase, with a great fraction of
lost Xe apparently recaptured by Al phosphates. It was
found earlier that Oklo Al phosphate is enriched in fission
Zr, Ce, and Sr, while adjacent uraninite is depleted in
those elements [10]. T his unique abi lity of A l phosphate t o
capture fission products may be useful in man-made re-
actors and long-term nuclear waste storage.
The second interesting finding in Al phosphate was
130Xe excess (Fig. 2). This isotope is not produced by
fission since it is shielded by stable 130 Te in the fission
chain. 130Xe is a likely product of the reaction
129In; 130 I!130Xe, previously observed only in Oklo
uraninites from Zones 2and 3[14,15]. The enhancement
of 130Xe in Al phosphates is much higher than in U-rich
phases (Fig. 2), suggesting that 129 Ihas been displaced
from its parent uraninite into adjacent U-free Al phos-
phate during exposure to the thermal neutrons. The shift
in 130Xe=129 Xe ratio allows us to estimate the effective
neutron fluence of 1:71021 n=cm2 more than was
previously determined in the sa me reactor zone (1:0
1021 n=cm2and 0:78 1021 n=cm2) [16,17], suggesting a
possibility that Al phosphate may attract fission 129 I.
The most rema rkable Xe anomalies were observed in
the heavy isotopes (Fig. 3). Xenon in U phase is a rela-
tively normal mixture of fission products of 235 Uand
239Pu (by thermal neutrons) and 238U(by fast neutrons).
Spontaneous fission of 238U, dominant in common rocks,
is negligible in Oklo samples. However, Al phosphate
once again carries the most extreme anomalies, which
are impossible to explain in terms of the mixing of
known fissile nuclei and n-capture reactions. The appa rent
feature of Xe in Al phosphate seems to be a deficit of
136Xe, the end product of the shortest fission chain. The
only -active precursor of 136 Xe is 86 s 136I;so,afterthe
onset of the fission chain reaction, 136 Xe appears first and
hence has more chance of being lost before the other Xe
isotopes start to accumulate. This, in itself, suggests a
cycling operation of the natural reactor. As the tempera-
ture rises during a pulse, diffusion of volatile Xe in Al
phosphate accelerates. During dormant periods, the tem-
perature returns to normal, slowing down the diffusion.
However, a sole deficit of 136Xe cannot explain experi-
mentally observed Xe isotopic anomalies in Al phosphate
(dotted lines, Fig. 3). Evidently, a more complex process
was responsible for the transformation of the relatively
normal fission Xe in U-rich phase into the anomalous Xe
observed in Al phosphate. Such processes must generate
isotopes in the following proportions: 131 Xe=134Xe 3:4,
132Xe=134 Xe 7:0,and129Xe=134Xe 0:95 (slopes of
solid lines, Fig. 3).
Tellurium is known to be the most retentive fission
product in Oklo reactors [18]. Measured yields of fission
Te isotopes precisely match the fission product yield curve
[19]. This implies that Te -active precursors, such as
2:8m 132mSb,23 m 131 Sb,4:4h 129Sb,2:4m 129 Sn,
6:9m129mSn, also retained well in the reactor material.
We assume that fission isotopes of iodine, including the
long-lived 129 I, were retentive as well, otherwise we
would not find the excess of 130Xe produced by 129 I
neutron capture. In addition, numerous observations of
129Iand 129Xe in meteorites clearly demonstrate that
iodine is much more retentive than xenon (e.g., [20]).
Therefore, during a reactor pulse, the radioactive fission
tellurium and iodine migrate from U oxide into Al phos-
FIG. 2. Excess 130Xe (fission shielded) provides a mean for
an estimation of neutron fluence from the slope of the dashed
line. Data corrected to account for atmospheric contamination.
Negative 130Xe=136 Xe values are due to slight overcorrections.
FIG. 1. Fission 136Xe and 86Kr in U oxides (open ci rcles) and
Al phosphates (solid squares) after minor correction to account
for atmospheric contamination using 128 Xe and 82Kr. Xe con-
centrations were ca lculated from measured amounts of 136Xe
and the amount of degassed material which was estimated from
the specific density and geometry of the extraction crater.
Fission 136Xe concentration and 86Kr=136 Xe ratios in U-rich
phases tend to be lower, suggesting a migration of fission
products to Al phosphate.
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phate, where they subsequently decay to Xe. The latter,
however, is volatile and is retained in Al phosphate only
when the reactor cools down between operational pulses.
There is only one apparent problem: why is not the Xe
produced in the first pulse given off at the next one?
This problem can be resolved considering the conditions
at which Al phosphate has been formed. The high con-
centration of short-lived intermediate fission products in
Al phosphate without significant quantities of uranium
implies that it precipitated during the operation of the
Oklo reactor. Hydrothermal experiments demonstrate that
Al phosphate grows fast at relatively low temperatures
(270300 C) [21]. After being captured by growing
Al phosphate, fission products decay into Xe which re-
mains imprisoned in Al phosphate because of its frame-
work crystalline structure [22] (similar to the cagelike
structure of zeolite). Then Xe can be released from the
alumophosphate only after destroying its cr ysta lline
structure, which requires temperatures higher than those
during the operational pulse of the Oklo reactor.
To calculate the evolution of isotope ratios of accumu-
lating Xe precursors during and after the active pulses of
the fission chain reaction, we considered only those fis-
sion fragments which have a fission yield greater than
0.1% and/or a half-life shorter than 1 min. We also
assumed that the three major fissile nuclei in the Oklo
reactor 235 Uth,239 Puth (thermal neutron fission) and
238Ufn (fast neutron fission) —have relative contributions
of 75%, 7%, and 18%, accordingly. These numbers were
determined [16] using Ru, Pd, Nd, Sm, and Gd isotopic
compositions measured in the same reactor zone where
our sa mple ca me from. Then , using known i ndividual and
cumulative fission yields [23] for 235Uth,239Puth,and
238Ufn, we calculated independent and cumulative yields
for each fission fragment relevant to Xe production in our
reactor zone. Finally, cumulative accumulation of Sn, Sb,
Te, and I in each isoba ric chain was computed and plotted
in the form of isotope ratios on Fig. 4.
Evidently, the calculated isotopic ratios are changing
with time, and the final Xe composition will depend on
how long the operational pulses last (d) and when the Al
phosphate cools down enough to retain Xe (p). We tried to
vary these two free parameters dand puntil all three
measured Xe ratios (determined from Fig. 3 and shown as
gray horizontal lines on Fig. 4) matched the calculated
isotope ratios. This turned out to be impossible. However,
if we considered only two ratios h131i=h134iand h132i=
h134i, there is one single solution d30 m and p
2:5h. And there is no solution for h129i=h134icombined
with eit her one of the two others. To match all three ratios
the value of 129 Xe=134Xe needs to be adjusted from the
measured 0.95 to about 1.5 (light gray line on Fig. 4). This
can be done assuming that 37% of 129Ihas been lost by A l
phosphate subsequent to the termination of the reactor
2 Ga ago, which is not unreasonable. 129Ihas a 16
106yr half-life, several orders of magnitude longer than
all other Xe precursors, is chemically active, forms water
soluble compounds and, therefore, has a chance of being
par tia lly leaked out from Al phosphate in the aqueous
environment of the reactor. Indeed, there is clear evidence
for 129Imigration from uranium deposits [24].
Interestingly enough, the 30 min pulses of natural
nuclear reactor activity and 2:5h dormant periods re-
corded in the Oklo Al phosphate resemble a typical geyser
operation. Similar time scales suggest similar processes.
This simila rit y suggests that 0.5 h after the onset of the
chain reaction, unbounded water was converted to steam,
decreasing the thermal neutron flux and making the
reactor subcritical. It took at least 2:5h for the reactor
to cool down until fission Xe began to retain. Then the
water returned to the reactor zone, providing neutron
moderation and once again establishing a self-sustaining
chain.
FIG. 3. Xe isotopic composition in U oxides and
Al phosphates are relat ed by a process that shifted points along
the solid lines. Dotted lines illustrate a sole deficit of 136 Xe,
which cannot explain the Xe anomalies in Al phosphate.
Migration of all isotopes in each isobaric chain must be
considered. Also shown are Xe components produced by the
three potential progenitors (235 Uth,239 Puth,238 Ufn).
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It is fascinating that Xe in Al phosphate measured
today provides us with such pristine timing records for
a natural reactor operated 2 billion yr ago.
We are grateful to Donald Bogard and the late Paul
Kuroda, with whom the idea of cycling operation of Oklo
reactor was discussed. A precious sa mple from Zone 13
was kindly provided by Maurice Pagel (GREGU, France).
This work was suppor ted by NASA (Grant No. NAG5-
12776).
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FIG. 4. T he calculat ed evolution of isotopic composition of
intermediate fission products Sb, Te, and I, which hold up the
production of stable Xe isotopes. Bold black lines correspond to
the active period of the reactor, with the numbers indicat ing the
duration of that period. Dashed lines illustrate free decay of Xe
precursors. The gray horizontal lines show the compositions
required to ma ke Xe in Al phosphate from Xe in U oxides (as
inferred by the slope of the solid lines observed in Fig. 3). The
light gray line represents adjusted h129i=h134iratio assuming
37% losses of 129I.
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... Oklo RZ13 experienced the shortest duration and highest flux of the natural reactors, lasting 24 ka with a neutron fluence of up to 7.8 × 10 20 -1.1 × 10 21 n⋅cm -2 (Hidaka and Holliger, 1998;Meshik et al., 2000). The average power was roughly 100 kW, comparable to a small research reactor (Meshik et al., 2004). The neutron flux was roughly 5-30× higher than other RZs. ...
... Excess 130 Xe indicated that neutron capture occurred on fissionogenic 129 I. The isotopic composition of the captured Xe further revealed a record of the reactor cycling on for ~30 min and off for 2.5 h over the duration of its operation (Meshik et al., 2004), with water-to-steam conversion likely providing reactivity control to prevent a runaway thermal event. This sample also provided the first in-situ evidence of fissionogenic Cs and Ba capture within the reactor core (Groopman et al., 2018), with fissionogenic Cs and Ba being found in association with the metallic aggregate ε-phase. ...
... Secondary aluminous hydroxy phosphate minerals in RZ13 that formed during criticality exhibit large Cs and Ba abundances, but these are of terrestrial isotopic composition, likely reflecting exchange after criticality (Dymkov et al., 1997;Groopman et al., 2018). This contrasts to the large abundances of sequestered noble gases, such as Xe, in the aluminous phosphates, which retain their fissionogenic signatures (Meshik et al., 2000(Meshik et al., , 2004. Isotopic equilibration of Cs and Ba is apparent throughout the rest of the reactor sites, but does not appear to have strongly influenced the ε-phase. ...
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Previously proposed hypotheses on the origin of life are reviewed and it is demonstrated that none of them can provide the energy flux of ionizing radiation (UV/X/γ photons, and high-energy charged particles and neutrons) required to synthesize organic materials as demonstrated by the experiments by Miller and Urey in 1953. In order to overcome this difficulty, Ebisuzaki and Maruyama, in 2017, proposed a new hypothesis called the “Nuclear Geyser Model” of the origin of life, in which high-energy flux from a natural nuclear reactor drives chemical reactions to produce major biological molecules, such as amino acids, nucleotides, sugars, and fatty acids from raw molecules (H2O, N2, and CO2). Natural nuclear reactors were common on the surface of Hadean Earth, because the ²³⁵U/²³⁸U ratio was as high as 20%, which is much higher than the present value (0.7%), due to the shorter half-life of ²³⁵U than ²³⁸U. Ebisuzaki and Maruyama further posited that aqueous electrons and glyceraldehyde play key roles in the networks of chemical reactions in a nuclear geyser and suggested that primordial life depended on glyceraldehyde phosphate (GAP) from the nuclear geyser system as energy, carbon, and phosphate sources, pointing to a possible parallelism with the anaerobic glycolysis pathway; in particular, the lower stem path starting from GAP through Acetyl Coenzyme A to produce ATP and reduction power. It is shown that microbes (members of candidate division OD1) inhabiting high alkali hot springs, a modern analogue of the Hadean Earth environment, do not possess genes associated with conventional metabolisms, such as those of the TCA cycle, but only have genes in the lower stem path of the glycolysis. This is named the “Hadean Primordial Pathway”, because it is believed that this striking result points to a plausible origin of metabolic pathways of extant organisms. Also proposed is a step-by-step scenario of the evolution of the metabolism: 1) Chemical degradation of GAP supplied from the nuclear geyser to lactate; 2) Catalytic reactions to produce reductive power and acetyl coenzyme A (or its primitive form) and self-reproductive reactions by ribozymes on the surface of minerals (pyrite and struvite), which precipitate in a nuclear geyser (RNA world); 3) Enzymatic reactions by proteins with pyrites and the struvite in their reaction centers (RNP world); and, 4) Metabolism of extant organisms with the full assembly of enzymes produced by translating molecular machines with information stored in DNA sequences (DNA world). It is further inferred that relics of primordial metabolic evolution in the Hadean nuclear geyser can be seen at the reaction centers of enzymes of both pyrite and struvite types, nucleotide-like molecules as a cofactor of the enzymes, Calvin Cycle of photosynthesis, and chemical abundance of cytoplasm.
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Chemical evolution, starting from simple compounds that eventually transform into complex mixtures, is a theme long recognized to be of fundamental importance to origins of life research. Ionizing radiation on simple and small molecules, such as hydrogen cyanide (HCN) and acetonitorile (CH3CN), is proposed to be one of the geochemically plausible mechanism driving multiple reactions yielding prebiotic precursors essential to emergence of life on early-Earth. Water radiolysis forming radicals, proton, and solvated electron also assists developing a reaction network in a one-pot system. Radiolytic mechanisms on an experimental basis are reviewed. Particularly, proposed synthetic strategies and mechanisms of amino acids and nucleotides are presented.
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Nuclear power plants split uranium atoms in a process called fission. In a nuclear power plant, heat is generated to produce steam that spins a turbine to generate electricity. Nuclear energy has been proposed in response to the need for a clean energy source compared to CO2 production plants. However, nuclear energy is not necessarily a source of clean energy as nuclear power plants release small amounts of greenhouse emissions in activities related to building and running the plant. Moreover, even if all safety measures are followed, there is no guarantee that an accident will not occur in a nuclear power plant. In the case of an accident involving a nuclear power plant, the environment and the people around it may be exposed to high levels of radiation. Another important environmental problem related to nuclear energy is the generation of radioactive waste that can remain radioactive and dangerous to human health for thousands of years. There are also several issues with burying the radioactive waste. Here, we describe different types of radioactive waste pollution from nuclear power plants, their environmental effects, nuclear regulations, and nuclear power plant incidents. Moreover, two case studies on nuclear power plant accidents and their consequences are discussed.
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Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less 129Xe (or more 128Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the 128Xe/132Xe ratio in trapped is normal planetary, so that a 129Xe/132Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on 129Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both 129Xe and 127I, and subsequently in 128Xe after n-capture during reactor irradiation. Enrichment in radiogenic 129Xe, but with a 129Xe/127I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.
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Isotopic ratios and elemental abundances of rare earth elements (REE), Rb, Sr, Zr, Mo, Ru, Pd, Ag, Te, Ba, U in four samples from two Oklo Reactor Zones 7, 8, 9 and 10 were determined by thermal ionization mass spectrometry. These elements have unusual but reasonable isotopic anomalies due to the effect of fission and/or neutron capture reactions. The results are in agreement with previous Oklo work in that 1) Ru, Pd, Te and most of the series of REE (except for La and/or Ce) have been well retained in the samples; 2) Rb, Sr and Ba have been lost to a great extent, and their isotopic ratios are nearly the same as those of terrestrial standard values; and 3) Zr, Mo and Ag have been partially removed from the reactors. The present work reveals that, among REE, La and Ce might have been partially removed in contrast to the good retention of other REE. Also, there is a possible remobilization of 90Sr during operation of the Oklo reactors. -from Authors
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CONTENTS 1. Introduction 937 2. History of the Discovery of the Natural Reactor 938 3. Reactor Parameters 939 4. Consequences of the Discovery of the Oklo Phenomenon 942 Literature 943
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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.
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CONTENTS 1. Introduction 937 2. History of the Discovery of the Natural Reactor 938 3. Reactor Parameters 939 4. Consequences of the Discovery of the Oklo Phenomenon 942 Literature 943
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THE possibility that fundamental nuclear constants may vary slowly while the Universe expands has been discussed by several authors1-5. I try here to show that the well known resonance properties of the `heavy nucleus plus slow neutron' system make it a sensitive `receiver', sharply tuned to the current values of nuclear constants.