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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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182302-1 0031-9007=04=93(18)=182302(4)$22.50 2004 The A merican Physical Societ y 182302-1
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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182302-2 182302-2
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
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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182302-3 182302-3
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-
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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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182302-4 182302-4
... 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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We present the results of a coordinated NAUTILUS and NanoSIMS isotopic study of epsilon (ε) phase metallic aggregates from the Oklo natural nuclear reactor zone (RZ) 13. We observed that fissionogenic Tc and Cs were heterogeneously sequestered within the aggregates. Isotopes of these elements are relevant for improving the safety of spent nuclear fuel storage and reactor operation on generational timescales. Like the noble metals, nearly all of the Tc was retained within the reactor, though its abundance relative to Ru in the metallic aggregates varied by a factor of 10. The neutron fluence estimated from the production of ¹⁰⁰Ru from neutron capture on ⁹⁹Tc was estimated to be up to 1.2 × 10²¹ n·cm⁻². In contrast to Tc, nearly all of the fissionogenic Cs in the reactors was lost from the reactor fuel. The metallic aggregates contain the only phases yet identified to have sequestered radiocesium. Fissionogenic Cs isotopes decay over vastly different timescales, but were incorporated and retained within the ε-phase in proportions similar to stable ¹³³Cs. This indicates that retention began during criticality and sequestration lasted billions of years, despite local geologic activity and the presence of nearby magmatic dikes. Using fissionogenic Ba isotopes, we estimated that the metallic aggregates continually incorporated their radioactive Cs parents during criticality, though the majority of Cs was flushed out of the reactor on a characteristic timescale of 2.7 ± 0.6 years. We found that the abundance of Bi was correlated to Rh and Pd, and speculate that this may have been due to primary Np–Rh and Np–Pd alloys forming during or shortly after criticality. Using Pb–Pb data from uraninite and galena grains surrounding the metallic aggregates, we also inferred a final Pb mobility age of 298 Ma for RZ13, which is more recent than most estimates from other RZs.
... The deep underground situation of these geological layers imply specific initial pressure and temperature conditions (P 200 bars, T~150°C), astonishingly similar to the Pressurized Water Reactor (PWR) conditions [16,17,21,27]. The criticality conditions have recently been revisited using different methods [19], and in more recent work (Ibekwe et al. 2020), the proposed cyclic operating mode of some Oklo reactors [28] was investigated within a simplified approach without geometrical effects, based on the coupling of neutron physics with heat and mass transfer. a e-mail: (corresponding author) Furthermore, the multi-parametric feature of this phenomenon implies investigating as far as possible the limits and contours of relevant parameters and their contribution to the occurrence and maintenance of criticality. ...
Since their discovery, the Oklo natural nuclear reactors were subject of many detailed field studies, sample analysis and criticality simulations. The present article is dedicated to advanced simulations of Oklo cores using a custom Python code to generalize and automate MCNP criticality calculations. The impacts of both the initial neutron absorbers and the clay fraction, which define the initial and evolving mineralogical environment, were studied by means of parametric simulations.
... Not only has uranium been purified by natural processes on earth, but natural chain reactions have also occurred. The Oklo natural nuclear reactors operated 2 Gy ago in very rich uranium deposits in Africa (Gauthier-Lafaye et al. 1996;Meshik et al. 2004;Cowan 1976). ...
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Type Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain reaction in an isolated white dwarf (WD). The first solids that form as a WD starts to freeze are actinide rich and potentially support a fission chain reaction. In this Letter, we explore thermonuclear ignition from fission heating. We perform thermal diffusion simulations and find at high densities, above about 7 × 10 ⁸ g cm ⁻³ , that fission heating can ignite carbon burning. This could produce an SN Ia or another kind of astrophysical transient.
... Not only has uranium been purified by natural processes on earth, natural chain reactions have occurred. The Oklo natural nuclear reactors operated 2 Gy ago in very rich Uranium deposits in Africa Gauthier-Lafaye et al. (1996); Meshik et al. (2004);Cowan (1976). ...
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Type-Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain reaction in an isolated white dwarf (WD) [PRL 126, 1311010]. The first solids that form as a WD starts to freeze are actinide rich and potentially support a fission chain reaction. In this letter we explore thermonuclear ignition from fission heating. We perform thermal diffusion simulations and find at high densities, above about 7x10^8 g/cm^3, that the fission heating can ignite carbon burning. This could produce a SN Ia or another kind of astrophysical transient.
... As a result, one may incorrectly deduce their distance. This case would be analogous to the Oklo natural nuclear reactor which only operated ≈ 2 Gyr ago when f 5 was larger Gauthier-Lafaye et al. (1996); Meshik et al. (2004); Cowan (1976). Such a natural reactor is likely impossible today because f 5 is too small. ...
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The origin of life on Earth remains enigmatic with diverse models and debates. In previous studies, several hypotheses for the birthplace of life are proposed, such as (1) Darwin's “warm little pond,” leading to a “prebiotic soup” for life; (2) panspermia or neopanspermia (succession model of pan‐spermia); (3) transportation from/through Mars; (4) a deep sea hydrothermal system; and (5) an on‐land subduction‐zone hot spring. Here, we introduce a new hypothesis, named the “nuclear geyser model,” as the birthplace of life. The existence of natural reactors on Hadean Earth is suggested by the Earth formation process and geological knowledge, and ionizing radiation from natural reactors can satisfy all the conditions necessary for the birthplace of life, such as (1) an energy source (ionizing radiation and thermal energy); (2) a supply of nutrients (P, K, REE, etc.); (3) a supply of life‐constituting major elements; (4) a high concentration of reduced gases, such as CH 4 , HCN, and NH 3 ; (5) dry–wet cycles to create membranes and polymerize RNA; (6) a nontoxic aqueous environment; (7) Na‐poor water; (8) highly diversified environments; and (9) cyclic conditions, such as day‐to‐night, hot‐to‐cold, etc. Based on these nine requirements, we evaluate previously proposed locations for the origin of Earth's life. We conclude that the nuclear geyser model is the most ideal candidate because of its efficiency in continuously supplying both the energy and the necessary materials for life, thereby maintaining the essential “cradle” for life's initial development. In addition, we propose a three‐step model for the emergence of the first life on Earth that explains how the first life was born in a nuclear geyser system. According to this model, the first proto‐life is born in an underground environment through the prebiotic chemical evolution that occurs inside the geyser. After that, due to the material cycle by geysers, the first proto‐life adapts to the surface environment and evolves into the second proto‐life that can utilize solar energy. They further evolve into the third proto‐life through exposure to the toxic ocean. This third form is the first life on Earth known as the prokaryotes. The key to the three‐step model is the material cycle that occurs inside the nuclear geyser system, in which a process called “cell dynamics” leads to the birth of life via the coevolution of three important vital functions of metabolism, membranes, and self‐replication.
In order to promote a sound basis for considering the role of nuclear in climate change, this review spans the technical topics of social and political debate surrounding nuclear energy with a focus on the objective science of these issues including nuclear waste, accidents and overall risk. Novel aspects include the emergence of nuclear energy as being potentially renewable and the antithesis of Fukushima being an argument for the unacceptable risks associated with the use of nuclear energy. The purpose of this review is to present the facts about nuclear energy divorced from political, social or comparable bias. The results argue nuclear as effectively the most attractive option from almost every possible perspective in which common social discourse would have these painted as unfavourable if not horrific.
The evidence for a natural uranium fission reactor in Oklo, Gabon (Africa) begs the question as to whether this was the only one to have ever occurred on earth or elsewhere. Modern nuclear terminology classifies uranium as highly enriched uranium when the U235 content exceeds 20% which is shown comparable to projecting back ca 4.5e9 yr for terrestrial uranium isotopic abundances. At that time, the natural uranium content of the earth would have been classified as highly enriched uranium. With one verified natural criticality event, more events are postulated both on earth and throughout our galaxy. The latter effect should result in some background contribution to gamma ray burst events placing nuclear reactors as a potentially ubiquitous natural phenomenon.
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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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.
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
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
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.
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.