Natural nuclear fission reactor

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A natural nuclear fission reactor is a

fissile 235
U
in gaseous uranium made from Gabonese ore.

Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized

ore layers. There, self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, during the Statherian period of the Paleoproterozoic. Fission in the ore at Oklo continued off and on for a few hundred thousand years and probably never exceeded 100 kW of thermal power.[2][3][4] Life on Earth at this time consisted largely of sea-bound algae and the first eukaryotes, living under a 2% oxygen atmosphere. However even this meager oxygen was likely essential to the concentration of uranium into fissionable ore bodies, as uranium dissolves in water only in the presence of oxygen. Before the planetary-scale production of oxygen by the early photosynthesizers groundwater-moderated natural nuclear reactors are not thought to have been possible.[4]

Discovery of the Oklo fossil reactors

In May 1972, at the Tricastin uranium enrichment site at Pierrelatte, France, routine mass spectrometry comparing UF6 samples from the Oklo mine showed a discrepancy in the amount of the 235
U
 isotope. Where the usual concentrations of 235
U
 were 0.72% the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less 235
U
 than expected.[5] This discrepancy required explanation, as all civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted into the construction of unsanctioned nuclear weapons. Further, as fissile material is the reason for mining uranium in the first place, the missing 17% was also of direct economic concern.

Geological situation in Gabon leading to natural nuclear fission reactors
  1. Nuclear reactor zones
  2. Sandstone
  3. Uranium ore layer
  4. Granite

Thus the

fast neutron
induced (n,2n) reactions in nuclear reactors. In Oklo any possible deviation of 234
U concentration present at the time the reactor was active would have long since decayed away. 236
U must have also been present in higher than usual ratios during the time the reactor was operating, but due to its half life of 2.348×107 years being almost two orders of magnitude shorter than the time elapsed since the reactor operated, it has decayed to roughly 1.4×10−22 its original value and thus basically nothing and below any abilities of current equipment to detect.

This loss in 235
U
 is exactly what happens in a nuclear reactor. A possible explanation was that the uranium ore had operated as a natural fission reactor in the distant geological past. Other observations led to the same conclusion, and on 25 September 1972 the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.[4]

Nd 143 144 145 146 148 150
C/M 0.99 1.00 1.00 1.01 0.98 1.06

Fission product isotope signatures

Isotope signatures of natural neodymium and fission product neodymium from 235
U
 which had been subjected to thermal neutrons.

Neodymium

The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% 142
Nd
, while that of Oklo contains less than 6%. The 142
Nd
 is not produced by fission; the ore contains both fission-produced and natural neodymium. From this 142
Nd
 content, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of 235
U
. The two isotopes 143
Nd
 and 145
Nd
 lead to the formation of 144
Nd
 and 146
Nd
 by neutron capture. This excess must be corrected (see above) to obtain agreement between this corrected isotopic composition and that deduced from fission yields.

Ruthenium

Isotope signatures of natural ruthenium and fission product ruthenium from 235
U
 which had been subjected to thermal neutrons. The 100
Mo
 (an extremely long-lived double beta emitter) has not had time to decay to 100
Ru
 in more than trace quantities over the time since the reactors stopped working.

Similar investigations into the isotopic ratios of

ppb) decay to 100
Ru
 will have occurred. Other pathways of 100
Ru production like neutron capture in 99
Ru or 99
Tc (quickly followed by beta decay) can only have occurred during high neutron flux
and thus ceased when the fission chain reaction stopped.

Mechanism

The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with

light water reactors. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of neutron poisons
, no longer could sustain a chain reaction.

Fission of uranium normally produces five known isotopes of the fission-product gas

barium-135. Meanwhile, xenon-136, the product of neutron capture in xenon-135 decays extremely slowly via double beta decay
and thus scientists were able to determine the neutronics of this reactor by calculations based on those isotope ratios almost two billion years after it stopped fissioning uranium.

A graph showing the exponential decay of Uranium-235 over time.
Change of content of Uranium-235 in natural uranium; the content was 3.65% 2 billion years ago.

A key factor that made the reaction possible was that, at the time the reactor went

fissile isotope 235
U
 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was 238
U
 and roughly 55 ppm 234
U, neither of which is fissile by slow or moderated neutrons.) Because 235
U
 has a shorter half-life than 238
U
, and thus decays more rapidly, the current abundance of 235
U
 in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.[7]

The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water, and physical conditions needed to support the chain reaction was unique, as far as is currently known, to the Oklo ore bodies. It is also possible that other natural nuclear fission reactors were once operating but have since been geologically disturbed so much as to be unrecognizable, possibly even "diluting" the uranium so far that the isotope ratio would no longer serve as a "fingerprint". Only a small part of the continental crust and no part of the oceanic crust reaches the age of the deposits at Oklo or an age during which isotope ratios of natural uranium would have allowed a self sustaining chain reaction with water as a moderator.

Another factor which probably contributed to the start of the Oklo natural nuclear reactor at 2 billion years, rather than earlier, was the

increasing oxygen content in the Earth's atmosphere.[4] Uranium is naturally present in the rocks of the earth, and the abundance of fissile 235
U
 was at least 3% or higher at all times prior to reactor startup. Uranium is soluble in water only in the presence of oxygen.[citation needed
] Therefore, increasing oxygen levels during the aging of the Earth may have allowed uranium to be dissolved and transported with groundwater to places where a high enough concentration could accumulate to form rich uranium ore bodies. Without the new aerobic environment available on Earth at the time, these concentrations probably could not have taken place.

It is estimated that nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of 235
U
 and elevated temperatures to a few hundred degrees Celsius.

palladium-107 (since decayed to silver), 86 kilograms (190 lb) of strontium-90 (long since decayed to zirconium), and 185 kilograms (408 lb) of caesium-137
(long since decayed to barium).

Relation to the atomic fine-structure constant

The natural reactor of Oklo has been used to check if the atomic

149
Sm
 captures a neutron to become 150
Sm
, and since the rate of neutron capture depends on the value of α, the ratio of the two samarium
isotopes in samples from Oklo can be used to calculate the value of α from 2 billion years ago.

Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most have concluded that nuclear reactions then were much the same as they are today, which implies that α was the same too.[10][11][12]

See also

References

  1. doi:10.1063/1.1743058
    .
  2. PMID 16318030
    .
  3. ^ Mervin, Evelyn (July 13, 2011). "Nature's Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa". blogs.scientificamerican.com. Retrieved July 7, 2017.
  4. ^
    doi:10.1016/S0016-7037(96)00245-1
    .
  5. S2CID 118394767
    .
  6. PMID 15525157
    .
  7. ISBN 978-0-08-037941-8
    .
  8. doi:10.1016/0012-821X(80)90135-1
    .
  9. doi:10.1016/S1631-0705(02)01351-8
    .
  10. ^ New Scientist: Oklo Reactor and fine-structure value. June 30, 2004.
  11. S2CID 118272311
    .
  12. S2CID 119227720
    .

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