Oklo natural reactor

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Coordinates: 1 ° 24 ′  S , 13 ° 10 ′  E

Relief Map: Gabon
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Oklo natural reactor​
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Gabon
Geological conditions of the natural reactor in Oklo mine:
the first  reactor zones
2.  Sandstone
3.  Ore flöz
4.  granite stock

The Oklo natural reactor in Mounana in the Gabonese province of Haut-Ogooué is a uranium deposit in which a naturally occurring uranium concentration and water set off a nuclear chain reaction . In the meantime, the remains of a total of 15 natural reactors have been found in the Franceville basin , 14 of which are in Oklo and one in Bangombé, 30 km away .

About two billion years ago, in the Proterozoic Age , the proportion of the fissile uranium isotope 235 U in natural uranium (0.7% today) was still about 3%; the decrease since then can be explained by the shorter half-life of 235 U compared to 238 U. This 235 U concentration was sufficient to achieve criticality with natural water as moderator . The natural nuclear reactor was about 500,000 years active and sat at a thermal output of up to 100 kW of energy during this period of a few hundred tera - watt hours free. This corresponds roughly to the amount of energy that an average nuclear power plant generates over a period of several decades. In the course of this, a total of about ten tons of uranium-235 ( 235 U) were split and at the same time about four tons of plutonium-239 ( 239 Pu) were hatched from uranium-238 ( 238 U) . The chain reaction came to a standstill at least around 1.5 billion years ago when the 235 U was depleted enough.

history

The possibility of natural reactors was postulated in 1953 by George Wetherill and Mark Inghram. In 1956 Paul K. Kuroda (USA) examined the possibility of their existence more closely. The necessary prerequisites for maintaining a nuclear fission process are:

  • the sufficient amount and concentration of easily cleavable isotopes such as B. 235 U
  • the absence of substances such as B. boron or cadmium, which absorb free neutrons strongly and thus withdraw them from the process
  • the presence of a moderator composed of light atoms such as B. Water to slow down the fast neutrons

Kuroda's thesis was controversial because it was considered too improbable that these conditions would coincide in the wild.

Such possibilities had been discussed in France for a long time. A pioneer in this field was Jacques Noetzlin , who immediately after the discovery of nuclear fission in 1939 saw it as a possible source of energy for volcanism. (To what extent this thesis has been supported or refuted by radioactivity measurements on the numerous active volcanoes is not known here.)

In 1972 the Frenchman Henri Bouzigues discovered in the uranium enrichment plant of Eurodif in Pierrelatte ( France ) an anomaly in the isotope ratio of UF 6 , which had been extracted from Oklo ore (publication by laboratory manager H. Bouzigues). In particular, the isotope 235 U had a lower proportion compared to all other deposits in the world. Instead of the usual amount of 0.7204%, only 0.7171% of 235 U (99.54% of the normal concentration) was measured. Since the proportion of 235 U in natural uranium on earth, in lunar rocks and also in meteorites found is very exactly 0.7204%, this difference was interpreted as a “significant deviation”. Later in other samples from the Oklo opencast mine, even lower 235 U contents were measured. The half-life of 235 U is approx. 704 million years, 238 U is much longer-lived. The proportion of 235 U thus decreases over time. When the reactor was active about two billion years ago, its 235 U share was about 3%.

The first attempt at an explanation considered earlier above-ground nuclear weapons experiments as the cause. However, the associated short-lived nuclides could not be found, so this theory was rejected. The subsequent investigations led to the discovery of the reactor. Since the reactor has long been extinguished, all reaction products with short half-lives are missing in its vicinity. Reaction products with longer half-lives exist in exactly the same isotope ratio as one would expect in spent reactor fuel. For example, the isotope ratio of neodymium in Oklo ore differs significantly from the world average: only 6% 142 Nd instead of the usual 27%.

Description of the reactors

The total of 14 reactors at Oklo are today partly complete, partly largely exhausted. The opencast and underground mines have been flooded, so that only the smallest of the known reactors in Bangombé has been preserved for further scientific studies regarding the behavior of fission products and actinides in geological formations.

The size of the reactors varies. The largest known reactor is 12 m long, 18 m deep and 20 to 50 cm thick. The smallest reactor is 5 m long, 1 m wide and a few centimeters thick. This smallest reactor is located very close to the earth's surface and is therefore exposed to severe weathering. The actual reactor core consists of 5 to 20 cm thick layers of uraninite embedded in clay .

functionality

New research suggests that this natural reactor by inflow of (ground) water moderated been, resulting in a cyclical process: The fission reaction began when the sandstone matrix had reached a uranium content of 10%. The reactor core heated up to 400 ° C, so that thermal conduction led to hydrothermal water currents. These hot waters loosened the surrounding silicate rock and transported it away, so that there was a relative enrichment of uranium to 40 to 60%. For about 30 minutes, the water slowed the neutrons down to the speed required for nuclear fission. It heated up and evaporated. Without water, the system was subcritical , so that the chain reaction died down. Water then ran for about 2 to 2.5 hours until the system reached criticality again and the cycle started again.

research

Natural reactors provide extremely interesting findings for modern science. Among other things, they allow conclusions to be drawn about how radioactive substances spread in nature within extremely long periods of time (here 2 billion years), which is of great importance with regard to the planning of nuclear repositories . However, the Swiss Nagra writes that such natural analogues should not be regarded as one hundred percent “evidence”, but merely as “indications” for the behavior of repositories.

The found ratio of the nuclides allows a conclusion that two billion years ago the nuclear reactions took place exactly as they do today, and thus sets narrow limits to a possible thesis of changes in natural constants, in particular the fine structure constants .

Others

On December 15, 1976, Gabon issued a stamp on the Oklo natural reactor.

Individual evidence

  1. ^ A b c F. Gauthier-Lafaye: 2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa) . In: CR Physique 3, No. 7, pp. 839-849 (2002). doi: 10.1016 / S1631-0705 (02) 01351-8
  2. ↑ After Alex Meshik: The workings of an ancient nuclear reactor . Scientific American, November 2005.
  3. ^ Kuroda: On the Nuclear Physical Stability of the Uranium Minerals . Journal of Chemical Physics, Vol. 25, No. 4, 1956, pp. 781-782.
  4. ↑ Étienne Roth, René Létolle: Jacques Noetzlin (1898–1972) et la “géologie nucléaire”: un pionnier méconnu . Comité français d'histoire de la géologie, Volume 20, 2006, website Annales des mines.
  5. ^ R. Bodu, H. Bouzigues, N. Morrin, F. Pfiffelmann: Sur l'existence d'anomalies isotopiques rencontrées dans l'uranium du Gabon. CR Acad. Sci., Paris, Vol. 275, 1972, pp. 1731-1732.
  6. ^ R. Loss: Oklo Fossil Reactors - Who discovered these Natural Fossil Reactors? ( July 18, 2009 memento on the Internet Archive ) Curtin University of Technology Australia, October 25, 2005, accessed February 8, 2009.
  7. ↑ National Cooperative for the Storage of Radioactive Waste (NAGRA): How nature conserved and disposed of. In: Nagra info 22, November 2006, p. 3. PDF, 948 kB
  8. ^ CR Gould, EI Sharapov, SK Lamoreaux: Time-variability of alpha from realistic models of Oklo reactors In: Phys. Rev. C 74, 024607 (2007), arxiv : nucl-ex / 0701019 .
  9. ^ SK Lamoureux, JR Torgerson: Neutron moderation in the Oklo natural reactor and the time variation of alpha . Physical Review D, Volume 69, 2004, 121701, Arxiv
  10. ↑ Reacteur fossile d'Oklo postage stamp 60 FCFA, Gabon. colnect.com 2003–2017, accessed July 5, 2017.

literature

  • Michael Schaaf: Nuclear fission in the heart of darkness. Africa and the origins of the nuclear age . In: Vera Keizer (Ed.): Radiochemistry, diligence and intuition. New research on Otto Hahn . Berlin 2018, ISBN 978-3-86225-113-1 .
  • AP Meshik, CM Hohenberg, OV Pravdivtseva: Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo / Okelobondo Area in Gabon. In: Phys. Rev. Lett. 93, 182302 (2004), doi: 10.1103 / PhysRevLett.93.182302 .
  • AP Meshik: Natural Nuclear Reactors . In: Spektrum der Wissenschaft 2006/06, pp. 84–90.
  • Paul K. Kuroda: The origin of the chemical elements and the Oklo phenomenon. Springer, Berlin 1982, ISBN 3-540-11679-6 .
  • George Cowan : A natural fission reactor . Scientific American, Volume 235, January 1976.

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