Uranium deposit, Oklo

Fig. 1. (a) Cross-section of reactor 9 in the Oklo uranium deposit (b) bore-hole BAX3 crossing the reactor of Bangombe. 

Dymkov, Y. M, Holliger, P Pagel, M., Gorshkov, A. Artyukhina, A. 1997. Characterization of a La-Ce-Sr-C aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon). Mineralium Deposita, 32, 617-620. 

Gancarz, A. J. 1978. U-Pb age (2.05 x 109 years) of the Oklo uranium deposit. In Proceedings of the Technical Committee Meeting, Paris, 19-21 December 1975. Natural Fission Reactors, IAEA, Vienna, 513-520. 

Sunder, S., Miller, N. H. Duclos, A. M. 1994. XPS and XRD studies of samples from the natural fission reactors in the Oklo uranium deposits. Materials Research Society Symposia Proceedings, 333, 631-638. 

The Oklo anomaly, which was discovered as a result of isotopic assays of uranium from the Mounana mill, was first recognized as being the result of a natural fission reactor in 1972 (1,2). The Oklo uranium deposit, located in the southeast part of Gabon, occurs in one of the Francevillian series of sedimentary deposits. In the deposit the uranium concentration is... 

It may seem unlikely that all these conditions could have been met, but at least one deposit of uranium ore has characteristics indicating that, long ago, it operated as a natural nuclear reactor. At Oklo in the Gabon Republic near the western coast of equatorial Africa (see photo), there are uranium deposits of high purity... 

Oklo and Bangombe shows that the geological location, the mineralogy of the hosted rocks, and the chemistry of the fluids are important parameters for the preservation of the uranium deposits and of the natural nuclear reactors and that this information could be used to design and select materials for high-level waste disposal. 

Drozd, R. X, Hohenberg, C. M., Morgan, C. X (1974) Heavy rare gases from Rabbit Lake (Canada) and the Oklo Mine (Gabon) Natural spontaneous chain reactions in old uranium deposits. Earth Planet. Sci. Lett., 23, 28-33. 

At Oklo, the uranium deposits behaved as natural fission reactors in the Precambrian. 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

There are indications that this method will isolate the waste until the radioactivity decays to safe levels. One reassuring indication comes from the natural fission reactor at Oklo in Gabon, Africa. Initiated about 2 billion years ago when uranium in ore deposits there formed a critical mass, the reactor produced fission and fusion products for several thousand years. Although some of these products have migrated away from the site in the intervening 2 billion years, most have stayed in place. Another indication of the possible success of... 

In 1972 it was found that uranium in ore deposits at Oklo, Gabon, contains significantly smaller concentrations of than other deposits of natural uranium (<0.5% compared with 0.72%). At these places, the isotopic composition of other elements is also different from the mean composition in nature. For instance, natural Nd contains 27% " Nd and xl2% " Nd, whereas Nd at Oklo contains <2% Nd and up to 24% Comparison with the yields of nuclear fission leads to... 

Nd. From the high isotope ratio " Nd/ " Nd and the low isotope ratio it must be concluded that chain reactions have occurred. The age of the Oklo deposits was found by Rb/ Sr analysis to be a 1.7 10 y. At that time the concentration of in natural uranium was 3%. The presence of water in the sedimentary ore deposits led to high values of the resonance escape probability p (eq. (11.1)) and to criticality of the systems ( eff > ) ... 

Having an enriched uranium sample is only one of the requirements for starting a controlled chain reaction. There must also have been a sufficient amount of the ore and an appropriate moderator present. It appears that as a result of a geological transformation, uranium ore was continually being washed into the Oklo region to yield concentrated deposits. The moderator needed for the fission process was largely water, present as water of crystallization in the sedimentary ore. 

Because has a shorter half-life than " U, all uranium ores were richer in in the past. From Rb— Sr analysis the age of the Oklo dqxisit is known to be 1.74 X 10 y at that time the content of natural uranium was 3%. Although the fission factor i rapidly increases with content (about 1.8 for 3 % U), conditions in the natural Oklo deposit were such (e 1.0, p 0.4,/ 1.0) that < 1. The deposit is sedimentary and was formed in the presence of water, which greatly increases the resonance escape probability factor p for an atomic ratio H20 U of 3 1, p 0.8, and > 1. Thus conditions existed in the past for a spontaneous, continuing chain reaction to occur in the Oklo deposit. 

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