The Natural Reactors at Oklo

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 

The natural reactors at Oklo have been in operation for about 10 y, probably with intermissions, depending on the presence of water. The neutron flux density, the power level and the temperature were relatively low ( 10 cm s 10 kW and about 400-600 °C, respectively). About 6 tons of have been consumed and about 1 ton of Pu has been produced. Since then, the latter decayed into 

If can be assumed that natural nuclear reactors have been in operation about (1 to 3) 10 y ago at many other places containing uranium-rich ore deposits in the presence of water. 

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Analysis of the natural reactors at Oklo gives valuable information about the migration behaviour of fission products and actinides in the geosphere. Uranium and the lanthanides have been redistributed locally. Plutonium produced in the Oklo reactors did not move during its lifetime from the site of its formation 85-100% of the lanthanides, 75-90% of the Ru and 60-85% of the Tc were retained within the reactor zones. Small amounts of U, lanthanides, Ru and Tc moved with the water over distances of up to 20-50 m. 

Unusually high fission track densities are found in the vicinity of nuclear explosions and at the natural reactors at Oklo. 

Abstract The chapter is devoted to the practical application of the fission process, mainly in nuclear reactors. After a historical discussion covering the natural reactors at Oklo and the first attempts to build artificial reactors, the fimdamental principles of chain reactions are discussed. In this context chain reactions with fast and thermal neutrons are covered as well as the process of neutron moderation. Criticality concepts (fission factor 77, criticality factor k) are discussed as well as reactor kinetics and the role of delayed neutrons. Examples of specific nuclear reactor types are presented briefly research reactors (TRIGA and ILL High Flux Reactor), and some reactor types used to drive nuclear power stations (pressurized water reactor [PWR], boiling water reactor [BWR], Reaktor Bolshoi Moshchnosti Kanalny [RBMK], fast breeder reactor [FBR]). The new concept of the accelerator-driven systems (ADS) is presented. The principle of fission weapons is outlined. Finally, the nuclear fuel cycle is briefly covered from mining, chemical isolation of the fuel and preparation of the fuel elements to reprocessing the spent fuel and conditioning for deposit in a final repository. 

In this context, it is worth remembering the natural reactor at Oklo discussed at the beginning of this article. Apparently the increase in reactivity due to the influx of water was limited to k < 1.0065. Otherwise an explosion would have been the consequence. 

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... 

What convinces scientists that sustained fission once occurred at Oklo is the presence of characteristic fission products in the ore. Elements of mass numbers between 75 and 160 occur in the ore in larger amounts than elsewhere. Furthermore, mass analysis of the elements in Oklo ore shows that they are distributed in the characteristic pattern shown in Figure 22-12. This isotopic signature, which is not found in any other naturally occurring materials, is so characteristic that it has convinced most scientists that the ore deposits at Oklo once formed a huge nuclear reactor. 

Indications from both microscopic analyses of metallic particles from corrosion tests and evidence from the Oklo natural reactors indicate that performance assessment calculations should not assume 99Tc is easily mobilized. It is entirely inappropriate to use "Tc release as a marker for fuel corrosion because Tc is not located in the fuel matrix. The TEM examinations of corroded e-particles have shown that Mo is preferentially leached from these phases, a behaviour that is similar to the one observed at Oklo. It is interesting to note that laboratory dissolution of billion-year old 4d-metallic particles for a chemical analysis required a corrosive mix of peroxide and acid (Hidaka Holliger 1998) similar to the experience at SNF reprocessing plants. It is doubtful that the oxidation potential at the surface of an aged fuel will be sufficient to move Tc(0) from the e-metal particles. 

Hidaka, H. Holliger, P. 1998. Geochemical and neutronic characteristics of the natural fossil fission reactors at Oklo and Bamgombe, Gabon. Geochimica et Cosmochimica Acta, 62, 89-108. 

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 any case, the primordial radioactivity on the earth was appreciably higher than at present. The ratios of the aetivities at the time of the birth of the earth to those at present are listed in Table 15.4 for some long-lived radionuclides that represent the main radioactive inventory on the earth. The relatively high activity of about 2 10 y ago is the reason for the operation of the natural nuclear reactors at Oklo at that time (seetion 11.8). 

These conclusions are confirmed by the results of the investigations of radionuclide behavior in natural analog sites. Even more relevant are the results of the natural reactor region at Oklo. 

Introducing the values into the equation, using a minimum Kd-value of >300, gives a retention factor of >750. If this value is combined with a representative water transport time from repository to recipient (>1000 years for a distance >100 m), the transport equation indicates that it will take the plutonium >750,000 years to reach the recipient which is the water man may use. This estimate is supported by findings at the ancient natural reactor site at Oklo in Gabon (67). 

Information on the interaction of radionuclides with ground-water in deeply-buried, high-level, long-term "waste repositories" is available at only a few locations. One is the OKLO natural reactor in Gabon which has for over 1. 7 billion years retained some of the radionuclides also present in nuclear wastes (5). Another is the Nevada test Site, where radionuclides were first deposited underground on September 19, 1967 during the 1.7 kt... 

The shorter half life of compared with that of meant that at this time, ca. 1.8 x 10 y ago, natural uranium contained some 3% rather than its current level of 0.7%. Thus, in regions of the ore body where uranium concentrations greater than 10% were present in seams over 0.5 m thick, there was sufficient fissile material for the moderating effect of percolating groundwater to lead to a fission chain reaction. When the heat released was sufficient to boil the water and expel it, the moderating effect would be lost, so that the reactor zone would cycle between critical and subcritical conditions depending upon its water content. It has been estimated that the principal reactor zone at Oklo must have operated in this way for at least 1.5 x lO years at a power level between 10 and 100 kW. 

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