More about OKLO, the natural fission reactor
February 19, 2014
Fission of course occurs as a natural process, through the spontaneous decay of certain heavy nuclei. An element of interest is natural uranium. Its natural abundance has been carefully and precisely measured in minerals from variety of sources, especially owing to the need for exact control of 235 U enrichment. The accepted value is 0.00720 +- 0.00001 for the fraction of 235 U in natural uranium. The small limit of uncertainty not only represents the precision of the measured values , but is also indicative of the variation in samples from very different locations, where uranium is mined- the western United States, Canada, Australia, and Africa, even the rocks returned from the Moon by the Apollo missions show the same value.
Of cause this ratio vary over geological time because both 235 U and 238 U are radioactive, with 235 U having a shorter half-life: 700 million years, and 238 U having half- life 4.5 milliard years. Because of the shorter half-life, 235 U decays more rapidly than U 238, and in the times past 235 U must have had a greater relative abundance. One can calculate the present abundance and the previous abundance ( using decay constants). If one then makes a figure, a plot of this ration, one can notice, that about 2 milliard years ago, the 235 U fraction was of order 3%. As we know from water-moderated reactors, with 235 U enrichment of 3% the neutron absorption in natural water as moderator does not prevent the construction of a critical assembly.
Going further back, one can postulate that the extreme conditions in a supernova ( where the heavy elements may have been produced) would not be expected to discriminate significantly between 235 U and 238 U.by extrapolation one can estimate the time of formation of these elements. and the age of solar system. The ratio 235 U / 238 U one can be used in dating the galaxy.
Because of the extreme precision and uniformity of the measured abundance of 235 U, even small anomalies are particularly apparent. In 1972, a sample of uranium, mined at OKLO in what is now the Republic of Gabon on the west coast of Africa, was analyzed by the French Atomic Energy Commission and showed 235 U abundance of only 0.00717, about 3 standard deviations below the accepted value. This small deviation was enough to excite their curiosity, and analysis of additional samples showed even smaller 235 U abundances, as low as 0.00440. Since the ONLY KNOWN PROCESS that could lead to reduction in 235 U concentration is fission by low-energy neutrons, the French workers theorized that the natural nuclear reactor operated in OKLO site about 2 milliard years ago, when the 235 U abundance was high enough ( about 3%) to permit the operation of a reactor moderated by groundwater.
Of cause, the OKLOreactor could have operated even before 2 milliard years ago. However, the formation of deposits of uranium such as in OKLO region requires the transport of uranyl ions (UO2)++ in groundwater, and it is now believed that before the evolution of oxygen producing bacteria about 2 milliard years ago there were insufficient oxygen in the water to have formed the uranium deposit. Thus the need of highly oxygenated water and for 235 U enrichment of 3% place fairly rigid limits on the age of OKLO reactor. Estimating the total size of the uranium deposit and its deficiency of 235 U, it has been calculated that about 5 tons of 235 U were fissioned; with 200 meV released per fission, this amounts to a total energy release of 2 *10E30 MeV or about 1000 MW. However, it is unlikely that the OKLO reactor operated at this power level for the boiling of the water would have removed the moderator and terminated the reactor until groundwater could collect again. The average power level was probably more like 0,01 MW, suggesting that the OKLO reactor may have operated 1 000 000 years.
In OKLO minerals there were an abundance of fission products, relative abundance of Neodymium isotopes in natural Nd and residues of 235 U fission products.Also Ruthenium isotopes were abundant 100 Ru abundance differes slightly from that of ordinary fission of 235 U ( because of neutron capture fission product 99 Tc, Half life 210 000 years) The observed excess of 100 Ru can be used to estimate the duration of the OKLO reactor, and values obtained are about 1 million years, in agreement with the estimate based on the 235 U depletion.
Interesting feature: The fission products of OKLO reactor are still in place in the reactor zone and have migrated very little. Despite climatic changes, no substantial movement of these fission products has taken place over the past 2 milliard years. This suggests that there may be merit in the present schemes for burying waste products of power reactors in geologically stable formations.
Source:Kenneth S. Krane. Introductory Nuclear Physics 1988.Pages 516- 520