|
|
| Fig. 1: A diagram of samples taken from a vein of Uraninite at Oklo. (Image Source: S. Erickson, after Cowen. [2]) |
In Gabon, there are rich Uranium deposits in the Franceville basin. Within this basin, several deposits formed natural fission reactors. The first to be discovered was the Oklo deposit. (See Fig. 1.) This natural fission reactor formed roughly 2 billion years ago, when the abundance of the fissionable isotope Uranium 235 (U-235) was much higher. [1] The main evidence of natural fission occurring at Oklo is the depletion of U-235, and the anomalous abundance of fission decay products, particularly the rare earth metal Neodymium. [2]
In 1972, during routine measurement of samples, French researchers discovered an anomaly. In a sample from Oklo, U-235 was measured to have 0.7171% abundance rather than the expected 0.7202%. [2] Although natural fission reactors had been theorized since 1953, it wasn't until fission decay products were also discovered in the Oklo samples that the source of the depletion could be confirmed to be nuclear fission. [3]
Fission of Uranium results in many stable decay products. One in particular, Neodymium (Nd), is a useful marker, since it is very rare in nature, and the background can be subtracted easily. [2] A comparison of the observed isotopic fractions of Nd at Oklo and the expected isotopic fractions in nature is shown in Table 1. These fractional abundances are higher at Oklo than expected in nature, because the baseline isotope, Nd-142, is not a fission decay product, while the other isotopes are. These significant anomalies confirmed that fission must have occured.
|
|||||||||||||||||||||
| Table 1: Relative abundances of Neodymium isotopes. [4] For relative abundance A/B, the number quoted is the abundance of isotope A divided by the abundance of isotope B. |
There is irrefutable evidence that natural fission occurred in the Oklo deposit, thanks to the depletion of U-235 and the anomalous isotopic abundances of fission products like Nd.
© Sydney Erickson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[2] R. T. Ibekwe et al., "Modeling the Short-Term and Long-Term Behaviour of the Oklo Natural Nuclear Reactor Phenomenon," Prog. Nucl. Energy. 118, 103080 (2020).
[2] G. A. Cowen, "A Natural Fission Reactor," Sci. Am. 235, No. 1, 36 (July 1976).
[3] A. Zhao, "Oklo: Nature's Nuclear Reactor," Physics 241, Stanford University, Winter 2016.
[4] M. Loubet, and C. J. Allegre, "Behavior of the Rare Earth Elements in the Oklo Natural Reactor," Geochim. Cosmochim. Acta 41, 1539 (1977).