Oklo: Nature's Nuclear Reactor

Andy Zhao
February 20, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016

Historical Background

Fig. 1: Gabon (Source: Wikimedia Commons)

In the Central African state of Gabon (see Fig. 1), lays a natural nuclear fission reactor. Neither man made nor constructed from modern steel and concrete, Oklo is a nuclear reactor zone of sandstone, granite, and uranium ore found in nature (see Fig. 2). It was discovered in May 1972 by a nuclear fuel-processing plant in France. The percentage of U-235 in the samples of uranium from the Oslo deposit in Gabon (0.717%) was slightly off from the 0.720% found in uranium elsewhere in the earth's crust. The French Atomic Energy Commission analyzed that this discrepancy meant that they were missing around 200 kilograms of U-235, enough to make 6 nuclear bombs. [1] The difference comes from the uranium reacting billions of years ago which changed the makeup of the uranium ore in Oklo.

Natural Fission Reactors

19 years before the discovery of the Oklo natural nuclear reactor, in 1953, scientists from UCLA and the University of Chicago published predictions that some uranium deposits may be operating as self operating nuclear fission reactors. [2] Calculations from Paul K. Kuroda, a chemist from the University of Arkansas found 3 conditions necessary for a natural fission reactor. [3] First, the width of the uranium deposit must be more than two thirds of a meter. This allows neutrons to travel far enough to be absorbed, to continue the nuclear chain reaction, and to do so before leaving the uranium deposit. Second, the percentage of U-235 must be above 3% in the uranium ore to properly fuel a nuclear reactor. U-235 decays over time, so two billion years ago there was enough U-235 in Oklo to allow a natural fission reactor to exist. Today, the composition of U-235 is too dilute. Third, a neutron moderator (such as water) to control the nuclear reaction is necessary - one without too much boron, lithium, or anything that will absorb too many neutrons.

Lessons from Oklo

Fig. 2: (1) Nuclear reactor zones; (2) Sandstone; (3) Ore layer; (4) Granite. (Source: Wikimedia Commons)

Oklo provides a time machine to examine how nuclear waste can be handled. Scientists have examined how the nuclear waste from reactions 2 billion years ago has affected surrounding areas. Oklo has demonstrated that aluminum phosphate minerals are able to contain radioactive gases that are normally very dangerous byproducts of nuclear reactions, such as Xe-135 and Kr-85, for billions of years. [1]

There have also been new hypotheses about the time variation of the fine-structure constant alpha assuming "a Maxwell-Boltzmann low energy neutron spectrum." [4] These are based on an analysis from isotopic analysis of smaples from Oklo.

In the hundreds of thousands of years operating as a nuclear reactor, Oklo never had a meltdown or explosion. [1] The safety of present day nuclear power plants can be informed by billions of years in nature. The combination of aluminum phosphate grains to trap radioactive materials and the groundwater to regulate the reaction allowed for an extremely safe reactor.

© Andy Zhao. 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.


[1] A. P. Meshik, "The Workings of an Ancient Nuclear Reactor," Scientific American, 1 Nov 05.

[2] G. W. Wetherill, "Spontaneous fission Yields from Uranium and Thorium," Phys. Rev. 92, 907 (1953).

[3] P. K. Kuroda, "Nuclear Fission in the Early History of the Earth," Nature 187, 36 (1960).

[4] S. K. Lamoreaux and J. R. Torgerson, "Neutron Moderation in the Oklo Natural Reactor and the Time Variation of α," Phys. Rev. D 69, 121701(R) (2004).