Two billion years ago, a natural nuclear fission reactor formed in Africa
I just finished posting a fascinating article about a teenage boy who put together a real, if small-scale, nuclear reactor in his mother's back-yard shed -- so while I'm on the subject of nuclear reactors, I thought I'd post this equally fascinating article about a naturally occurring nuclear reactor that came into being two billion years ago in Africa, and continued operating for many millenia.
Here's a brief description of this natural nuclear reactor from the Wikipedia article: A natural nuclear fission reactor can occur under certain circumstances that mimic the conditions in a constructed reactor. The only known natural nuclear reactor formed 2 billion years ago in Oklo, Gabon, Africa.  Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction. The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years.
For other articles on this very interesting, if little-known, topic, see here, here , and here.
Finally, here's the description of the illustration: The remains of the OKLO 15 reactor, however, are intact and can be viewed. The yellow spots, which are well visible in the picture, are uranium oxide of 70% concentration in sandstone.
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Fission reactors on the Earth
It is not necessary to search the sky if we want to find ancient, several billion years old reactors. In 1972 a French engineer made one of the most astounding discoveries in the history of science. Bougzigues worked as an analyst for a company producing fuel for nuclear power plants in Provance, France. In the course of his routine measurements he found a strange anomaly.
Discovery of the Oklo reactors
There are two naturally ocurring isotopes of uranium, whose mass numbers are 235 and 238, respectively. (Actually, there is a third isotope, with mass number 236, but its abundance is negligible.) At the time the Earth and the solar system were born, the ratio of the isotopes 235U and 238U was a given value. This ratio has changed significantly since the two isotopes decay with different half-lifes. The half-life of 235U is 700 million years, while that of 238U is 4.5 billion years. Correspondingly, the amount of 235U decreases faster. It is obvious that the half-life of the isotopes is exactly the same everywhere in the world, in all the rocks and stones, which implies that the ratio of the isotopes has changed to the same degree everywhere over the millions of years. Until 1972 several measurements verified this assumption and the ratio of the isotopes with mass numbers 235 and 238 in uranium ores was always found to be 0.7202% with an accuracy of 0.00004%.
In the Pierrelatte factory in 1972, nuclear fuel was manufactured from uranium ore that originated from the Gambian Oklo. Bougzigues made routine measurements during which he found that ratio of isotopes in the uranium ore to be processed is somewhat less than the earlier measured value: 0.717%. Different possibilities were investigated that could cause the measured anomaly. First they ascertained that no spent nuclear fuel had been deposited in the mine. The activity of the ore was too little for that. There were certain theories which tried to explain the phenomenon with the crash of an extraterrestrial space ship or with the existence of an ancient civilization that utilized nuclear energy. However, reality was far beyond even these romantic imaginations. The researchers found the remnants of an ancient, natural nuclear reactor in Oklo!
Remnants of the core of the OKLO 15 natural reactor
During the investigations performed in 1972, the remnants of six natural reactors were found in the Oklo mine and its vicinity. Until now marks of altogether 17 natural reactors have been discovered. Nine of them have been completely mined out. The remains of the OKLO 15 reactor, however, are intact and can be viewed. The yellow spots, which are well visible in the picture, are uranium oxide of 70% concentration in sandstone.
Conditions of the operation of natural reactors
As early as in 1956 a Japanese physicist, Paul Kuroda pointed out the possibility of the existence of natural reactors. There was not much reflection to his statement, even in the scientific life. He worked out the conditions which are necessary for the operation of natural reactors.
Kuroda estimatedthe historical era when natural reactors could be born. Of the two uranium isotopes the one with mass number 235 is fissile. Nowadays the abundance of 235U is only about 0.7%, natural reactors cannot operate. The reason for this is that the 238U isotopes and the other nuclei of the ore catch too many of the neutrons which are necessary to sustain the chain reaction. Moreover, some moderator is also needed to slow down the neutrons to make them easier to cause fission and this moderating material also absorbs neutrons. The usual moderator materials in today's nuclear reactors are water, heavy-water and graphite. Although using presently mined natural uranium and graphite or heavy-water as moderator one can build an operable reactor, neither of these materials are present in nature. On the other hand, the neutron absorbtion ability of water is so high that it is not possible to use it with natural uranium (in which the percentage of the isotope with mass number 235 is not increased, that is not enriched) in a reactor. However, the situation was different 2 billion years ago. At that time the ratio of 235U in uranium was 3% and using this material as fuel, one can in fact build a reactor, even with water moderator! (This is the most widespread enrichment of the power reactors today.)
Kuroda determined the necessary concentration of uranium atoms in the carrying agent of the ore: according to his calculations it should be 70%.
He also determined the critical size of the reactor. If the dimensions are smaller, no self-sustaining chain reaction can start because too many neutrons would escape from the so called reactor core.
Finally, he pointed out that the ore must be porous in order that water can remain in the small pores and play the role of a neutron moderator.
How did the Oklo reactors evolve?
The uranium content of the Gabon uranium ore probably got to the surface by volcanic activity. Later the surface waters dissolved it from the volcanic rock. About 1.7 million years ago there was enough oxigen in the atmosphere for uranium to oxidize. Uranium oxide, however, is insoluble in water and thus it could settle down in high concentration layers.
Operation of the reactors
The Oklo reactors perfectly fulfilled the requirements determined by Kuroda. The reactor cores of large mass and high uranium concentration embedded into porou carrying material, mainly sandstone. The concentration of neutron absorbing materials in the rocks was negligible and the 235U/238U ratio was around 3%. In the porous sandstone there was sufficient amount of water for neutron moderation. Moreover, the water present in the rocks was thermohydraulically coupled to the surface and ground water.
Finally, about 1.7 billion years ago the reactors went critical. The chain reaction was probably started by the spontaneous fission and cosmic radiation. The chain reaction was not at all continuous. As the number of fissions per unit time was increasing, more heat was produced, which lead to the warming up and possible boiling of the water. However, as the amount of moderator decreased, the reactors became sub-critical and thus the rate of fission went down. Accordingly there was a cooling down period, in which water could again filtrate into the rocks and make the reactor critical. This pulsating chain reaction could last for about a million years. The neutron flux in the reactors was probably in the range of 109 to 1021 /cm2s (this means that in a piece of surface of 1 cm2 in one second 109 to 1021 neutrons pass trough in any direction). In today's power plant reactors this quantity is around 1013 to 1014 /cm2s.
When the concentration of 235U dropped below the critical value due to the operation of the reactor and natural radioactive decay, the Oklo reactors stopped for good. Their ancient operation is only observable from the lower 235U/238U ratio.