Oklo: the Impossibly Ancient Nuclear Reactors of Gabon
Nuclear power is among the most complex and advanced technologies we have developed as a species. Our fission reactors are some of the most complicated machines and systems ever built, which is hardly surprising given what occurs at their heart.
Nuclear fission, whereby atoms of uranium are bombarded by neutrons which they absorb, causing them to become unstable and split apart, produces an astonishing amount of energy. There are currently some 440 nuclear reactors worldwide, and between them they generate about 10% of our entire energy production.
In practice, the reaction requires constant monitoring. Nuclear power plants in effect are maintaining a giant and unstable chain reaction as neutrons from the splitting of the initial atoms go on to bombard further uranium.
None of this happens easily, either. Most elements exist in stable states which resist this bombardment, with only uranium (and its evil brother plutonium) possessing atoms which are large, unstable and otherwise suitable enough for fission to be viable. Uranium is also the only naturally occurring element that could you used for fission with “thermal” neutrons.
Even then it is only under specific conditions that this can occur. The reaction must be tightly controlled, or there is the risk that a runaway reaction could lead to an explosion.
And the process is, historically speaking, brand new. It was less than a century ago in December 1938 that two industrial chemists named Otto Hahn and Fritz Strassmann were able to confirm that fission had occurred, building on four decades of research into radioactivity by the scientific community.
Such a rare, complex and unstable process should be considered among mankind’s greatest achievements, and only a few countries possess the infrastructure and expertise to consider nuclear power. There certainly shouldn’t be any evidence that nuclear reactors existed millions of years ago in the African country of Gabon, right?
… right?
Oklo and the Fission Mine
Uranium was discovered near the city of Franceville in Haut-Ogooué in the southeast of Gabon (historically a French colony, hence the names) in 1956. The region suddenly became of intense interest to the French Alternative Energies and Atomic Energy Commission, who set out to mine the metal for their reactors.
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France has historically long been a supporter of nuclear power, and the French mined uranium from the region for the next forty years, only stopping when the mines were exhausted. However there was something odd about the uranium that they were digging out of the ground:
Uranium is required to be “enriched” before it can be used in a reactor, and this process was undertaken in France at one of their power plants. Uranium exists in three types in nature, depending on the number of protons it has: 234, 235, and 238.
In 1972 during the enrichment process something odd was discovered. The uranium ore extracted from the mine contained less than the expected level of uranium 235, with amounts up to 40% lower in the metal. The only explanation was also impossible: the uranium pulled out of the ground had already been used for nuclear fission.
Further investigation only confirmed this wild hypothesis. Geologists who analyzed the materials from the Oklo mine found quantities of waste products from a nuclear reactor in the natural bedrock.
To complicate things even more, the state of these products and the uranium in the sample suggested that the nuclear reaction had occurred long ago. 1.7 billion years ago, to be precise.
Had the uranium been tampered with? Had there been some deception, had the useful uranium been stolen and replaced? It would take a two-decade old theory to provide the answer, and it sounded like something out of science fiction.
In 1956 the Japanese-American nuclear scientist Paul Kuroda had predicted the possibility of the existence of natural nuclear reactors. The 1972 discovery was tangible proof of these theories: the Oklo deposits, 1.7 billion years ago, were in a self-sustaining state of nuclear reaction.
How could this happen in the natural bedrock? It seems that it started with the uranium deposits being inundated with groundwater, which acts as a “moderator” or a medium through which neutrons moved more slowly than in air. This reduction in speed allowed for the nuclear reaction to take place.
The reaction was highly exothermic, producing lots of heat which would have then boiled away the groundwater, stopping the reaction. This led to a self-sustaining cycle where water entered the deposit, started the reaction which then stopped as the water turned to steam.
These cycles were estimated to each last around 3 hours, and this process is estimated to have occurred for hundreds of thousands of years. Only the eventually exhaustion of the uranium 235 along with the build up of waste products which absorbed the neutrons before they could act finally stopped the nuclear reaction.
These deposits at Oklo are the only known location on Earth where such a self-sustaining reaction is known to have occurred. About five tons of uranium 235 is estimated to have been consumed in the process, producing the rough energy equivalent of 100 megatonnes of TNT.
This is twice the amount produced by the Tsar Bomba, the largest nuclear bomb ever detonated. However because the energy production was over such a long period of time, the only effect of the nuclear reaction was to elevate the surrounding rock by a few hundred degrees Celsius.
The likelihood of such an event happening is considered extremely slight, but as we can see with Oklo such a thing is not impossible. There is, in theory, no reason that a similar set of circumstances could not occur elsewhere, and it is chiefly the scarcity of uranium 235 which is likely to mean there are no more ancient, natural nuclear reactors out there.
… right?
Top Image: What French scientists discovered about the uranium from Oklo was almost unbelievable. Source: nighthawk101stock / CC BY 3.0.
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