Slide 1
Rocks at Oklo once sustained nuclear fission
Nuclear fission splits atomic nuclei and releases energy. About two billion years ago, uranium-rich rock at Oklo in present-day Gabon sustained it naturally.
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Roughly two billion years ago, rocks at Oklo in what is now Gabon sustained nuclear fission. No builder was required. An unusual combination of uranium and water allowed a physical process now associated with engineered reactors to occur in a geological setting.
Slide 2
Unusual uranium provided the clue in 1972
In 1972, uranium with an unexpected composition revealed a clue to Oklo’s ancient activity. Explaining that difference meant looking at how fission works.
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The ancient activity was recognized in 1972 through unusual uranium evidence. The material did not have the composition investigators expected. That discrepancy became a clue to a past process, rather than evidence that the rocks contained an inexplicable exception to physics.
Slide 3
One split can trigger further splits
Splitting a uranium nucleus can release neutrons that trigger further fissions.
This creates a chain reaction; water can help sustain it by slowing neutrons under suitable conditions.
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When a uranium nucleus splits, it can release neutrons that cause further fissions. Water can slow neutrons, helping sustain this chain under suitable conditions. Slowing a neutron is called moderation; it is different from absorbing the neutron and removing it from the chain.
Slide 4
Ancient uranium made those conditions different
The chain also needed suitable fuel. Ancient uranium contained a larger share of the isotope needed for this kind of fission than uranium contains today.
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Ancient uranium contained a greater proportion of the isotope that could sustain this kind of fission than uranium does today. The fuel and water conditions mattered together. The explanation depends on that past combination, not on a claim that the site is operating now.
Slide 5
The right fuel and water came together naturally
Oklo connects atomic nuclei, groundwater and a mineral deposit in one natural process. Geology assembled conditions that no engineer had designed.
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Oklo joins processes at very different scales: atomic nuclei, groundwater and a mineral deposit. Its natural origin does not make the mechanism simple. It shows that the relevant physical conditions can sometimes come together without anyone designing the system.