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We get nearly all our power from nuclear reactions. Solar energy is nuclear-based. From 150,000,000 kilometers away, the sun sends us the energy released from fusing atoms. For millenia, the sun has heated our homes. More recently solar energy has been used for passive cooling and electricity production.

credit: macdonaldarchitects.com

credit: macdonaldarchitects.com

The sun also drives evaporation which eventually enables hydroelectric dams. However nuclear reactions in the sun also released the energy now stored in fossil fuels. Closer to home, radioactivity within the earth nearly matches the energy consumption of humans on the surface. Around 20 TW of heat is generated by radioactive decay, while 17 TW was consumed by humans in 2013.

In modern nuclear reactors we can sustain nuclear fission reactions by bringing certain materials together in the right proportions, but did you know that earth also had its own fission reactors about 2 billion years ago?

In the central African country of Gabon, in a region called Oklo, miners discovered that the uranium ore they were mining was deficient in the U-235 isotope. That was the first clue. U-235 is the natural isotope responsible for most of today’s nuclear power. Besides fuel though, a reactor needs a moderator. The moderator slows down neutrons to speeds where they can split additional atoms.

Today, most reactors use light water which is just water with hydrogen nuclei that consist of a single proton. Using this moderator, it is necessary to enrich U-235 to around 3% before it can sustain a fission reaction. Natural uranium today is only 0.7% U-235. However U-235 decays faster than U-238. 1.7 billion years ago, the U-235 fraction was around 3%.

This enabled the reactors at Oklo to periodically “switch on” when water was present. The uranium deposit would generate a lot of heat and the water would eventually evaporate or otherwise escape. The reactor would “switch off” and cool down. When the water returned, the reactor would cycle again.

credit: gsapubs.org

credit: gsapubs.org

Besides being an interesting puzzle, the Oklo reactors can and have informed today’s nuclear engineers and scientists. Since they operated for thousands of years without destroying themselves, the reactors can inform modern reactor design. Additionally, they can tell us how radioactive materials can be immobilized for billions of years. Scientists have even used Oklo to test whether the fine structure constant has changed over time.