1.7 billion years ago, Earth had a natural nuclear reactor - by StartsWithABang
If you were hunting for alien intelligence, looking for a surefire signature from across the Universe of their activity, you’d have a few options. You could look for an intelligent radio broadcast, like the type humans began emitting in the 20th century. You could look for examples of planet-wide modifications, like human civilization displays when you view Earth at a high-enough resolution.
By the time you get to the present day, U-235 makes up only about 0.72% of all naturally-occurring Uranium, meaning it has to be enriched to at least about 3% levels in order to get a sustaining fission reaction, or a special setup is required. But 1.7 billion years ago was more than two full half-lives ago for U-235. Back then, in ancient Earth, U-235 was about 3.7% of all uranium: enough for a reaction to occur.
Geologic cross-section of the Oklo and Okélobondo uranium deposits, showing the locations of the nuclear reactors. The last reactor is located at Bangombé, ~30 km southeast of Oklo. The nuclear reactors are found in the FA sandstone layer. A selection of some of the original samples from Oklo, discovered in 1972. This is a piece of high-grade ore from the Oklo mines mysterious contained 0.4% less U-235 than all other naturally occurring samples relative to U-238, evidence that some sort of previous fission reaction had depleted the U-235.Five isotopes of the element xenon are produced as reaction products.U-235, when split apart, produces large amounts of neodymium with a specific weight: Nd-143.
The groundwater acts as a neutron moderator, allowing more than 1 out of 3 neutrons to collide wtih a U-235 nucleus, continuing the chain reaction. This three hour cycle would repeat itself for hundreds of thousands of years, until the ever-decreasing amount of U-235 reached a low-enough level, below that ~3% amount, that a chain reaction could no longer be sustained. At that point, all that both U-235 and U-238 could do is radioactively decay.
Ludovic Ferrière, curator of the rock collection, holds a piece of the Oklo reactor in Vienna’s Natural History Museum. A sample of enriched ore from the Oklo reactor is now on permanent display in the Vienna museum as of 2019.
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