The Oklo fission reactors are the only known examples of a natural nuclear reactor here on Earth … For approximately 30 minutes, the reactor would go critical, with fission proceeding until the water boils away. Over the next ~150 minutes, there would be a cooldown period, after which water would flood the mineral ore again and fission would restart.
Source: Mother Nature Created A Natural Nuclear Reactor on Earth 1.7 Billion Years Ago – Neatorama
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