Humans have dug up radioactive elements from the Earth for over a hundred years. We have created horrible weapons with it to create large destructive force. But long before humans fiddled with Uranium or anything else, nature herself was playing with the effects of radioactivity. And not just among the stars, but here on Earth.
Using a large expanse of light-sensing photomultiplier tubes, scientists at the KamLAND neutrino detector made an interesting observation: about half the heat in the Earth’s atmosphere comes from radioactive decay. It was determined that 20 Terawatts of energy are imparted from radioactivity. A lightning strike has about 1 Terawatt of energy for 30 microseconds. In the case of uranium, potassium, and thorium, however, the energy output doesn’t stop.
In addition to the continuous decay that heats our planet, there is evidence that nuclear fission reactions (similar to the ones in bombs) have occurred naturally. During a 1972 routine inspection of the Oklo Mine (in Gabon), it was discovered that what should have been a concentration of 0.7202% uranium-235, was actually as low as 0.440%. That drastic change is something only seen in nuclear reactors.
With further observation, it was determined that the Oklo Mine was the site of a natural nuclear reactor 1.7 billion years ago. Eventually, sixteen other sites were found in the area. In order for a natural uranium fission reactor to occur, there has to be a large amount of highly-concentrated uranium. Today there is not enough reactor grade uranium to sustain such a reaction naturally. How did it work back then?
At the time, the Oklo uranium would have been at around an amazing 3% concentration and the reaction would have been very spontaneous. When a stray neutron strikes a single uranium atom, the nucleus splits to release more neutrons. These neutrons strike other atoms, leading to a chain reaction. With groundwater moderating the neutrons (and boiling off to slow the reaction), these nuclear reactors are estimated to have continued for a hundred thousand years!