The world’s only natural nuclear fission reactor.

Not in the sun or the stars, but here right on earth, mother earth too has her little nuclear reactor. Constructed billions of years before man made the first one. Unlike our complicated reactors, earth only needed water, uranium and blue-green algae. Really! It's in Africa at a place called Oklo and scientist estimate it started about 1.7 billion years ago. Algae in a river near Oklo produced excess oxygen after undergoing photosynthesis. This excess oxygen made the water acidic. As the water seeped underground, it dissolved the uranium from the bedrock. Some underground algae filtered this water and this caused the uranium to concentrate in one spot, achieving a critical mass. A critical mass only is not sufficient to start a chain reaction, Uranium nuclei should be struck by neutrons and they must absorb these neutrons. When uranium fissions, the neutrons emitted are very fast and so they cannot be absorbed to cause a chain reaction. Oklo uranium went nuclear only because the river water slowed the neutrons down enough for the neighbouring nuclei to catch them. Without this water the reaction would never have happened. Now fission produced heat, this heat boiled the water away. With no water, the neutrons became too fast to absorb and the process came to a halt. Only when the Uranium cooled down, did the water trickle back, this slowed the neutrons and restarted the reaction. The reaction was so self controlled that there is no big crater, in Oklo today. The reactor has consumed 6000 Kilos of Uranium over 1,50,000 years, in on/off cycles of 150 minutes.