While Americans pride themselves on unlocking the secrets of the atom for military purposes as well as sustainable energy, it seems that the mother nature is still able to solve even the most complex problems well before us. In 1972, French physicist Francis Perrin made an interesting discovery in Oklo, Africa. While mining uranium ore, he discovered that the concentration of uranium 235 was less than what is typically found in uranium ore samples. Uranium 235 can be found to make up 0.720 percent of any given uranium sample but samples taken in Oklo showed a decrease to 0.717 percent. With the large amount of ore in the mine, the decrease showed that nearly 200 kilograms had been missing.
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Further research showed that the mine had the proper conditions for a nuclear reaction to take place. Since uranium 235 reacts only to relatively slow moving neutrons before it fisses into more stable elements, a medium more dense than air must be present to absorb the kinetic energy of the neutrons. Uranium 235 released energy in the form of heat when interacting with its surroundings and caused the water in the cave to heat up enough to evaporate. With the medium for the neutrons gone, the reaction would stop and the cave would then cool. Ground water would flow back to where it had been before it evaporated. This continued until the concentration of uranium-235 was too small to sustain a reaction.
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With 16 sites in the Oklo mine, the estimated power output was 100 kilowatts. This energy was never put to good use and the heat was dissipated into the atmosphere. While this was not a just cause, the fact that nature figured out how to set up the proper conditions for uranium to undergo fission in a controlled cycle before us is fascinating. This leaves a sense of wonder and also a sense of fear with the possibility of other naturally occurring nuclear reactors.
Source: Scientific American