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6 Natural Nuclear Reactors, The Oklo Phenomenon

6 Natural Nuclear Reactors, The Oklo Phenomenon - CHAPTER 6...

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CHAPTER 6 NATURAL NUCLEAR REACTORS, THE OKLO PHENOMENON © M. Ragheb 3/2/2009 6.1 INTRODUCTION In 1972, natural nuclear reactors were found in West Africa in the Gabon Republic, at the adjacent Oklo and Okelobondo mines, and at Bangombe 35 kilometers away. The reactors were critical about 1.8 eons or billion years ago. One of the reactors released 15,000 MWth.years of energy and consumed six metric tonnes of uranium. It operated over several hundred thousand years at a low average power of 100 kWth. Sixteen reacting zones where these natural reactors occurred have been identified. From 1942 until 1972, humans thought that they have achieved a feat that nature did not, when the first sustained nuclear chain reaction initiated at the Chicago Pile number one: CP1. In CP1, graphite, which is a form of carbon, was used as a moderator containing spheres of natural uranium metal and oxide UO 2 . Scientists have believed for a long time that the elements were synthesized only in the stars. The chemist Francis W. Aston in 1922 speculated that the process of nucleo- synthesis could also occur on Earth. He suggested a situation in which: “In this event the whole of the hydrogen on the earth might be transformed at once and the success of the experiment published at large to the universe as a new star.” Fig. 1: The Oklo uranium mine in the Gabon Republic, West Africa.
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Fig. 2: Past location of the Oklo site in the Pangean super continent. Presently, it is located at the Gabon Republic, Africa. In 1953, George W. Wetherhill of The University of California at Los Angeles, and Mark G. Inghram of the University of Chicago suggested that some uranium deposits may have once operated as natural fission reactors. Paul K. Kuroda, a chemist from the University of Arkansas in 1956 suggested that an assembly of uranium and water could become a self sustained chain reacting system in the early history of the Earth. He calculated that the size of the deposit should exceed the average diffusion length of a fission neutron at about 2/3 of a meter. He was proven right in May 1972, when it was discovered that humans were not first in achieving a self sustained fission chain reaction. Nature had preempted them about 1.8 eons or billions years earlier. Aston was also proven right to a certain extent, where the process of transmutations of the elements had existed naturally on Earth billions of years ago in the form of fission rather than in the form of fusion as he speculated. The fissile isotope U 235 has an natural abundance of 0.7202 percent, but some samples from Oklo have a reduced U 235 abundance of 0.7171 percent. Other samples are depleted down to 0.44 percent. This difference could only be explained if some of the U 235 had been consumed in a fission reaction. Like present day power reactors, a natural reactor would require the existence of several special conditions, namely fuel, a moderator, a reflector, lack of neutron absorbing poisons and some way to remove the heat generated. At Oklo, the area was naturally loaded with uranium by water transport and deposition. The concentration of U 235 is artificially enriched for most modern reactors to a level of 3-5 percent, but at the
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6 Natural Nuclear Reactors, The Oklo Phenomenon - CHAPTER 6...

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