By Allison Kubo Hutchsion
However, in 1972 workers at a nuclear fuel processing plant in France discovered an unexplainable anomaly. They found an abnormally low ratio of U235 to U238 of only 0.717%. Now this may seem like a small difference but in terms of uranium, it is a huge chasm. At the processing plant, they came up short 200 kilograms of U235 since they had assumed the normal ratio. 200 kilograms of U235 is equivalent to almost 600 million kilograms of coal and enough to sustain all the electricity needs of 800,000 people in France for a year. How could the ratio be lower? Did it decay faster? Something must have occurred to burn off more U235 than normal. It was as if it had already been a nuclear reactor.
A natural nuclear reactor.
The ore had come from the Oklo mine in Gabon. The conundrum was dubbed the “Oklo Phenomena” and stumped scientists because the conditions necessary for a nuclear reactor are very specific. Examining the deposits found that surprisingly Oklo mine was able to maintain the delicate balance necessary to sustain a chain reaction 2 billion years ago. First, due to the decay rate of uranium 2 billion years ago the natural ratio of uranium was approximately 3 percent. Since then much of the U235 has decayed but for a period of time far in the past, the natural levels were enough to produce a reaction. Once this was satisfied, scientists searched for a moderator. The Oklo uranium deposits are hosted in a layer of permeable rocks; the high permeability allowed groundwater to flow through the deposit and act as moderator. Water is a very efficient moderator and is used in many reactors today. The clay minerals around the deposits recorded the heat of the reactor and the original host rock became hydrothermally altered due to the immense heat of fission. The unique structure of the deposit allowed sustained fission to occur. By measuring the different isotope present, scientists developed an operational model for the reactor. Fission started in the pockets of uranium ore when a stray neutron, which is naturally produced and elements decay, hit a U235 atom. Then this started a chain reaction and water acted as a moderator for approximately 30 minutes. After a time, the water would become so hot that it became steam and escaped through the permeable rock. Then it would take several hours for the reactor to cool then start the cycle again in a process similar to eruptions from the geyser “Old Faithful”.
The reactor had produced radioactive isotopes such as plutonium but since then it has decayed leaving just a strange ratio of isotopes to indicate what had occurred. This phenomenon convinced scientists that similar things might have occurred elsewhere and discovered a similar occurrence at Bangombé, 30 kilometers southeast of Oklo. It could be that 2 billion years ago this phenomenon was common. The Oklo mine provided not only a stumping science question but also a reminder of the immense power of nature.
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