Uranium has three isotopes: uranium-238, uranium-235, and uranium-234. Uranium is the most common fuel used in commercial nuclear power plants. The energy produced by nuclear fission is generally used to heat water and produce steam, which turns large turbines that produce electricity. When carefully controlled, a self-sustaining “critical” reaction of nuclear fission can generate power for a long time-until the nuclear fuel becomes depleted of fissionable atoms. If slowed down by a moderating substance (typically water or graphite), these neutrons may induce other atoms to undergo fission. As a result of this fission, fast neutrons are produced. In manmade nuclear reactors, power is generated when uranium (or sometimes plutonium) atoms fission or break into parts, releasing nuclear energy. These conditions are very similar to the conditions under which nuclear reactions are sustained in manmade nuclear reactors. The nuclear reactors are found in the FA sandstone layer. The last reactor (#17) is located at Bangombé, ~30 km southeast of Oklo. Notably, in a 1956 paper Paul Kuroda theorized the conditions under which nuclear fission could spontaneously develop and be sustained.įigure 2: Geologic cross-section of the Oklo and Okélobondo uranium deposits, showing the locations of the nuclear reactors. The possibility that natural nuclear reactors may have operated on the ancient Earth was first hypothesized by scientists in the 1950s, when commercial nuclear reactors were first being developed and becoming popular. Further, at the Gabon reactors many of the radioactive products of the nuclear fission have been safely contained for two billion years, providing evidence that long-term geologic storage of nuclear waste is feasible. Figure taken from Mossman et al., 2008.ĭespite their modest power output, the Gabon nuclear reactors are remarkable because they spontaneously began operating around two billion years ago, and they continued to operate in a stable manner for up to one million years. Other uranium deposits (which did not host natural nuclear reactors) are found at Boyindzi, Okélobondo, and Mikouloungou. The natural nuclear reactors are located at Oklo and Bangombé. As a comparison, commercial pressurized boiling water reactor nuclear power plants produce about 1,000 megawatts, which would power about ten million lightbulbs.įigure 1: The geology of the Franceville Basin. The average power output of the Gabon reactors was about 100 kilowatts, which would power about 1,000 lightbulbs. The energy produced by these natural nuclear reactors was modest. There are two types of light-water reactors operating in America.Two billion years ago- eons before humans developed the first commercial nuclear power plants in the 1950s- seventeen natural nuclear fission reactors operated in what is today known as Gabon in Western Africa. This means they use normal water as both a coolant and neutron moderator. Types of Light-water Reactors in the United StatesĪll commercial nuclear reactors in the United States are light-water reactors. The heat created by fission turns the water into steam, which spins a turbine to produce carbon-free electricity. The moderator helps slow down the neutrons produced by fission to sustain the chain reaction.Ĭontrol rods can then be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. Inside the reactor vessel, the fuel rods are immersed in water which acts as both a coolant and moderator. A reactor core is typically made up of a couple hundred assemblies, depending on power level. Typically, more than 200 of these rods are bundled together to form a fuel assembly. The uranium is processed into small ceramic pellets and stacked together into sealed metal tubes called fuel rods.
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