![]() Removing the water would have shut down the reactor until the rocks cooled enough to allow water to reenter the deposit, at which point the chain reaction would begin again. It is thought that these natural nuclear reactors operated only intermittently, however, because the heat released would have vaporized the water. As a result, the entire deposit “went critical” and became an uncontrolled nuclear chain reaction, which is estimated to have produced about 100 kW of power. When rainwater or groundwater saturated one of these deposits, the water acted as a natural moderator that decreased the kinetic energy of the neutrons emitted by radioactive decay of 235U, allowing the neutrons to initiate a chain reaction. An unusual combination of geologic phenomena in this region apparently resulted in the formation of deposits of essentially pure uranium oxide containing 3% 235U, which coincidentally is identical to the fuel used in many modern nuclear plants. The natural abundance of 235U 2 billion years ago was about 3%, compared with 0.72% today in contrast, the “fossil nuclear reactor” deposits in Gabon now contain only 0.44% 235U. There is compelling evidence that uncontrolled nuclear chain reactions occurred naturally in the early history of our planet, about 1.7 billion years ago in uranium deposits near Oklo in Gabon, West Africa ( Figure 20.19 "A “Fossil Nuclear Reactor” in a Uranium Mine Near Oklo in Gabon, West Africa"). Dismantling the plant and decontaminating the site is estimated to require 30 years at a cost of approximately $12 billion. To put this in perspective, drinking one liter of fresh water with this level of contamination is the equivalent to receiving double the annual dose of radiation that is typical for a person. ![]() Radioactive iodine levels in contaminated seawater from the plant were over 4300 times the regulated safety limit. When the water supply was disrupted, so much heat was generated that a partial meltdown occurred. ![]() ![]() The plant used fresh water for cooling nuclear fuel rods to maintain controlled, sustainable nuclear fission. The most recent event resulted from the damaged Fukushima Dai-ichi plant after the March 11, 2011, earthquake and tsunami that devastated Japan. Uncontrolled nuclear fission reactions are relatively rare, but they have occurred at least 18 times in the past. Many levels of control are required, along with a fail-safe design, because otherwise the chain reaction can accelerate so rapidly that it releases enough heat to melt or vaporize the fuel and the container, a situation that can release enough radiation to contaminate the surrounding area. A variety of techniques can be used to control the flow of neutrons from such a reaction, which allows nuclear fission reactions to be maintained at safe levels. When a critical mass of a fissile isotope is achieved, the resulting flux of neutrons can lead to a self-sustaining reaction. ![]()
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