The most powerful neutron poison is xenon-135, but since it is a gas, it relatively quickly diffuses out of the uranium ore after it’s made (isn’t that convenient!).įinally, there must be a neutron moderator that makes the free neutrons more likely to be absorbed by the U-235 atoms. Some occur naturally, such as boron and silver, while others are produced in the fission process. In addition, there can’t be too many “neutron poisons,” elements that absorb neutrons otherwise absorbed by uranium atoms. Any thinner and too few of the emitted neutrons would be absorbed by the uranium. They must be at least two-thirds of a meter thick. Had oxygen arisen much earlier, natural nuclear reactors would have been more wide-spread, perhaps violently energetic, and depleted much more U-235.Īnother condition relates to the geometry of the uranium-bearing veins. Had the rise of oxygen been slightly delayed, the natural nuclear reactors would not have been possible. What is surprising is the remarkable coincidence in the timing of the rise of oxygen and the minimum required ratio of the uranium isotopes for self-sustained fission. So, the fact that the Oklo ore had the required ratio of these isotopes is not too surprising. This condition will be met at some point in Earth’s history given the steady decline in the U-235 to U-238 ratio with time. But, two billion years ago it would have made up about the same fraction as enriched uranium does today in man-made reactors (about 3 percent). This is not possible today in nature, given that too little U-235 is available (due to its shorter half-life). Thus, life is largely responsible for the uranium ore deposits we mine, which date to at most two billion years.Ī second condition for a nuclear reaction is the sufficient abundance of the short-lived fissionable isotope, U-235, relative to the more abundant isotope, U-238. And, photosynthetic life caused the rise of oxygen. Organics greatly accelerate the rate of uraninite ore formation, whether from dead matter, algal matts, or living bacteria. It is interesting that life apparently played an important role in the formation of uranium ore. Once dissolved in groundwater, uranium can concentrate in aquifers in the crust where it is reduced to uraninite and precipitates out of solution to form ore. This is because uranium is soluble in water only when oxygen is present. However, this was not possible prior to the rise of oxygen in the atmosphere just over two billion years ago. First, the uranium minerals needed to be concentrated sufficiently for the fission reactions to be self-sustaining. They even predicted in 1956 the conditions that would have to be met to have one. Geologists and physicists have a pretty good idea how the Oklo reactors operated, and they have been able to model them. He suggested the reactors were not very improbable. He didn’t actually give a probability, however, simply because the probability is not yet known. He admitted that there were several highly specific conditions they had to satisfy. Any specified event or structure with a probability less than 1 part in 10 150 exhausts the probabilistic resources of the cosmos and cannot be attributed to chance.ĭembski considered whether the natural nuclear reactors at Oklo satisfied the complexity-specification criterion. The criterion involves knowing the universal probability bound, which is the maximum number of specified natural events in the cosmos. Dembski describes his explanatory filter and how it helps us understand the complexity-specification criterion to detect designed events. William Dembski asked this question in his 2001 book, No Free Lunch: Why Specified Complexity Cannot Be Purchased without Intelligence. How can such a process occur naturally? Must a natural nuclear reactor have been designed? No Free Lunch In addition, a moderator slows the fission-generated fast neutrons, which induce the fission of other uranium atoms. This is achieved with a coolant to remove heat from the reactor core. They have to control and contain the heat generated from the fission of enriched uranium. Modern nuclear fission power plants are complex machines. In 1972 French physicists discovered that natural nuclear fission reactors were in operation about two billion years ago in Oklo, Gabon in Africa.
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