In 1972 French physicists discovered that natural nuclear fission reactors were in operation about two billion years ago in Oklo, Gabon in Africa. Modern nuclear fission power plants are complex machines. They have to control and contain the heat generated from the fission of enriched uranium. This is achieved with a coolant to remove heat from the reactor core. In addition, a moderator slows the fission-generated fast neutrons, which induce the fission of other uranium atoms. How can such a process occur naturally? Must a natural nuclear reactor have been designed?

No Free Lunch

William Dembski asked this question in his 2001 book, No Free Lunch: Why Specified Complexity Cannot Be Purchased without Intelligence. Dembski describes his explanatory filter and how it helps us understand the complexity-specification criterion to detect designed events. The criterion involves knowing the universal probability bound, which is the maximum number of specified natural events in the cosmos. He quotes a very generous 10¹⁵⁰. Any specified event or structure with a probability less than 1 part in 10¹⁵⁰ exhausts the probabilistic resources of the cosmos and cannot be attributed to chance. 

Dembski considered whether the natural nuclear reactors at Oklo satisfied the complexity-specification criterion. He admitted that there were several highly specific conditions they had to satisfy. He didn’t actually give a probability, however, simply because the probability is not yet known. He suggested the reactors were not very improbable.

Geologists and physicists have a pretty good idea how the Oklo reactors operated, and they have been able to model them. They even predicted in 1956 the conditions that would have to be met to have one. First, the uranium minerals needed to be concentrated sufficiently for the fission reactions to be self-sustaining. However, this was not possible prior to the rise of oxygen in the atmosphere just over two billion years ago. This is because uranium is soluble in water only when oxygen is present. 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. 

It is interesting that life apparently played an important role in the formation of uranium ore. Organics greatly accelerate the rate of uraninite ore formation, whether from dead matter, algal matts, or living bacteria. And, photosynthetic life caused the rise of oxygen. Thus, life is largely responsible for the uranium ore deposits we mine, which date to at most two billion years.

A 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. This is not possible today in nature, given that too little U-235 is available (due to its shorter half-life). 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 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. So, the fact that the Oklo ore had the required ratio of these isotopes is not too surprising. 

The Rise of Oxygen

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. Had the rise of oxygen been slightly delayed, the natural nuclear reactors would not have been possible. Had oxygen arisen much earlier, natural nuclear reactors would have been more wide-spread, perhaps violently energetic, and depleted much more U-235.

Another condition relates to the geometry of the uranium-bearing veins. They must be at least two-thirds of a meter thick. Any thinner and too few of the emitted neutrons would be absorbed by the uranium. In addition, there can’t be too many “neutron poisons,” elements that absorb neutrons otherwise absorbed by uranium atoms. Some occur naturally, such as boron and silver, while others are produced in the fission process. 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!).

Finally, there must be a neutron moderator that makes the free neutrons more likely to be absorbed by the U-235 atoms. Just as in many man-made nuclear reactors, water was the moderator in the Oklo reactors. The surrounding porous sedimentary rock allowed water to interact with the uranium ore. The reactors probably operated in a cycle, shutting off for a few hours when the water boiled away from the nuclear heat generated, and then started again after it cooled down and water returned to the uranium core. This process has been compared to the natural cycling of geysers, such as Old Faithful in Yellowstone.

The Oklo reactors continued operating without meltdowns or explosions for up to a million years. Since then, they have remained relatively undisturbed. This is the final lesson the reactors are teaching us — how to deal with nuclear waste. The long-term preservation of the reactors is evidence that geological solutions exist for this problem.

It is interesting that natural nuclear reactors were predicted to exist less than two decades after physicists built the first one. Had they been discovered in the 1930s, they could have been an instance of “geonucleomimetics,” the geological and nuclear equivalent of biomimetics. 

Evidence of Design and Purpose?

Let’s return to Dembski’s question about the design status of the Oklo reactors. Application of his explanatory filter and the universal probability bound led him to doubt that they were designed. Known physical laws could explain their operation, and the reactors weren’t very improbable. The reactors would trigger the “physical necessity” node of his filter. One needs to remember, however, that Dembski’s filter applies to patterns and events within the history of the universe. For this reason, one can’t apply his filter to the physical laws, since they form the necessary backdrop against which contingent events in the universe are compared. 

At one level, then, one can conclude that the Oklo reactors are “natural.” But, what about a higher level of explanation? In this case, we can ask, “Is it logically necessary to find ourselves living on a planet with natural nuclear reactors?” If the answer to this questions is yes, then the Oklo reactors are not designed. Notice that this is a more specific question than asking, “Is it logically necessary that there should exist any natural nuclear reactors in the cosmos?” Even if the answer to this question were yes, they still could be designed.

Natural nuclear reactors are not logically necessary. Can they be explained by chance? Here’s where we can bring in additional information. The reactors don’t seem very improbable on Earth, but the timing of their appearance is fine-tuned. They appeared as late in Earth’s history as they possibly could have.

While we don’t know this for certain, natural nuclear reactors might, in the context of the Solar System, also be unique to Earth. If this is shown to be the case, then it would firm up the relationship between life and nuclear reactors. We know enough to conclude that planets and moons lacking an oxygen-rich atmosphere and a hydrological cycle inclusive of uranium-bearing rocks should be ruled out. 

Today, biomimetics inspires biologists and engineers to reinvent for our benefit technologies already present in nature. Although natural reactors didn’t inspire physicists and engineers to build the first one, that certainly could have been the case. Nuclear power plants are useful to modern civilization. Earth appears to be specially set up for energy generation in general. Much had to go right for fossil fuels to be abundant and accessible. Earth receives abundant solar energy at its surface (only the Moon and Mercury receive more solar energy at their surfaces). Earth also permits practical energy generation from wind and hydro. Of all the planets and moons in the Solar System, Earth provides the most diverse energy resources.

An Archeologist and a Rock

Sometimes we can determine that something is designed, but we are not able to discern its purpose. In those cases where a purpose can be discerned, we can be more confident that it was indeed designed. For example, an archeologist digs up a rock that looks like it was shaped in an unnatural way. He may decide to keep it for that reason. Upon closer inspection, consideration of the context of the find, and comparison with similar-looking artifacts with known provenance and purpose, he eventually concludes that it was fashioned and used as a hand ax. Now that he has assigned a purpose for the rock, he can have greater confidence that it is indeed an artifact of an ancient intelligence.

Natural nuclear reactors, by themselves, are like the interesting-looking rock. Perhaps they are designed, but more information is needed. Within the context of Earth’s history, the timing of natural reactors is fine-tuned. Within the broader context of the Solar System, natural reactors appear to be unique to Earth. There is a tight relation between the existence of natural reactors and life, but not in the sense that life requires natural reactors for its survival. And, finally, natural nuclear reactors fit within the broader context of Earth’s uniquely diverse energy resources, which are so important to modern civilization.

Looking at natural nuclear reactors, then, as part of the larger purpose of providing us with diverse energy resources gives me confidence that they are designed. This has profound policy implications. Perhaps we were meant to make use of Earth’s energy resources. There are safe and economically sensible ways to use them. Precisely how we develop and use them, however, is a question for specialists in multiple fields, and informed people can disagree on the details.

What about those geysers? It is instructive to compare them to natural nuclear reactors. First, they don’t seem to fit a broader pattern or purpose. They are not very improbable. They are not unique to Earth. Finally, they don’t require fine-tuning. For these reasons, I think we are safe in classifying geysers as natural.

Photo: A human-designed nuclear power plant, by mzter from Pixabay.