1.7 billion years ago, the Earth had a natural nuclear reactor

If you’re looking for alien intelligence, and looking for a surefire signature from across the universe for their activity, you have a few options. You can look for smart radio broadcasts, like the kind humans started broadcasting in the 20th century. You can look for examples of planet-wide adaptations, such as shows of human civilization when you view Earth in high enough resolution. You can look for artificial lighting at night, such as viewing our cities, towns, and fisheries, that can be seen from space.

Or you might be looking for a technical achievement, like creating particles like antineutrinos in a nuclear reactor. After all, this is the first way we have discovered neutrinos (or antineutrinos) on Earth. But if we make that last option, we may be deceiving ourselves. Earth created a nuclear reactor, of course, long before humans even existed.

Experimental nuclear reactor RA-6 (Republica Argentina 6), en marcha, showing characteristic Cherenkov radiation from emitted faster-than-light particles in water. The neutrinos (or, more accurately, antineutrinos) were first hypothesized by Pauli in 1930 from a similar nuclear reactor in 1956.

(Credit: Bariloche Atomic Center/Beck Dario)

In order to create a nuclear reactor today, the first ingredient we need is reactor fuel. For example, uranium comes in two different naturally occurring isotopes: U-238 (with 146 neutrons) and U-235 (with 143 neutrons). Changing the number of neutrons does not change the type of element, but it does change how stable the element is. For U-235 and U-238, both degrade via a radioactive chain reaction, but U-238 lives about six times as much on average.

By the time you get to the present day, uranium-235 makes up only about 0.72% of the total naturally occurring uranium, which means it must be enriched to at least about 3% in order to have a sustainable fission reaction, or special preparation (includes water mediators) is required. heavy). But 1.7 billion years ago there was more than a full half-life since before Uranium-235. At that time, in the ancient Earth, Uranium-235 represented about 3.7% of total uranium: enough for a reaction to occur.

This diagram shows the chain reaction that can occur when an enriched sample of uranium-235 is bombarded with a free neutron. Once uranium-236 forms, it quickly splits, releasing energy and producing three additional free neutrons. If this reaction escapes, we get a bomb; If this reaction can be controlled, we can build a nuclear reactor.

(Credit: Fastfission/Wikimedia Commons)

Between the different layers of sandstone, before reaching the granite bedrock that makes up most of the Earth’s crust, you often find veins of mineral deposits rich in a particular element. Sometimes these are very profitable, as when we find golden veins underground. But sometimes, we find other rare materials there, such as uranium. In modern reactors, enriched uranium produces neutrons, and in the presence of water, which acts as a neutron modifier, part of these neutrons will strike another uranium-235 nucleus, causing a fission reaction.

When the nucleus splits, it produces lighter daughter nuclei, releasing energy, and also producing three additional neutrons. If conditions are right, the reaction will lead to additional fission events, resulting in a self-sustaining reactor.

Geological cross-section of the uranium deposits Oklo and Okilobondo, showing the sites of nuclear reactors. The last reactor (No. 17) is located at Bangombé, 30 km southeast of Oklo. Nuclear reactors are found in the FA sandstone layer.

(Credit: DJ Mossman et al., Deep Geologic Repositories, 2008)

Two factors came together, 1.7 billion years ago, to create a natural nuclear reactor. The first is that above the granite bedrock, groundwater flows freely, and this is only a matter of geology and time before water flows into uranium-rich areas. Surround the uranium atoms with water molecules, and that’s a strong start.

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But to make your reactor run well, in a self-sustaining way, you need an extra component: You want the uranium atoms to dissolve in water. In order for uranium to dissolve in water, oxygen must be present. Fortunately, aerobic bacteria that use oxygen evolved in the wake of the first mass extinction in Earth’s recorded history: the Great Oxygenation Event. With oxygen in the groundwater, dissolved uranium could be possible when water flooded mineral veins, and it could have especially created a uranium-rich substance.

A selection of some of the original specimens from Oklo, discovered in 1972. This piece of high quality ore from Oklo’s obscure mines contains 0.4% less U-235 than all other naturally occurring specimens relative to U-238, evidence of That kind of previous fission reaction had depleted uranium-235.

(Photo source: Ludovic Ferrière/Vienna’s Natural History Museum)

When you have a uranium fission reaction, you end up producing a number of important signatures.

  1. Five isotopes of xenon are produced as reaction products.
  2. The remaining uranium-235 / uranium-238 ratio should be reduced, because only uranium-235 is fissile material.
  3. U-235, when dissociated, produces large amounts of neodymium (Nd) with a specific weight: Nd-143. Usually, the ratio of Nd-143 to other isotopes is about 11-12%; Seeing the boost indicates uranium fission.
  4. Same deal for Ruthenium 99 (Ru-99). Fission occurs naturally with an abundance of about 12.7%, and nuclear fission can increase to about 27-30%.

In 1972, French physicist Francis Perrin discovered a total of 17 sites scattered across three ore deposits at the Oklo Mines in Gabon, West Africa, which contain all four of these signatures.

This is the site of the Oklo natural nuclear reactor in Gabon, West Africa. Deep in the Earth, in hitherto unexplored regions, we may find other examples of natural nuclear reactors, not to mention what can be found on other worlds.

(Credit: US Department of Energy/Sandia National Laboratories)

Oklo fission reactors are the only known examples of a natural nuclear reactor here on Earth, but the mechanism by which they occurred leads us to believe that these could occur in many locations, and could occur elsewhere in the universe as well. When groundwater floods deposits of minerals rich in uranium, fission reactions can occur, to split uranium-235.

Groundwater acts as a neutron medium, allowing (on average) more than 1 in 3 neutrons to collide with a U-235 nucleus, continuing the chain reaction.

With the reaction going on for a short period of time, the groundwater that tempers the neutrons boils away, stopping the reaction completely. However, over time, without fission occurring, the reactor cools naturally, allowing groundwater to enter again.

The terrain surrounding the natural nuclear reactors at Oklo suggests that the introduction of groundwater, over a bed of shale, may be a necessary component of rich uranium ore capable of spontaneous fission.

(credit: Curtin University/Australia)

By examining the concentrations of xenon isotopes that are trapped in the mineral formations surrounding uranium ore deposits, humanity, like an outstanding investigator, has been able to calculate the exact timeline for the reactor. For about 30 minutes, the reactor becomes critical, with fission continuing until the water boils away. Over the next 150 minutes, there will be a slowdown period, after which the water will sink into the ore again and fission will begin again.

This three-hour cycle repeats itself for hundreds of thousands of years, until the ever-decreasing amount of uranium-235 reaches a low enough level, below this ~3%, that the chain reaction cannot continue. At this point, all U-235 and U-238 can do is radioactive decay.

There are many natural neutrino signals produced by stars and other processes in the universe. For some time, it was thought that there would be a unique and unambiguous signal coming from the antineutrinos in the reactor. But we now know that these neutrinos can also be produced naturally.

(Credit: IceCube Collaboration/NSF/University of Wisconsin)

Looking at the Oklo sites today, we find a natural abundance of uranium-235 which has depleted from its natural ratios of 0.44% to 0.60%. Although the naturally occurring abundance found is usually incredibly low, at 0.720% U-235, compared to 99.28% U-238 (looking at uranium alone), the Oklo samples only display an abundance of uranium-235 that ranges from 0.7157% to 0.7168 %: All is well below the normal value of 0.72%.

Nuclear fission, in one form or another, is the only natural explanation for this discrepancy. Combined with evidence of xenon, neodymium and ruthenium, the conclusion that this was a geologically constructed nuclear reactor is inevitable.

Ludovic Ferrier, curator of the rock collection, holds a piece from the Oklo reactor at the Natural History Museum in Vienna. A sample of enriched ore from the Oklo reactor is now on permanent display in the Vienna Museum from 2019.

(Source: L. Gill / IAEA)

Interestingly, there are a number of scientific findings that we can derive by looking at the nuclear reactions that occurred here.

  • We can determine the time ranges of on/off cycles by looking at different xenon deposits.
  • The sizes of the uranium veins and the amount they have migrated (along with other reactor-affected materials) over the past 1.7 billion years could give us a useful natural isotope for how nuclear waste is stored and disposed of.
  • The isotopic ratios at the Oklo sites allow us to test the rate of different nuclear reactions, and determine whether they (or the fundamental constants that drive them) have changed over time.

Based on this evidence, we can determine that the rates of nuclear reactions, and therefore the values ​​of the constants that define them, were the same 1.7 billion years ago as they are today.

Finally, and perhaps most important to understanding the natural history of our planet, we can use the proportions of the various elements to determine the age of the Earth as well as its composition at the moment of its inception. The levels of isotopes of lead and isotopes of uranium have taught us that 5.4 tons of fission products were produced, over a period of approximately 2 million years 1.7 billion years ago, on a planet that is 4.5 billion years old today.

elements

This image from NASA’s Chandra X-ray Observatory shows the location of the various elements in Cassiopeia – a supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple), as well as an overlay of all of these elements. Elements (top). Supernova remnants expel heavy elements from the explosion into the universe. Although not shown here, the ratio of uranium 235 to 238 in supernovae is roughly 1.6:1, indicating that Earth was born largely of raw uranium, rather than recently.

(Credit: NASA/CXC/SAO)

When a supernova explodes, as well as when neutron stars merge, both U-235 and U-238 are produced. By examining supernovae, we know that we actually produce more uranium-235 than uranium-238 in a roughly 60/40 ratio. If the uranium on Earth had been created from a single supernova, that supernova would have occurred 6 billion years before the Earth was formed.

In any world, as long as there is a rich vein of near-surface uranium ore in a ratio greater than 3/97 from U-235 to U-238, mediated by water, a normal and natural nuclear reaction can occur. These conditions can arise at any time, and as long as a few half-lives have passed relative to the time of uranium-235 decay, the discovery of “reacting antineutrinos” from another world may point to a natural nuclear reaction just as easily occurring. It can refer to the existence of an intelligent and technologically advanced civilization that creates its own nuclear reactions.

In one serendipitous place on Earth, in more than a dozen cases, we have compelling evidence of the history of nuclear fission. In the natural energy game, never leave nuclear fission off the list again.

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