If you’re looking for extraterrestrial intelligence, looking for reliable signatures of their activity from across the universe, you have a few options. You can look for a smart radio broadcast of the type that humans started transmitting in the 20th century. You can look for examples of modifications on a global scale, such as human civilization shown when you observe the Earth at a high enough resolution. You can look for artificial lighting at night, such as our city, town and fishing displays, visible from space.
Or, you might look for technological achievements, such as creating particles like antineutrinos in nuclear reactors. After all, this is how we first detected neutrinos (or antineutrinos) on Earth. But we might be kidding ourselves if we choose the last option. Earth naturally created a nuclear reactor long before humans existed.
To make a nuclear reactor today, the first ingredient we need is reactor-grade fuel. For example, uranium has two different natural isotopes: U-238 (with 146 neutrons) and U-235 (with 143 neutrons). Changing the number of neutrons will not change your element type, but will change the stability of your element. For both U-235 and U-238, they both decay through a radioactive chain reaction, but the average lifespan of U-238 is about six times longer.
As of today, U-235 makes up only 0.72% of all natural uranium, which means it must be enriched to a level of at least 3% for a sustained fission reaction, or special setup (involving a heavy water medium) is required. But 1.7 billion years ago was two full half-lives of U-235. At that time, on ancient Earth, U-235 made up about 3.7% of all uranium: enough for a reaction to occur.
Between different layers of sandstone, you’ll often find veins rich in specific elements before you reach the granite bedrock that makes up most of the Earth’s crust. Sometimes these are very lucrative, like when we find gold mines underground. But sometimes, we find other rarer materials in it, such as uranium. In modern reactors, enriching uranium produces neutrons, and in the presence of water, water acts like a neutron moderator, and some of the neutrons hit another U-235 nucleus, causing a fission reaction.
When an atomic nucleus splits, it creates lighter daughter nuclei, releasing energy and three additional neutrons. If the conditions are right, the reaction will trigger additional fission events, creating a self-sufficient reactor.
Two factors combined to create a natural nuclear reactor 1.7 billion years ago. The first is that above the granite bedrock layer, groundwater flows freely and it is only a matter of geology and timing for water to flow into the uranium-rich region. Surround your uranium atoms with water molecules and that’s a solid start.
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But in order for your reactor to function properly in a self-sustaining manner, you need an additional component: you want the uranium atoms dissolved in water. In order for uranium to dissolve in water, oxygen must be present. Fortunately, aerobic, oxygen-consuming bacteria evolved following the first mass extinction in Earth’s recorded history: the Great Oxidative Event. With the oxygen in the groundwater, whenever the water floods the veins, it has the potential to dissolve the uranium and possibly even produce a particularly uranium-rich material.
When you have a uranium fission reaction, you end up with a number of important features.
- Five isotopes of the element xenon are produced as reaction products.
- The remaining U-235/U-238 ratio should be reduced since only U-235 is fissionable.
- U-235 produces a large amount of neodymium (Nd) when separated, its specific gravity is: Nd-143. Typically, the ratio of Nd-143 to other isotopes is about 11-12%; seeing an enhancement indicates uranium fission.
- The same is true for ruthenium with a weight of 99 (Ru-99). The naturally occurring abundance is about 12.7%, and fission can increase this to about 27-30%.
In 1972, French physicist Francis Perrin discovered a total of 17 sites spread across three deposits at the Oklo mine in Gabon, West Africa, containing all four of these features.
Oklo fission reactors are the only known examples of natural nuclear reactors on Earth, but the mechanisms by which they occur lead us to believe that these could happen in many places, and possibly elsewhere in the universe. The fission reaction that splits U-235 occurs when groundwater floods uranium-rich deposits.
Groundwater acts as a neutron moderator, allowing (on average) more than a third of the neutrons to collide with the U-235 nucleus, continuing the chain reaction.
Because the reaction is only going on for a short time, the groundwater that slows the neutrons boils, stopping the reaction entirely. However, over time, if fission does not occur, the reactor cools naturally, allowing groundwater to re-enter.
By examining the concentration of xenon isotopes in mineral formations trapped around uranium deposits, humans, like an excellent detective, have been able to calculate the exact timeline of the reactor. In about 30 minutes, the reactor will enter criticality and fission will continue until the water boils. For the next approximately 150 minutes, there will be a cooldown period, after which the water will flood the ore again and fission will start over.
This three-hour cycle repeats for hundreds of thousands of years, until the declining U-235 population reaches a low enough level, below about 3 percent, that the chain reaction will no longer continue. At that time, all U-235 and U-238 could do was radioactive decay.
Looking at today’s Oklo site, we found that the natural U-235 abundance was reduced by 0.44% to 0.60% from its normal rate. Although the natural abundance is usually found to be very low at 0.720% U-235 and 99.28% U-238 (looking at uranium only), Oklo samples only show U-235 abundances ranging from 0.7157% to 0.7168%: both far low 0.72% of the normal value.
Some form of nuclear fission is the only naturally occurring explanation for this difference. Combined with the evidence for xenon, neodymium and ruthenium, the conclusion of a geologically fabricated nuclear reactor is almost inevitable.
Interestingly, many scientific discoveries can be made by looking at the nuclear reactions taking place here.
- We can determine the time scale of the on/off cycle by looking at various xenon deposits.
- The size and migration of uranium veins (along with other materials affected by reactors) over the past 1.7 billion years could provide us with a useful, natural analogue for the storage and disposal of nuclear waste.
- The isotope ratios found at the Oklo site allow us to test the rates of various nuclear reactions and determine whether they (or the fundamental constants driving them) vary over time.
Based on this evidence, we can determine that the rates of nuclear reactions and the values of the constants that determine them were the same 1.7 billion years ago as they are today.
Finally, and perhaps most important for understanding our planet’s natural history, we can use the ratios of various elements to determine the age of the Earth and its composition at the moment of its creation. Lead isotope and uranium isotope levels tell us that 5.4 tons of fission products were produced on today’s 4.5 billion-year-old Earth over a time span of about 2 million years, about 1.7 billion years ago.
Both U-235 and U-238 are produced when supernovae explode and neutron stars merge. By examining supernovae, we know that we actually created more U-235 than U-238, with a ratio of about 60/40. If all uranium on Earth was produced by a supernova, that supernova would have occurred 6 billion years before Earth formed.
Spontaneous natural nuclear reactions will occur on any world where there are abundant near-surface uranium veins mediated by water with a ratio of U-235 to U-238 greater than 3/97. These situations could arise at any time, as long as a sufficiently few half-lives have elapsed relative to the decay time of U-235, the discovery of a “reactor antineutrino” from another world could indicate that natural nuclear reactions are as easy as it could indicate the existence of a Intelligent, technologically advanced civilizations create their own nuclear reactions.
In a serendipitous place on Earth, in a dozen instances, we have overwhelming evidence of a history of nuclear fission. In the game of natural energy, nuclear fission should never be ruled out again.