Two billion or so years ago there was a brief moment of time (geologically speaking) when nature managed to make something that would not be seen again until the middle of the last century – a natural nuclear reactor. Earlier and there wasn’t enough oxygen in the atmosphere to mobilize uranium (uranium is only soluble in oxygen-bearing water) and later the shorter-lived fissionable isotope (U-235) had decayed away to concentrations that made it increasingly hard to sustain a fission chain reaction. But for a time it was possible for water to move uranium and concentrate it in what are now ore deposits while the fraction of U-235 was about the same as today’s reactor fuel.
Of course there’s more to making a nuclear reactor than simply putting a bunch of uranium together – there also has to be a way to slow down neutrons to energies that are more likely to cause fission (a process called moderation). But in at least one location there was a wonderful confluence of events – uranium precipitated out of solution in a mass of sandstone; when the porous rock became saturated with water, conditions were right, and nuclear fission ensued. For over 100,000 years the uranium deposit happily fissioned away off and on with the reactor shutting down when the heat from fission boiled away the water and then restarting when the rocks cooled to the point where water could re-saturate the rocks.
Fast-forwarding a few billion years – to 1972 – French scientists noticed that uranium from a particular mine in the Oklo region of the African nation of Gabon had a different isotopic makeup than any other uranium ore on Earth, being depleted in U-235. After some great scientific detective work they realized that the only plausible explanation for the discrepancies they found was that this ore body had undergone fission – that the Earth had once had a natural nuclear reactor. Since that time studies have continued – there have been tons of findings but, to me, there are three that are particularly intriguing (in addition to the obvious one that nature beat us to the punch in this particular development):
- The type of rock formation in which the Oklo reactor was found is hardly unique
- The Oklo reactor formed in a sandstone deposit saturated with water in a geologic formation called a sedimentary basin. This kind of rock formation has been fairly common on Earth throughout its history – what is unusual is its preservation for so long a period of time. According to geologists Laurence Coogan and Jay Cullen (both of the University of Victoria) there might have been a fairly large number of such reactors at that point in Earth’s history. It could be that the Earth of a few billion years ago was filled with bubbling and steaming reactor zones, pumping radiation into the nearby environment. Not only that, but there has even been speculation that the same thing might have happened on Mars in the distant past. Pretty much anyplace where enough uranium with reactor-level concentrations of U-235 could collect and be immersed in water could have supported a fission chain reaction – on Earth, on Mars, or anywhere else in the universe.
- Virtually all of the fission products are still in place in the rocks that once hosted the reactor
- Uranium fission produces radioactive waste, whether the fission takes place in a natural or an artificial nuclear reactor. Surprisingly, Australian geologists J.R. de Laiter, K.J.R. Rosman, and C.L. Smith found that virtually all of the radioactive waste produced by the Oklo reactors can be accounted for. What makes this remarkable is that this radioactive waste has been sitting in porous rock that has been saturated with water for two billion years – and it’s still largely in place. This bodes well for our trying to isolate radioactive waste for a mere hundred thousand or million years in a specially designed repository dug into much less permeable rock that is only occasionally waterlogged.
- And some have speculated that natural reactors might have affected the evolution of ancient life.
- Coogan and Cullen also point out in their paper that radiation from ancient reactors might have had both positive and deleterious effects on nearby living organisms. The negative impact is easy to guess at – periodic blasts of high-level radiation could certainly kill all but the most radiation-resistant organisms. On the other hand, radiation dose rate drops off quickly with distance and shielding – move just a few meters from a location with deadly radiation levels and you can find yourself in an area that is easily survivable. It is entirely possible that radiation from early natural nuclear reactors not only killed whatever migrated closest, but that it also might have induced mutations in slightly more-distant organisms, accelerating the rate of evolution.
The Oklo reactor may or may not have been unique but, regardless, it is fascinating. And if it turns out to have lessons that can help us better understand the evolution of life or the disposition of radioactive waste then so much the better.