Lessons from Oklo

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.

 

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5 Responses to “Lessons from Oklo”

  1. Bob Finch March 5, 2012 at 11:04 AM #

    The first and second findings mentioned in this article are in potential conflict. If ancient natural reactors were once more common than the single known occurrences at & near Oklo, then fission products from (and all other evidence for) one or more ancient natural reactors may well have been dispersed in their entirety. That is, the uniqueness of Oklo may well be the retention of those fission products, rather than retention being the norm. Indeed, exploring the reasons for why those fission products (and other radionuclides) were retained at Oklo – but possibly not elsewhere – might lead to a better understanding of how to achieve a similar result for modern radioactive-waste disposal.

  2. Dr Y March 5, 2012 at 11:17 AM #

    Good point – but not necessarily directly applicable. The problem is that most sedimentary basins such as the one where the Oklo reactors formed are not preserved for so long a time. They erode away or they are subducted, melted, and recycled. So it’s not a question of the fission products having migrated away so much as the basin itself no longer existing – we can’t even study them.

    If we can find another ancient reactor site (indicated by depleted U-235) that has lost its fission products then you’re correct that we’d have a great chance to look for differences that we can learn from.

    • Bob Finch March 7, 2012 at 7:02 PM #

      Erosion is certainly one of several potential means of dispersal (relevant to, but probably not a desirable fate for, a high-level waste repository).

  3. katesisco February 14, 2013 at 12:16 PM #

    http://www.world-nuclear.org/info/inf37.html

    Tokaimura Criticality 1999

    The accident

    At the point of criticality, the nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation, triggering alarms. There was no explosion, though fission products were progressively released inside the building. The significance of it being a wet process was that the water in the solution provided neutron moderation, expediting the reaction. (Most fuel preparation plants use dry processes.)

    The criticality continued intermittently for about 20 hours. It appears that as the solution boiled vigorously, voids formed and criticality ceased, but as it cooled and voids disappeared, the reaction resumed. The reaction was stopped when cooling water surrounding the precipitation tank was drained away, since this water provided a neutron reflector. Boric acid solution (neutron absorber) was finally was added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to some radioactivity. The next task was to install shielding to protect people outside the building from gamma radiation from the fission products in the tank. Neutron radiation had ceased.

    Oklo has many times been stated without knowledge that neutrons were produced. How could bacteria survive this?

  4. Dr. Y February 14, 2013 at 7:57 PM #

    Well – any nuclear fission produces neutrons, so I guess I always assumed that the radiation dose from neutrons was a given. But you’re right that it is not always explicitly stated. But I have read a few scientific papers that have calculated the neutron flux and radiation dose from both gammas and neutrons at Oklo, with the implication being that any organisms in the vicinity would have been dosed. But, again, this is implied and not stated. What would be interesting would be to calculate the radiation dose – gamma plus neutron – to organisms at different distances from the reactor zone, just to see what doses would be.

    With regards to your question – radiation dose from neutrons is more damaging than dose from gammas, but neutron exposure is survivable, provided it’s not too high. Neutrons cause anywhere from 5-20 times as much genetic damage as do gammas – this is called the “relative biological effectiveness” (or RBE). Without getting into all the gory details, any organism can survive neutron irradiation; just not quite as much as with gammas. It could well be that the zone in which neutron radiation was fatal could have been larger than the zone in which gamma radiation would have killed all the microbes – but there would have been a point at which the dose would have been low enough to be survivable.

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