It is absurd to say that "high concentrations" of curium, americium and plutonium were found in these widely studied natural reactors, which operated for a few thousand years in a cyclical fashion. The longest lived curium isotope, which is very rare even in commercial nuclear fuels, Curium-147, has a half-life of 15.6 million years. It has a very high fission cross section, with resonances in the thousands of barns, and thus does not survive in reactors in an appreciable amount. This is a problem in campaigns at Oak Ridge to produce Californium-252, the yield is very low.
Even in modern reactors, which operate at higher power than the Oklo reactors did, the amount of americium and especially curium can never be said to be present in "high concentrations."
It is possible to imagine that plutonium-244 might survive long enough for a few atoms of it to remain in the Oklo formations, but the branch ratio from its formation from the decay of Cm-244 is low. To my knowledge, the only natural Pu-244 ever found on Earth was discovered by Darlene Hoffman in the lanthanide ore mines in California. It is believed to be primordial; an artifact of the supernovae that produced most of the heavy elements on Earth. It is very difficult to make Pu-244 even using modern technologies.
No Americium isotope has a half-life over that of Am-243, which has a half-life of only 7,370 years. It would not have survived at Oklo, nor would its daughter, Pu-239. Its occurrence would tend to increase the concentration of U-235, not deplete as at Oklo.
These were thermal reactors, not fast reactors, and in fact their cyclic power performance was tied to thermalization and dethermalization has water converted to steam.
Interestingly, the cesium did not migrate all that far, but there is a clear indication of the rate of migration given the isotopic distributions of the barium daughters of Cs-135 and Cs-137, the former being fairly long lived - but not long enough to have survived in the period after the Oklo reactors shut down.
The Oklo reactors operated only because oxygen appeared in Earth's atmosphere. Uranium assumes the mobile U(VI) oxidation state only in the presence of oxygen, and this is the only mobile form of uranium, and in fact, represents the state of the 4.5 billion tons of uranium that naturally occurs in the ocean. Before oxygen appeared on Earth, the seas had essentially no uranium in them.
The Oklo reactors, which functioned in porous sandstone, are often raised as an indicator of the absurdity of worries about so called "nuclear waste" dumps, particularly on a planet where millions of people die each year from fossil fuel waste. Very few nuclides migrated more than 100 meters over billions of years in a formation through which water flowed regularly, an exception being the pertechnate ion. The Oklo formations show relatively low concentrations of the technetium daughter, Ru-99.
However, despite the fact that Oklo makes anti-nuke paranoia look even more stupid than is commonly recognized, there is no real justification for "nuclear waste" dumps at all, since all of the materials in used nuclear fuel are exceptionally valuable, even if the general public is too badly informed to recognize this as such.
The authors of this news item should be excused on the grounds that they do not have a clear appreciation of nuclear materials, as they appear to be archeologists and not nuclear engineers. Still, the short excerpt contains significant errors.
I discussed the Oklo nuclear reactors years ago at Daily Kos: The Natural Nuclear Reactors At Oklo, and Fundamental "Constants." and Nuclear Waste and the Environment: What the History of the African Reactors at Oklo Can Tell Us.
I actually got to discuss Oklo and the fundamental constants with Freeman Dyson himself, in what may be one of the most memorable afternoons of my life.
Interestingly I was banned from Daily Kos for actually knowing something about nuclear energy, specifically for stating the fact that opposing nuclear energy kills people, which is a true statement. Facts matter. The attitude of Markos shows that even political liberals can reject truths based on uninformed, and frankly, ignorant biases. This particular bias of Markos, and many of us on the left, is extremely dangerous, given the crisis of climate change as well as the death toll associated with air pollution.
To repeat: Anti-nuke biases kill people, and are, in fact, destroying the entire planet.
The more we are aware of our own flaws, the better we will be able to govern. It is not enough to state that the opposition is bad, but it is more important to demonstrate that we have minds open enough to do good.
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