Oklo for half an hour

How we know…

How can we possibly know, two billion years later, that the fission in Oklo lasted half an hour at a time and then stopped for a few hours? That seems completely absurd. Nobody was there; nobody could see; do scientists just make these things up for breakfast?

Well, we were not there, and we can’t figure out all the details of what happened, but the reason for thinking it worked this way is completely logical and it has to do with the isotopes of xenon which are left in aluminum phosphate crystals in these deposits. You can read about it for yourself if you want to work through all the ideas more thoroughly, but here is the main scheme:

Xenon is a noble gas, an unreactive element like helium, and it has several isotopes that are produced at different points during the normal breakdowns of radioactive uranium and its various “daughter” elements.

Note: When uranium breaks down it does not become a different isotope of uranium, but a different element altogether, depending on how it breaks. The daughter elements can be identified as uranium breakdown products because each element has one isotope that is, you might say, its own “original,” and a few others that come at the end of various breakdown paths.

As scientists were looking over the material in Oklo, they were surprised to find that the several xenon isotopes commonly found after nuclear fission were found here in very different proportions from the norm; in particular, there wasn’t enough xenon-136 or xenon-134. Where could they be? How come the others were dominant?

After some consideration, researchers realized that the missing xenon isotopes were the daughter isotopes of elements that are soluble in water. Perhaps some of these isotopes (radioactive tellurium and iodine) had been carried away by water seeping through the sandstones around the uranium. Perhaps some had been carried away before they broke down into xenon. This water was certainly present for the fission because it is important to the fission itself, but it boils away when things get hot, and then the fission stops. So apparently it works like this:

1. Fission begins,
2. Radioactive tellurium and then radioactive iodine are produced, as well as other elements.
3. These two elements quickly produce xenon-136 and xenon-134 but since they are water-soluble, they are partly off-site by the time these xenons turn up.
4. The water heats and boils away.
5. Fission stops.
6. Other xenon isotopes are produced as things begin to cool, and some are produced very much later, when everything is stone cold.
7. Aluminum phosphate crystallizes out during the cooling and locks the later xenon isotopes (or their radioactive parents) in its crystals. (Otherwise they would float away.)
8. When things are cool enough, the water seeps back in.
9. Fission begins again.

Half an hour of fission and a few hours of crystal formation are what it would take to have part of the xenon-136 and xenon-134 missing while xenon-132, -131, and -129 are locked in the aluminum phosphate.

Something like that. Totally unexpected, complicated, but tidy.

Nice detective work, eh?

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The Oklo Reactor

The Oklo reactor is not in Japan. It is not in Denmark, in case you think it sounds like a Scandinavian name. Over its lifetime, it generated probably 15,000 megawatt-years of energy, about what you’d expect from 6 tons of Uranium-235, a four-year supply for a large reactor, or alternatively, the power supply for a few toasters for a few hundred million years.

Uranium has several different isotopes, mainly Uranium-238, which breaks down with a half-life of a few billions years, and Uranium-235 which is much more radioactive, breaking down at least 6 times faster. Odd that just three missing neutrons make such a difference, for that is all that distinguishes the two forms, but it has another effect as well:  the nucleus of a uranium-235 atom is unstable and apt to split if it is disturbed. All it takes is one wandering neutron, which might come from the natural radioactivity of either isotope or one of its breakdown products, and bam! — the uranium-235 can suddenly break into a couple of smaller unstable atoms, which may break down again, and their breakdowns yet again. Some of these breakdowns throw off neutrons, which, if there’s more uranium-235 in the area, can set off yet more chains of breakdown. All this breaking up is called fission and it can make the neighborhood quite hot if there’s enough of it. Interestingly, because the rate at which this series of decays takes place, the proportions of the fission-formed elements can tell us how long it’s been since the uranium-235 underwent a fission reaction.

At Oklo, it was 2 billion years ago. It was a rather lazy event, probably lasting half an hour at a time, cooling for few hours, and then continuing. And yes, it went on for maybe 200 million years.

Thus, the Oklo reactor was a natural event, and it produced its heat and died down again before men walked on the earth, before dinosaurs walked on the earth, before the shellfish left their households in the soil of Italy, and even before the trilobites who swam around for 250 million years and then died, leaving the sea to the fishes. All the animals we know go back maybe half a billion years, but the blue-green algae go back 3 billion years, and they were there to see Oklo. (Well, they didn’t have eyes.)

Where from, and why then?

The uranium came up from below, nobody knows how far below the crust of the earth, and spread over the edge of the Congo Craton of Africa, to which, incidentally, Brazil was still attached. At this time in the history of the world, the Great Oxygen Event was unfolding, when oxygen first became abundant in the atmosphere, apparently through the action of the blue-green algae, also called blue-green bacteria.

Incidentally, this surplus of oxygen had two very interesting effects:

1.   Oxygen reacted with iron in the water and laid down the banded iron formations which later gave the Bragas of Brazil their living.

2.   By reacting with the greenhouse gas, methane, already in the atmosphere, the new oxygen set off a worldwide glaciation, the Huronian, which very nearly brought life to a dead halt on earth before there were any footed creatures at all to walk its lands.

These amazing events belong to just this one period in earth’s history and they cannot happen again that way.

But the new flush of oxygen in those distant days was important to the uranium, too, because it allowed the formation of uranium oxides, which are water-soluble, and this allowed the uranium to be carried from one place to another. The place where the water dropped it, in Gabon, Africa, became a vast uranium deposit and is now a uranium mine. (Presumably, there are also uranium deposits “in” Brazil, but if so, they are probably 18-20 km underground, and are impossible to mine.)

It was in the context of mining operations in Gabon that the traces of this ancient, natural fission reaction were discovered in 1972, and an effort to understand their origin was slowly launched.

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