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:
- Fission begins,
- Radioactive tellurium and then radioactive iodine are produced, as well as other elements.
- 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.
- The water heats and boils away.
- Fission stops.
- Other xenon isotopes are produced as things begin to cool, and some are produced very much later, when everything is stone cold.
- 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.)
- When things are cool enough, the water seeps back in.
- 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?