Nuclear power was invented in Africa 2 billion years ago. Now scientists think they have figured out how geological processes conspired to create the equivalent of a 100-kilowatt nuclear plant that produced pulses of power every three hours for a period of about 150,000 years.

These natural nuclear reactors were discovered in the Oklo region of Gabon in 1972. Scientists found geological evidence that uranium in lens-shaped veins of uranium ore had undergone self-sustaining fission chain reactions, generating intense heat.

In this process, subatomic neutrons released by radioactive decay of uranium atoms induce decay of other uranium atoms, leading to a cascade of nuclear fission and substantial release of energy as heat. This is what modern nuclear reactors use to produce power.

The puzzle, however, is why the Oklo reactors didn't plunge straight into a runaway chain reaction, leading to meltdown of the veins or even to an explosion. In nuclear plants the reaction is kept under control by using 'moderators'. These are substances that either slow down the chain reaction by absorbing some of the fission neutrons or encourage it by adjusting the neutron energies.

Chained reaction

Alex Meshik and his colleagues, from Washington University in St Louis, Missouri, have found evidence that the Oklo reactors switched on and off cyclically; they publish their results in Physical Review Letters1. Active periods of about 30 minutes seem to have been followed by dormant spells of around two and a half hours.

The researchers think that this was related to the presence of water in the rocks. When a uranium nucleus undergoes nuclear fission, the ejected neutrons are travelling too fast to be absorbed by other nuclei and trigger fission. So there is no chain reaction. But water slows the neutrons down. In the Oklo reactors, water allowed the chain reaction to be sustained.

As the reaction proceeded, however, it generated heat, which would have boiled away all the water. The reactors dried up and shut down. Only after they cooled and their water was replenished by groundwater flowing into the uranium veins could the process start up again.

Xenon and krypton

Meshik and colleagues deduced all of this by measuring the amounts of xenon in the Oklo rocks, which preserve an imprint of the way the reactors operated. Xenon is a radioactive decay product of uranium fission. They found no xenon in the uranium minerals themselves, but lots of it in grains of aluminium phosphate dispersed throughout the reactor rocks. These grains, says Meshik, contain "the largest concentrations of xenon ever found in nature".

Xenon is a gas, and so it might be expected to escape from the hot mineral veins as soon as it was produced. But if the reactors cooled down periodically, this would have allowed the xenon to get locked inside phosphate grains. The researchers used their measurements to calculate how long these periods of heating and cooling must have been.

Radioactive xenon and the related gas krypton are both generated in modern nuclear reactors, but are simply released into the atmosphere. There is no good way of capturing them. But in the Oklo reactors they seem to have been trapped inside atomic-scale holes in the phosphate crystal structure. "Maybe this can give us a clue how to capture these gases in nuclear plants," says Meshik.