Nuclear reactors are naturally-occurring phenomena. Not only that, but ancient bacteria can be held responsible for the production of at least two tonnes of plutonium.
How so? Well, two billion years ago, a pair of separate evolutionary processes combined, for a geologically short period of time, to produce a remarkable phenomenon.
For several aeons, the Earth's atmosphere was unable to sustain the presence of free oxygen. This only changed two billion years ago, when photosynthesizing bacteria evolved. Such bacteria synthesize organic molecules from carbon dioxide, water, and photons of sunlight. And, as a by-product, they generate oxygen. Hence, when these lifeforms evolved, for the first time in the history of the Earth free oxygen accumulated in the atmosphere.
That's one half of this story, to which we'll return later. The other half concerns the evolving balance between the naturally occurring isotopes of uranium. Most natural uranium is U-238, a non-fissile isotope. A small fraction, however, consists of the fissile isotope U-235. Uranium-235 has a shorter half-life than uranium-238, hence the fraction of U-235 has been decreasing since the formation of the Earth. Two billion years ago, the natural abundance of uranium-235 reached 3%. Today, the natural abundance of U-235 has dropped to only 0.7%.
Now, nuclear reactors require a moderator to sustain a fissile chain reaction. In a uranium-fuelled reactor, this is because a U-235 nuclei is most likely to fission when it absorbs a low-energy neutron. Such neutrons are termed thermal neutrons, and possess energy levels of less than 1 electron-volt (eV). When a U-235 nucleus fissions, it releases other neutrons, but these neutrons have a mean energy of around 1 MeV (Mega electron-volt). Such neutrons are more likely to be absorbed by U-238 nuclei than they are to cause other U-235 nuclei to fission. Hence the necessity for moderators.
Moderators are materials made from light nuclei, which are likely to deprive fission neutrons of their energy in so-called elastic scattering reactions. Such reactions reduce fission neutrons to the thermal energies which will trigger further fission in U-235 nuclei.
Anthropogenic nuclear reactors employ a variety of moderators, but the most popular choice is water. Such reactors require U-235 levels of between 3-5%. On the surface of a planet which is 4.5 billion years old, this requires uranium-235 to be enriched from its naturally occurring preponderance. Two billion years ago, however, the natural abundance of U-235 was just right. Thus, to trigger a self-sustaining nuclear reactor on the surface of the Earth, all it needed was some geological means of concentrating uranium minerals in porous groundrock.
Which is where our photosynthesizing bacteria come in. Groundwater flow is capable of dissolving, transporting, and then depositing materials in concentrated zones. Uranium, however, is insoluble in anoxic water, and was therefore initially immune to this method of re-distribution. Until, that is, the evolution of photosynthetic bacteria generated free atmospheric oxygen, which in turn, produced oxygenated water, in which uranium is soluble.
Two billion years ago, in what is now the Gabon, Africa, groundwater flows containing dissolved uranium from nearby igneous deposits, met a zone of petroleum. The petroleum de-oxygenated the water, and the uranium precipitated out of solution. At least 16 separate naturally occurring reactors went critical for a period of around 100,000 years, producing heat, highly radioactive fission products, and approximately two tonnes of plutonium. These are the Oklo nuclear reactors.
The plutonium has subsequently decayed away, but the distinctive isotopic fingerprint of those fission products is still detectable. Moreover, this high-level nuclear waste appears to have been successfully sequestered underground for billions of years...