A typical man-made nuclear reactor requires a myriad number of skilled technicians and scientists to ensure safe operation. All the parameters of the reaction must be exactly controlled, and the hazardous post-reaction nuclear waste must be disposed off somehow. However, nature (as always) has found a clever, simpler way of doing it. It has been known for around 30 years that nature once did nuclear (fission) chain reactions. Researchers (Alexander Meshik, and colleagues) at Washington University in St. Louis have analyzed the isotopic structure of noble gases produced in fission in a sample from the only known natural nuclear chain reaction site in the world in Gabon, West Africa, and have found how she does the trick.

A handheld nuclear reactor! (Courtesy: Washington University)
In a fission reaction, an unstable (like Uranium) atom splits into two smaller (Barium and Krypton) atoms (U₂₃₅-->Ba₁₄₂+Kr₉₀). The subscript (e.g. 235 in U₂₃₅) is the number of nucleons (protons + neutrons) in the nucleus of the atom. This total number of nucleons (also known as Baryon Number) on the left and right of the nuclear reaction must be identical. In the above reaction, left has 235, and right has 232, and therefore, each such reaction also generates three free nucleons (in this case, neutrons). These neutrons, at high speed, collide with other uranium atoms, and thus triggering their breakup (a chain reaction, as in a fission bomb). In a typical nuclear reactor, these runaway neutrons are absorbed by a moderator (e.g. deuterium/heavy water, D₂O), and thus enabling a controlled reaction.
The researchers found that at the Gabon site, radioactive uranium converted into xenon and krypton, in the presence of a moderating mineral (an assembly of lanthanum, cerium, strontium, and calcium called alumophosphate). The alumophosphate also absorbed the waste material produced by the reaction. Ground water helped carry the runaway neutrons to other uranium atoms. As reaction proceeded, the rising temperature boiled off the ground water. With no water, the reaction stopped. After a while, as the temperature dropped, new influx of ground water would re-start the reaction. In this sense, the whole apparatus operated like a geyser, switching on for half-an-hour after every two-and-a-half hours.
This reactor operated some 2 billion years ago, for about 150 million years, with a power output of 100 kiloWatts. So why did it not blow up? Researchers found that the boiling water provided an elegant feedback mechanism. Increasing temperature boiled off the water, and thus cutting off the reaction. Also, a high concentration of moderating chemicals slowed down the reaction process.
A lot can perhaps be learnt from this natural reactor. Perhaps we can find a way to utilize nuclear reactions, and also solve issues such as waste disposal, and runaway meltdowns.