The presence of natural nuclear reactors 2 billion years ago beneath Africa sounds like a hoax, but it actually happened – they are the only known natural nuclear reactors to have existed. By studying the behavior of the elements produced, scientists are learning lessons for the storage of waste from modern reactors.
The remains of the Gabon reactor in Oklo were first discovered in 1972, and since then 17 reactor sites have been found nearby. Scientists examining the site as a potential uranium mine realized the metal's isotope ratio was different from anywhere else on Earth, indicating induced fission of uranium-235. The deposit also contained the products of induced radioactive decay such as neodymium-143 and ruthenium-99.
A study in Proceedings of the National Academy of Sciences describes an examination of these products. Although the radioactive decay chain initiated during fission is well studied, we know less about how the resulting isotopes behave if released into the environment. With so many sites, and different conditions at each, the Oklo reactors provided Dr Evan Groopman of the US Naval Research Laboratory and co-authors with a rich natural laboratory.
Groopman found most cesium (more than 10 percent of the reactors' yield) migrated into the surrounding environment. Along with barium, much of the cesium was captured by ruthenium some five years after the reactor stopped operating, and the metals have remained together ever since. With cesium being the most important source of radioactivity after the nuclear disasters at Chernobyl and Fukushima, this observation could be used to prevent cesium contamination in the future.
Natural nuclear reactors could not occur today. Chain reactions usually require at least 3 percent of the uranium fuel to be U-235. Oklo aside, uranium deposits are 0.7 percent U-235 and 99.3 percent U-238. Nuclear power plants enrich U-235 with centrifuges, a process with no natural counterpart.
However, when the Earth first formed, it had almost as much U-235 as U-238. With a half-life of 700 million years, most of the U-235 has undergone radioactive decay, but half the original U-238 (half-life 4.5 billion years) survives.
Even when U-235 was more abundant, however, reactors required rich uranium deposits with water flowing through them to slow neutrons to make them more easily captured by U-235 nuclei. Even under these circumstances, continuous operation was impossible – Oklo's reactors ran for periods of about 30 minutes, before boiling away the water and shutting down for 2-3 hours.
So much U-235 underwent nuclear fission at Oklo that today the ratio is notably lower than in other uranium deposits. The extent of U-235 depletion depends on how long the reactor operated, with periods varying from 24,000-1 million years. The paper reports on the site dubbed Oklo reactor zone 13, where almost half the U-235 has been burned up, the highest proportion yet identified, indicating particularly intense reactions.