We tend to think that humans are the only possible source of complex machinery on Earth. Leaving aside the exquisite complexity of biologically evolved organisms, it does seem to be true that the Earth creates less complexity than its human inhabitants.
In 1972, a worker at a nuclear fuel processing plant noticed something suspicious in a routine analysis of uranium obtained from a mineral source from Africa. As is the case with all natural uranium, the material under study contained three isotopes- three forms with different atomic masses: uranium 238, the most abundant variety; uranium 234, the rarest; and uranium 235, the isotope that can sustain a nuclear chain reaction. For weeks, specialists at the French Atomic Energy Commission (CEA) remained perplexed
Elsewhere in the earth’s crust, on the moon and even in meteorites, we can find uranium 235 atoms that make up only 0.720 percent of the total. But in the samples that were analyzed, which came from the Oklo deposit in Gabon, a former French colony in West Africa, the uranium 235 constituted only 0.717 percent. That small difference was enough to alert French scientists that there was something very strange going on with the minerals. These little details led to further investigations which showed that at least a part of the mine was well below the standard amount of uranium 235: some 200 kilograms appeared to have been extracted in the distant past, today, that amount is enough to make half a dozen nuclear bombs. Soon, researchers and scientists from all over the world gathered in Gabon to explore what was going on with the Uranium from Oklo.
What was fund in Oklo surprised everyone gathered there, the site where the uranium originated from is an advanced subterranean nuclear reactor that goes well beyond the capabilities of our present scientific knowledge? Researchers believe that this ancient nuclear reactor is around 1.8 billion years old and operated for at least 500,000 years in the distant past. Scientists performed several other investigation at the uranium mine, and the results were made public at a conference of the International Atomic Energy Agency. According to News agencies from Africa, researchers had found traces of fission products and fuel wastes at various locations within the mine.
Modern reactors slow their fission-ejected neutrons by simply putting a large volume of water into the system. The water slows the neutrons enough that they can impact the uranium nuclei, making the Oklo formations technically light water reactors (LWR). So-called “heavy water” reactors use a much more expensive form of water with a heavy hydrogen isotope called deuterium (D2O). Heavy water reactors slow the neutrons even more, allowing us to actually use samples with lower U-235 ratios. (As an aside, I’ve never heard a good explanation of why the West doesn’t offer to sell Iran the plans for CANDU heavy water reactors, which can use natural, unenriched material.)
Incredibly, compared with this massive nuclear reactor, our modern-day nuclear reactors are not comparable both in design and functionality. According to studies, this ancient nuclear reactor was several kilometers long. Interestingly, for a large nuclear reactor like this, thermal impact towards the environment was limited to just 40 meters on the sides. What researchers found even more astonishing, are the radioactive wastes that have still not moved outside the boundaries of the site, as they have still held in place thanks to the geology of the area.
One would imagine that engineers working in the nuclear power industry could learn a thing or two from Oklo. And they certainly can, though not necessarily about reactor design. The more important lessons may be about how to handle nuclear waste. Oklo, after all, serves as a good analogue for a long-term geologic repository, which is why scientists have examined in great detail how the various products of fission have migrated away from these natural reactors over time. They have also scrutinized a similar zone of ancient nuclear fission found in exploratory boreholes drilled at a site called Bangombe, located some 35 kilometers away. The Bangombe reactor is of special interest because it was more shallowly buried than those unearthed at the Oklo and Okelobondo mines and thus has had more water moving through it in recent times. In all, the observations boost confidence that many kinds of dangerous nuclear waste can be successfully sequestered underground.
Oklo also demonstrates a way to store some forms of nuclear waste that were once thought to be almost impossible to prevent from contaminating the environment. Since the advent of nuclear power generation, huge amounts of radioactive xenon 135, krypton 85 and other inert gases that nuclear plants generate have been released into the atmosphere. Nature’s fission reactors suggest the possibility of locking those waste products away in aluminum phosphate minerals, which have a unique ability to capture and retain such gases for billions of years.
The nuclear reactor had a supply of a regulating substance as well: a flow of natural groundwater. As the atoms started to split, they released neutrons as well as energy. The water would slow down the neutrons, but the energy would heat up the water. After a time, the water would get so hot that it would start to boil off. Eventually, enough of the water would have boiled away until there wasn’t enough left in the reactor to slow down the neutrons. The neutrons started shooting off into the ground without reacting with anything, and the reaction would stop. Then, the natural flow of groundwater would trickle in until there was enough water to start the whole process again. This watery cycle probably continued for hundreds of thousands of years.
Sadly, all good days are numbered, even for a happy natural reactor: The levels of uranium 235 got used up and the level was too low to sustain any more meaningful reactions. The reactor eventually slowed to a stop, leaving only a few traces behind that it ever existed – including the enigma of the “missing uranium.”