If you could go back and read the news and scientific articles at the time, you could contrast the shock, dismay and confusion caused by the initial announcement that nuclear material had been mined in the past and that the byproduct plutonium had been created at Oklo.

In 1972, a French analyst named Bougzigues working at the Pierrelatte nuclear fuel processing plant detected a small but significant change in an important ratio between Uranium 235 and Uranium 238. The proper ratio is well known and well established in that field so that the change indicated that something way out of the ordinary was going on. The initial suspicion was that someone had included spent fuel in with the recently mined ore. This was quickly ruled out because there was no radiation signature associated with the mystery uranium.

The mystery uranium was eventually traced back to the Gabon mine at Oklo. You need to understand that what was so incredible was that a nuclear reaction had occurred such that plutonium was created and that the nuclear reaction itself had been “moderated”! This means that once a reaction is initiated, if you want to harness the power output in a controlled manner, you had better be able to keep it from exploding and releasing all the energy at once.

This particular group of reactors was, incredibly moderated using..., water. The first modern nuclear reactors used graphite and cadmium rods to moderate their reactions. At that time, people were speculating about aliens and advanced ancient civilizations. Some months after the discovery rocked the scientific world, someone came up with a somewhat dubious explanation which over time has taken on the patina of absolute certainty. It was a natural reactor which occurred billions of years ago.

There are at least six zones of depleted uranium (usually means mined) and with plutonium as a by product! You've got to really be trying to manufacture plutonium - its a complicated process. Yet, scientists studying these zones have a priori of course, decided not to consider that man was involved:

1) They place its occurrence safely "billions" of years ago

2) They assume that a breeding reaction could occur naturally when no such evidence of that exists and creating a breeder reactor was once a technological holy grail which was difficult to achieve on purpose

3) They theorize that although it must have happened naturally in the distant past, that we need not worry about it occurring now, as conditions differ

4) Nature designed the reactor such that its spent fuel and dangerous byproducts were "contained"

Scientists use the principle of uniformism to draw conclusions about the future and the past. If they cannot use this "principle", then they can not draw any conclusions because they could not state that the conditions in the universe that exist now, always existed. Here though as often happens, they have to assume that the past was vastly different as it related to the possibility of naturally occurring breeder reactions. Again, scientists from other countries were skeptical when first hearing of these "natural nuclear reactors". Some argued that the missing amounts of U-235 had been displaced over time, not split in nuclear fission reactions.

"How," they asked, "could fission reactions happen in nature, when such a high degree of engineering, physics, and acute, detailed attention went into building a nuclear reactor?"

Perrin and the other French scientists concluded that the only other uranium samples with similar levels of the isotopes found at Oklo could be found in the used nuclear fuel produced by modern reactors. They found that the percentages of many isotopes at Oklo strongly resembled those in the spent fuel generated by nuclear power plants, and, therefore, reasoned that a similar natural process had occurred.

Another View of the Phenomenon
Surprisingly, this uranium mine's nuclear reactor was well designed. Studies indicate that this reactor was several miles in length. However, for such a huge nuclear reactor, the thermal impact to its environment was limited to 40 meters on all sides. Even more astonishing is the fact that the radioactive wastes have still not migrated outside the mine site. They are held in place by the surrounding geology.

Faced with these findings, scientists consider the mine to be a "naturally occurring" nuclear reactor. The Oklo reactor has been documented for its importance as an analogue (a structural derivative of a parent compound) in the disposal of nuclear fuel wastes. But few people are bold enough to go one step further. As a matter of fact, many people today know that the reactor is a relic from a prehistoric civilization. It's probable that two billion years ago there was a fairly advanced civilization living at a place now called Oklo.

This civilization was technologically superior to today's civilization. Compared to this huge "natural" nuclear reactor, our current nuclear reactors are far less impressive. The question is: why did such a highly advanced civilization disappear? That's something to ponder about.

The textbook of Basic Radioactive Chemistry (C. Claire ed.) used by Tsinghua University has the following paragraph:

"The natural uranium in the Oklo mine in Gabon, West Africa, contains an abnormal amount of U235. It is as low as 0.29%, rather than the normal 0.72%. This means that many self-sustained nuclear fission chain reactions took place at this mine about two billion years ago. Thirteen nuclear reactors existed in prehistoric periods along the 200-metre mine bed, and they were comparable to the modern nuclear reactor in power and heat combustion. This mine had the capability of enabling self-sustained nuclear chain reactions...."

This discovery, that shocked the entire scientific community in 1972, has already been forgotten by people today.