Where are these Natural Fossil Reactors?

Fossil reactor sites

The site of the Oklo uranium deposit.

Natural fossil reactors have (so far) only been found in the country of Gabon in equatorial Africa. All but one of the reactors are located at a place known as the Oklo uranium deposit located in the south eastern corner of the country. Another fossil reactor has also been discovered in Gabon at another U deposit at Bangombe, some 35 km south east of the Oklo mine.

The Gabonese highlands

The Gabonese highlands.

Even though Gabon is an equatorial country, the countryside around Oklo consists of grassy highlands, as shown in this image. During the 1960’s, geochemical exploration by mining interests of these highlands at the boundary of what is known as the Franceville Basin revealed extensive deposits of highly enriched Uranium. At a place called Oklo a large U orebody was discovered. The main ore body was subsequently found to be several kilometers long and to contain pockets of highly enriched (up to 70% pure) uranium as UO₂ which they began mining shortly thereafter

The local Oklo countrside

The countryside surrounding the minesite.

To process the U ore the mining company built an ore processing plant (centre of image) and a village to house company officials and miners close to the minesite.

Map of the local area

The minesite showing the location of the 15 fossil reactors.

This map shows the layout of the Oklo minesite and the location of 15 of the fossil reactor zones. The ore body itself is several kilometres long and about half wide. The economically viable ore varies in thickness from metres to tens of meters.

Mining of the U started from the northern end of the deposit soon after the discovery of the ore in the 1960’s. In 1972 when the reactors were first discovered, mining was halted while a detailed geochemical survey and sampling (drilling of boreholes to obtain rock cores) of the deposit and surrounding or host rocks was performed.

During this time the first 6 of the reactor zones were discovered. Once the survey was completed, U mining recommenced. Many of the reactor zones are now completely mined out. However, rock core samples of these reactor sites are stored at Saclay in France.

Five of the reactors are still mostly intact and underground and several isotopically abnormal boreholes have been discovered at Oklobondo just south of the Oklo Deposit. The other major reactor zone is located at Bangombe some 35 km south of the Oklo deposit.

The Oklo mine today

The Oklo minesite looking south. Reactor zone 2 can be seen fixed in a concrete block.

The Oklo uranium ore pit. The majority of the mined uranium existed as a layer of U ore which covered the right hand side of the pit.

Of the first 9 reactors only about one third of one reactor (reactor zone 2) remains and is now fixed in concrete into the side of the mine pit. This was done not because the reactor zone is dangerous to life but to stop it from sliding down the slope of the pit. The reactor zones themselves were centimetre to metre thick layers of highly enriched U, buried within the U ore.

The remains of Zone 2

The north western side of the Oklo minepit.

The remains of Reactor zone 2 are clearly visible as a rectangular concrete block. On the right is the entrance to a tunnel which leads to one of the remaining and relatively unstudied underground reactor zones (zone 15).