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A Natural Reaction

Uranium is, of course, the stuff of nuclear reactors and atomic weapons, but it is also part of an intriguing detective story from 1972 that traces back to events two billion years ago – actually 4.5 billion years but at that age who’s counting.

First a little background. For either nuclear reactors or bombs, Uranium 235 is required. This isotope of Uranium has fewer neutrons in the nucleus and is present in small concentrations with U238, the most common isotope. U235 with a natural concentration of 0.72 %, must be concentrated further to make a fissile material that is used in reactors and weapons.

In the early seventies, there was something of a panic in France. France, then as now provides the lion’s share of their electricity from nuclear reactors. At the time France was buying Uranium ore called yellow cake from a mining region in the Oklo River basin in Gabon, Africa. Assays of some shipments showed that the ore was unnaturally low in U235, sometimes by as much as half the expected concentration.

During this period there was much civil unrest as the continent slowly emerged from under the yoke of colonialism. It was feared that Uranium was being stolen by a local tribe with the intention of making a crude bomb. It turns out that the problem was a rather unnatural event in nature. When scientists looked at an analysis of the shipments low in U235 they found several unnatural elements such as Americium, Curium, and Polonium.

These so-called transuranic elements were not known to exist in nature until this discovery. The only place they had been observed was as part of the waste from nuclear reactions, both controlled reactors and bombs. The French had discovered an extremely rare event, a natural nuclear reactor.

When the U235 atoms draw too close together a chain reaction occurs which produces heat. That heat is used to produce steam in nuclear reactors. In the process, the U235 reacts to turn in to other elements. Exactly the same process occurred in the Oklo River basin.

Over two billion years ago there was scant free Oxygen in the air, then along came cyanobacteria. Gradually the atmosphere changed and many minerals reacted with Oxygen. All the rusty looking soil across the planet is due to Iron Oxide which formed during this period.

In the case of Uranium, it became more water-soluble as it oxidized. In locations with rich Uranium deposits such as the Oklo River basin, this allowed for the dissolved Uranium to accumulate in shallow lakes. Over time some of these lakes became isolated and as the lakes evaporated the Uranium was concentrated. Another bacteria capable of taking the Oxygen away from the Uranium Oxide reduced the solubility even further.

When the Uranium in these pools reached critical mass – the concentration necessary for a chain reaction – the U235 fissioned producing heat and forming the transuranic elements. As the reactions proceeded the U235 was depleted. Altogether sixteen different sites in the river basin have been found to have undergone fission reactions. To date this is the only known place on earth where a natural fission reaction has occurred.

Dr. Bob Allen is Emeritus Professor of Chemistry, Arkansas Tech University.